*This article contains advertisements (affiliate links). The products and services introduced in this article are those that the author has found to be actually useful, but readers should make their own purchasing decisions.
Introduction
Hello, this is DIY Renova.
I want to replace the door with a larger one. When I looked under the floor with this in mind, I found that there was no foundation under the opening, so I was frozen. This time we will talk about such a scene.
Previously, I introduced a method for repairing misalignment between an existing foundation and foundation using chemical anchors (Renovation of wooden foundations and foundations using chemical anchors). This time's theme is "Adding new basics."
To be honest, the most difficult part of this construction work is not the dimensions or the reinforcement arrangement.On the day of pouring, how do you transport that heavy concrete to the foundation?is. If you can't imagine this, no matter how many numbers you list, you won't be able to start construction.
So this time, in addition to judgment, dimensions, and reinforcement,What to do on the day and what to buyI will write up to This article also covers the selection and fixing of plywood for formwork.
So, here you go.
Terminology mini dictionary
| term | reading | In one word |
|---|---|---|
| cloth foundation | Nunogiso | Foundation built in a strip along the wall |
| rising | stand up | The part of the wall that protrudes above the ground within the foundation |
| Rooting | Neire | Depth of the bottom of the foundation into the ground |
| Reinforcement | Haikin | Placing reinforcing steel in concrete |
| Main reinforcement/reinforcement reinforcement | Shukin/Hokyokin | Reinforcing bars that support the strength of the foundation and reinforcing bars that supplement it |
| Deformed reinforcing bars (D13/D10) | Iki Techkin | Rebar with knots on the surface. The thickness is called D with a number. |
| Post-installation anchor | Atosekou Anchor | A metal fitting that is drilled into the concrete after it has hardened. |
| lap joint | Overlap | How to connect old and new reinforcing bars by overlapping them to a certain length |
| Pitching | Uchitsugi | Joints when pouring concrete on different days |
| laitance | Reitance | A thin layer of weak strength floating on the concrete surface |
| throwaway con | Station | Thin concrete used as a base for formwork and reinforcement |
| Allowable stress level | Today's information | Guideline value for the force that can safely support the ground |
First, see the finished product in 5 minutes
It's quick to look at the actual construction before reading the text.
Reference video: [Housing Renovation ②] How to pour additional foundation concrete?(Akiyama Interior Co., Ltd., released in June 2023, approximately 4 minutes 49 seconds)
As the title suggests, the theme is "How concrete was transported." The video description also introduces this process as one of the steps to create an additional foundation during a complete home renovation project.
Think of this article as a way to track down what's happening in that video using dimensions and numbers.
Differences between foundations, concrete floors, and foundations
Even though they are the same concrete, the following three have completely different roles. If you confuse this, you will make the wrong decision on expansion.
| portion | Role | Reinforcement | load bearing |
|---|---|---|---|
| Foundation (rising/footing) | A structure that transmits the weight of a building and seismic force to the ground. | Arrange main bars and reinforcement bars according to legal standards | whole building |
| dirt floor concrete | Entrance dirt floor or porch floor | Wire mesh level (auxiliary) | Weight and capacity for people to ride |
| foundation | wooden crosspieces resting on the foundation | (No because of wood) | Transfer the load from the column to the foundation |
When people say that the foundation of a wooden house is damaged, it often refers to deterioration of the foundation and anchor bolts. This difference was also covered in the previous chemical anchor article.
Divergence of decision: Is it only necessary to install the earthen floor or do I need to start the foundation?
There is only one conclusion.Is there an existing foundation directly under the newly loaded column next to the opening?. This is it.
It is not "directly below the opening". It is "directly below the column that receives the load." There are so many people who get this wrong.
Judgment procedure
- Existing columns fall out or move when the opening is widened.
- The load of the missing column is distributed to the neighboring column or newly installed column.
- Check if there is an existing foundation rising directly below the column to which it will be distributed.
- If not, construct or extend the foundation to that location.
Cases where only reinforcement of the dirt floor is sufficient
- Existing pillar positions can be used as is.
- The width of the opening after expansion is acceptable only with the lintel (horizontal material at the top of the opening)
- The purpose is to repair cracks and subsidence in the dirt floor concrete itself.
Cases that require up to the basics
- Erect an additional load-bearing column next to the new opening.
- There is no existing foundation rising directly below it.
- The wall to be removed was a "load-bearing wall" containing braces and structural facings.
Situation where confirmation by a qualified person is required ①: Whether or not the load-bearing wall can be removed
Load-bearing walls are walls that resist the horizontal forces of earthquakes and typhoons. Article 46 of the Building Standards Act Enforcement Order specifies the amount and balance of load-bearing walls required (Building Standards Act Enforcement Order, n.d., Article 46).
If you remove one part at your own discretion, the earthquake resistance of the entire building may be compromised. Please check with a qualified person, such as an architect, to determine whether the wall you want to remove is a load-bearing wall and plan a replacement reinforcement before proceeding.
Whether a confirmation application is required
In most cases, it is not necessary if you just replace the door. However, the story changes when it comes to the scale of modifying the foundations and pillars.
- If you are just replacing one opening, there is often no need to apply for confirmation.
- Confirmation is required when new pillars and load-bearing walls are installed or removed.
- Due to the review of wooden building classification in April 2025 (reorganization to new category 2 and new category 3 buildings), there are some buildings where the confirmation procedures for large-scale repairs and remodeling have changed (Ministry of Land, Infrastructure, Transport and Tourism Housing Bureau, 2025)
The actual necessity will vary depending on the size of the building, scope of construction, and local government operations. Please check with your architect or local building official before starting construction. This article cannot determine whether or not it is necessary for individual buildings.
5 methods for adding foundations
| Construction method | Main uses | Involvement in structure |
|---|---|---|
| Extension of cloth foundation | Install new pillars along the wall | Load-bearing structure |
| independent foundation | New support for only one pillar | Load-bearing structure |
| extra shot | Add the thickness and height of the existing foundation | Reinforcement of existing structures |
| Post-installation anchors/insertion bars | Integration of old and new, fixing of hardware | Joint force transmission |
| Addition of dirt floor concrete | Level difference elimination/surface renewal | Non-structural (self-weight only) |
A cloth foundation is a foundation that is built in a strip along the wall.
An extension of a cloth foundation or an independent foundation is a structure that receives additional loads. Design decisions regarding location and dimensions must be confirmed by an expert.
Minimum dimensions and reinforcement arrangement for laid foundations｜Legal standard values
Reinforcement is the arrangement of reinforcing bars in concrete.
Here are the numbers. The legal minimum standards for the dimensions and reinforcement of cloth foundations are:Ministry of Construction Notification No. 1347 of 2000(Ministry of Land, Infrastructure, Transport and Tourism, 2000). This is a public notice based on Article 38, Paragraphs 3 and 4 of the Enforcement Order of the Building Standards Act.
The table below shows the "minimum standards stipulated by law." These are different from the recommended specifications indicated by the Japan Housing Finance Agency specifications, so please do not mix them. The actual design will be decided based on the results of the ground investigation and structural calculations.
Before that: It is decided first whether or not you can choose a cloth foundation.
Allowable stress is a guideline value of the force that the ground can safely support.
Notification No. 1347 Firstly, construction methods are divided into three stages depending on the allowable stress level of the ground.
- Less than 20kN/m²: Only structures using foundation piles
- 20 or more and less than 30kN/m²: Foundation pile or solid foundation (cloth foundation is not available)
- 30kN/m² or more: Foundation pile, solid foundation, or cloth foundation
In other wordsCloth foundations can only be selected for ground with an allowable stress of 30kN/m² or more.is. If the results of the ground investigation do not reach 30, the construction method described in this article cannot be selected. This is the first barrier.
Minimum standards for dimensions (fabric foundation)
| item | minimum standards | source |
|---|---|---|
| depth of rooting | 24cm or more and deeper than the freezing depth | Notification No. 1347 Paragraph 4, Item 1, Paragraph 3, Item 4 |
| Height of rising part (above ground part) | 30cm or more | Notification No. 1347 Paragraph 4, Item 1, Paragraph 3, Item 3 |
| Thickness of rising part | 12cm or more | Same as above |
| Thickness of the bottom plate | 15cm or more | Notification No. 1347 Paragraph 4, Item 1 |
The base plate is the plate-shaped part that extends at the bottom of the foundation.
The width of the base depends on the allowable stress of the ground and the type of building.
| Allowable stress level of the ground | Wooden construction/one-story house | Wooden construction, 2-story building | Other buildings |
|---|---|---|---|
| 30 or more and less than 50kN/m² | 30cm or more | 45cm or more | 60cm or more |
| 50 or more and less than 70kN/m² | 24cm or more | 36cm or more | 45cm or more |
| 70kN/m² or more | 18cm or more | 24cm or more | 30cm or more |
Source: Notification No. 1347, Paragraph 4, Item 2. The table starts with numbers above 30 for the reason stated in the previous section.
Minimum standards for reinforcement
| part | 径 | Placement/pitch |
|---|---|---|
| Main reinforcement of rising part | Deformed reinforcing bars with a diameter of 12 mm or more | At least one reinforcing bar at the top of the rising edge and one at the bottom of the bottom plate, tied with reinforcing bars. |
| Reinforcement bar at the rising part | Diameter 9mm or more | Vertical spacing of 30cm or less |
| Reinforcement bars for the bottom board (if the bottom board width exceeds 24cm) | Diameter 9mm or more | At intervals of 30cm or less, tied to the reinforcing bars at both ends of the bottom plate. |
Source: Notification No. 1347, Section 3, No. 5, I and B, Section 4, No. 3.
Main reinforcing bars are reinforcing bars that support the strength of the foundation, and reinforcing bars are reinforcing bars that supplement it.
If you search for these "diameter 12 mm or more" or "diameter 9 mm or more" in the store, you probably won't find it. How to buy it will be solved in the shopping guide in the second half.
Cover thickness (distance from the reinforcing bar surface to the concrete surface)
The legal minimum value is found in Article 79 of the Building Standards Act Enforcement Order (Building Standards Act Enforcement Order, n.d., Article 79).
| part | Enforcement Order Article 79 (legal minimum) | Public building construction standard specifications (unless otherwise specified) |
|---|---|---|
| Rising part of cloth foundation | 4cm or more | - (Adopting values close to outdoor classification for columns, beams, and load-bearing walls) |
| Foundation (excluding rising parts such as footings) | 6cm or more (excluding the thickness of abandoned concrete) | 60mm |
| Columns, beams, floors, and walls that are in direct contact with the soil | 4cm or more | 40mm |
Source: Article 79, Paragraph 1 of the Enforcement Order of the Building Standards Act, Standard Specifications for Public Building Works (Architectural Work Edition) 2020 Edition Table 5.3.6 (Ministry of Land, Infrastructure, Transport and Tourism, Minister's Secretariat, Maintenance Department, 2025). Since the figures are for public works, please treat them as a ``practical guideline unless otherwise specified'' when applying to single-family DIY projects.
The cover is a margin to prevent the reinforcing steel from rusting. If there is not enough, the rust will expand in a few years and crack the concrete from the inside.
In principle, unreinforced concrete foundations cannot be used.
With the amendments that took effect on April 1, 2025, reinforced concrete foundations are required regardless of the type of ground. The exception for unreinforced concrete construction that existed before the amendment is no longer applicable.
This point was confirmed on a specialized commentary site for structural regulations (Easy to Understand Structural Regulations, n.d.). The same content was also confirmed on another commentary site (blog-architect.me, 2024）。
The primary original text of the revised notification itself could not be identified at this time. Please note that the content of the two secondary sources is consistent. Before starting construction, be sure to check the latest notifications or operations of specified administrative agencies.
What is happening on the scene that day?
If you only look at the numbers, you won't be able to see the construction progress. Here, we will follow the process as it actually moves. First, there are three steps: excavation, reinforcing, and post-installation anchors.
1. dig (excavation)
Penetration is the depth of the foundation into the ground. Crushed stone is the process of laying and hardening crushed stone under the foundation. Thin concrete is thin concrete that is used as a base for formwork and reinforcement.
The depth of excavation is determined by addition.Rooting depth + crushed stone thickness + throw away concrete thickness.
- Rooting depth: 24cm or more (minimum standard of Notification No. 1347)
- Thickness of crushed stone: 60mm unless otherwise specified (Public Construction Standard Specifications 4.6.3)
- Thickness of waste concrete: 50mm unless otherwise specified (4.6.4)
If it meets the minimum standards, 24+6+5From about 35cmis. If you dig by hand by diving under the floor, this 35cm is farther than you might think. Since I can't swing a shovel, I'm mostly left with a hand hoe and a dustpan.
The excavated soil is packed in bags and taken outside. Even in a 2m section, there will be more soil than expected.If you don't decide where to put the soil first, it will get lost under the floor.
After leveling the crushed stone, compact it sufficiently using a rammer three times or a vibration compactor twice (see 4.6.3).
2. Assembling reinforcing bars (reinforcement and insertion bars)
Reinforcement involves drilling holes in existing concrete, fixing deformed reinforcing bars with adhesive, and connecting them to new reinforcing bars using lap joints.
A lap joint is a method of connecting old and new reinforcing bars by overlapping them to a certain length.
The length of the lap joint is determined by a multiple of the reinforcing bar diameter d. For SD295/concrete design standard strength Fc18, the standard is 45 d or more without a hook, and 35 d or more with a hook (Ministry of Land, Infrastructure, Transport and Tourism, Minister's Secretariat, Maintenance Department, 2025).
For D13 rebar, 45d=13×45Approximately 585mm. In other words, they overlap by about 60cm. It's not a matter of "just layer it up a little". If this is shortened, the old and new reinforcing bars will not be able to transfer the force.
Cover thickness is ensured by lifting the reinforcing bars from the formwork surface using spacers.
In an example of pouring moisture-proof concrete for a dirt floor under the floor, 50 concrete dice (spacers) were used (DIY Forest, 2021). The number will vary depending on the area and reinforcement density, but please estimate the quantity assuming that it will be distributed over the entire surface.
3. Drill a hole (later installed anchor)
Post-installation anchors are metal fittings that are drilled and driven into concrete after it has hardened.
The hole diameter and embedment length should be determined according to the specifications of the anchor used. Locations are limited.
- Edge clearance: 5.2 times or more of the anchor shaft diameter
- Edge clearance: 5 times or more of the anchor shaft diameter
(Ministry of Land, Infrastructure, Transport and Tourism, 2006). These numbers are for seismic retrofitting of RC structures, but the idea of avoiding splitting is also common to DIY foundation additions.
and,The most labor-intensive part of this process is cleaning the inside of the hole.is. The procedure is 4 steps (Ministry of Land, Infrastructure, Transport and Tourism, 2006).
- Vacuuming out chips from the hole with dust suction
- Check drilling depth
- Scrape off chips from the hole wall with a special brush
- suck dust again
Immediately after opening the hole, the inside is full of powder. If this powder remains, it will impair the adhesion of adhesive anchors and the tip expansion of metal anchors (Ministry of Land, Infrastructure, Transport and Tourism, 2006). Since it's not visible, the finish will look the same even if you don't do it. That's the scary part.
In the same example, 72 rebar anchors are used (DIY no Mori, 2021). Even if it is a small job to clean one by one, when it comes to cleaning dozens of pieces, the effort of cleaning becomes effective for the entire process.
Assembling formwork | Basics of plywood selection and fixing
Formwork is a temporary "mold" until the concrete hardens. The finish and safety will vary depending on the type of plywood and the strength of the fixing.
What is a control panel?
The plywood used for formwork is called ``konpane'' on site. Know-it-all eradication channel_1 minute course on architectural terminology explained in 2021. Abbreviation for plywood for concrete formwork.
The video covers the following items:
- What is a control panel?
- tree species
- Surface treatment
- size
- Adhesive water resistance
- Where used
Reference video: "Conpane (plywood for concrete formwork)_1 minute course". This is a video of approximately 1 minute and 50 seconds that was released in January 2021 by the know-it-all eradication channel_1 minute course on architectural terminology.
What is the difference in finish between painted panel and structural plywood?
There is a difference in the finish after demolding between painting control panels called "yellow plywood" and using general structural plywood. Kurimori Assembly Hall DIY Renovation channel will be introducing it in 2023. There is also an actual example where formwork was constructed using four types of plywood and compared (same).
In short, as long as the raised parts are invisible, there is no practical problem even if structural plywood is used. Please select a dedicated control panel for areas where you want to show off the finished surface. This is the same idea as the judgment in the "Parts used and alternative candidates" table above.
Reference video: "A beautiful finish that reflects the surrounding scenery!? ~ The power of yellow plywood was real! Old house DIY renovation #71". This is a video released by Kurimori Assembly Hall DIY Renovation channel in July 2023. This is a continuation of the previous article (#70) in which we assembled and compared four types of plywood, and we are checking the finished product after demolding.
How to assemble and fix the formwork (pieces and separators)
The formwork cannot stand on its own with plywood alone. It is supported from the back side with square timbers called ``sagi'', and the facing formworks are connected with metal fittings called ``separators.''
A rise is the part of the wall that rises above the ground within the foundation.
The separator has the role of maintaining the distance between the formwork on both sides and preventing the formwork from opening due to pressure during pouring. The spacing between the crosspieces should be determined based on the height of the stand and the thickness of the plywood, within a range that does not deform the formwork.
The thickness of plywood for formwork is 12 mm unless otherwise specified (Ministry of Land, Infrastructure, Transport and Tourism, Minister's Secretariat, Maintenance Department, 2025). The formwork shall be fixed with sufficient strength to withstand the concrete's own weight, lateral pressure, and vibration during pouring (same).
Reference video: “How to pour and construct dirt floor concrete that can be made DIY from formwork [Soil concrete DIY intermediate edition]”. This is a video of approximately 23 minutes and 30 seconds that was released by CROWN DIY CHANNEL in August 2021. There is a chapter on how to make the formwork in the episode where you make your own dirt floor concrete about 90cm square from formwork.
Reference video: “[Formwork Edition] Must-see before pouring concrete on the dirt floor vol.1”. This is a video of about 21 minutes and 40 seconds that was released by Gardening Channel in September 2024. This is a formwork edition written by a specialist in external construction and plastering, and covers formwork construction procedures.
How to prevent “harami”
The phenomenon in which the formwork expands during pouring is called ``harami.'' Once you get angry, you can't stop it right then and there.
If the spacing between the crosspieces and the number of separators are insufficient, problems will easily occur. Please check the following three points before pouring.
- Make sure that the spacing between the piers is sufficiently close to the height of the rise.
- Is the number of separators included commensurate with the expected pouring pressure?
- Is the bottom edge of the formwork securely fixed to the ground or concrete to prevent it from moving?
This is a different story from the numbers seen in the above section ``Minimum Dimensions and Reinforcement Arrangement for Cloth Foundations''. Even if you arrange the reinforcement according to the numerical values, if the formwork does not support it, it will not take shape.
From pouring to curing and anchor bolts
Once the formwork is fixed, it's time to pour the concrete. I'll follow up from here until the foundation is tightened.
1. Pour concrete (placement)
A pour joint is a joint created when concrete is poured on different days.
There are 3 points for processing the joint surface. The original text of the standard specification is as follows.
Prior to driving, clean the driving area to remove any foreign matter, and sprinkle water to keep the weir board and joint surface moist. Laitance and weak concrete are removed from pour joint surfaces to expose sound concrete.
Laitance is a thin layer of weak strength that floats on the surface of concrete.
In short, the three points of "cleaning," "wetting," and "removal of laitance" are checked again just before pouring (Ministry of Land, Infrastructure, Transport and Tourism, Minister's Secretariat, Maintenance Department, 2025).
For compaction, a rod vibrator is inserted vertically into each layer and vibrated until the paste floats on the top surface. When pulling it out, do it gradually so as not to leave any holes (same as above).
2. Wait (cure)
The period of moist curing varies depending on the type of cement (Ministry of Land, Infrastructure, Transport and Tourism, Minister's Secretariat, Maintenance Department, 2025).
| Types of cement | Duration of moist curing |
|---|---|
| Ordinary Portland cement, etc. | 5 days or more |
| Early strength portland cement | 3 days or more |
| Blast furnace cement type B etc. | 7 days or more |
The minimum retention period of the formwork (weir board) is determined by the temperature and age of the material (same).
| average temperature | ordinary portland cement | Early strength portland cement |
|---|---|---|
| 15℃ or higher | 3 days | 2 days |
| 5℃ or more | 5 days | 3 days |
| 0℃ or higher | 8 days | 5 days |
If you can confirm that the compressive strength is 5N/mm² or more, you can demold the mold regardless of the number of days (same).
I'm tempted to touch it the day after I hit it, but I have to hold back.
This is the "minimum number of days for demolding" stipulated by laws and specifications.
In practice, there are cases where the curing period is longer. There is an example in which the foundation formwork was constructed and poured, and the mold was removed after a curing period of about two weeks. Kurimori Assembly Hall DIY Renovation channel will be introducing it in 2023.
Judging the minimum storage period and the curing period on site are two different things. Don't rush and plan well.
Reference video: "A beautiful finish that reflects the surrounding scenery!? ~ The power of yellow plywood was real! Old house DIY renovation #71". This is a video released by Kurimori Assembly Hall DIY Renovation channel in July 2023. In the episode that covers the process up to the completion of the foundation, we check that the mold has come off after two weeks of curing.
3. Fasten the foundation (anchor bolts)
The following values are widely used for anchor bolts that fasten the foundation to a newly constructed foundation.
- Diameter: M12 (12mm) or more
- Embedded length: 250mm or more
- Spacing: 2.7m or less for 2-story buildings or less, 2.0m for 3-story buildings
- One within 200mm from the center of the pillars at both ends of the load-bearing wall
The source is the specifications of the Japan Housing Finance Agency. Since the primary specifications are commercially available, this article only confirms them with multiple structural explanation articles. Due to the nature of the numerical values, it is preferable to check directly with the primary specifications. Please be sure to check the latest version of specifications or the designer before starting construction.
In addition, the anchor boltFill it before the concrete hardens. If you get stuck in the wrong position, you won't be able to go back.
How to transport ready-mixed concrete | The most important part of this construction work
This is the theme of the video at the beginning. If the mixer truck can pour directly to the foundation, this chapter is not necessary. The problem is that this is not the case.
First, convert the amount carried into weight
The quantity is determined by "width x height x length". Calculate the rising part and the bottom plate separately.
Example (minimum standard dimensions/new section 2m):
- Rising part: width 0.12m x height 0.3m x length 2m = 0.072m³
- Bottom board: width 0.45m x thickness 0.15m x length 2m = 0.135m³
- Total: 0.207m³
- Loss premium (when approximately 10% is expected): Approx. 0.23m³
If you stop here, you won't feel it.Correct the weight.
The kneaded volume of a 25 kg bag of instant cement is approximately 13 to 14 liters depending on the product. Divide 0.23m³ by this: 0.23÷0.0135Approximately 17 bags. 25kg x 17 bags =425kg.
With a foundation of only 2 meters, it can carry about half the weight of a light vehicle. And it's under the floor.
An example of transportation under the floor
In an example of pouring moisture-proof concrete for an earthen floor below the floor, 2 cubic meters of ready-mixed concrete was transported (DIY no Mori, 2021). This is nearly 10 times the current estimate (approximately 0.23 cubic meters).
In this example, in addition to a unicycle, a transportation aid called a "concrete shooter" is used (same as above). A concrete shooter is a tool that guides ready-mixed concrete to a location away from the entrance using a gutter-like part.
Even in places where it is difficult to maneuver the unicycle, such as under the floor, a combination of the shooter and the unicycle has actually been successfully used. As the quantity increases, the transportation plan itself will determine the success or failure of the process.
Reference video: "[50 Years Old DIY] #9 Earthen floor (moisture-proof) concrete placement under floor moisture-proof measures ③". This is a video of approximately 11 minutes and 38 seconds that was released by DIY Forest in July 2021. This episode deals with the placement of damp-proof concrete under the floor.
How to choose a transportation route
| root | Conditions for establishment | Tightness |
|---|---|---|
| Directly in the chute of the mixer truck | The formwork is within reach of a car and a chute. | minimum |
| Lead with concrete shooter | The formwork is within reach of the shooter from the vehicle or loading entrance. Often used in conjunction with a unicycle | Medium (takes more time to install, but requires less physical effort than a unicycle) |
| Transporting pistons with a unicycle (cat cart) | From the car to the formwork, the width is wide enough for a unicycle to pass through, and there are few steps. | medium to large |
| Handed over in buckets and neribune | Places where a unicycle cannot fit, such as under the floor | maximum |
| Knead on site (instant cement) | Amount within which pouring can be divided into multiple steps | It takes time, but it can be divided |
There is one thing to be careful about when using a unicycle.Ready-mixed concrete is much heavier than soil or gravel, so it won't lift when fully loaded. If you aim for one cup to be two 25kg bags, or about 50kg, it will be a tough round trip, but you can carry it reliably. If it is 425 kg, it will be 8 to 9 round trips by simple calculation.
And ready-mixed concrete won't wait. If it arrives all at once, the amount must be discontinued within its pot life.The decision to order ready-mixed concrete depends on whether there are enough people to carry it.
Should I order ready-mixed concrete or mix it myself?
There are four judgment axes.
- Amount of pouring: If it exceeds 0.5 to 1 cubic meter, consider ready-mixed concrete.
- access: Can a mixer truck stop by? If you can't come, you'll need transportation personnel.
- Pitching: Ready-mixed concrete becomes inefficient if it is poured multiple times.
- pot life: If you are working alone, instant cement is more practical because you can cut the amount of kneading yourself.
This is how you compare the advantages and disadvantages. The quality of ready-mixed concrete is stable, so you don't have to manage variations in strength yourself. On the other hand, since large quantities are delivered at once, it requires manpower and is time-consuming. You can proceed with on-site training at your own pace, but uneven kneading will lead to uneven strength.
If you're doing it alone, it's safer to practice on-site so you can measure the amount.
Three numbers to say when ordering ready-mixed concrete
Ready-mixed concrete is manufactured and traded based on JIS A 5308 (Yamaguchi Prefecture Ready-mixed Concrete Industry Association, 2024). There are three things to tell when ordering:
- Nominal strength (N/mm²): 18-21 is the standard for foundation work.
- Slump (cm): Softness of ready-mixed concrete. Choose from 12 to 18
- Maximum dimension of coarse aggregate (mm): General foundation work is 20 or 25
At the site, these three are communicated side by side, such as "18-15-20." This means a nominal strength of 18, slump of 15 cm, and coarse aggregate of 20 mm. If you can say these three things, you can establish a conversation with the ready-mixed concrete factory.
In addition, with the 2024 JIS revision, the slump 10cm designation was abolished (Yamaguchi Prefecture Ready-Mixed Concrete Industry Association, 2024). Please be careful not to look at old materials and ask for 10cm.
Actual values should be determined in accordance with the structural calculations and specifications.
Shopping Guide | What should I buy?
I think this is the most confusing part. To avoid freezing at the sales floor,Only the deciding factor to chooseArrange them.
Reinforcing bars: ``12 mm or more'' as advertised is not available in stores.
The notice says "diameter 12 mm or more" or "diameter 9 mm or more," but reinforcing bars are not sold at those numbers.Deformed reinforcing bars are called "D + number"It is distributed in
- Main reinforcement (diameter 12mm or more) → D13 (Nominal diameter 13mm)
- Reinforcement bar (diameter 9mm or more) → D10 Buy (nominal diameter 10mm)
D13 and D10. If you keep these two things in mind, you will be able to meet the minimum standards of the notification. Remember that D13 is the main reinforcement and D10 is the reinforcement.
They are distributed in fixed lengths (generally standard lengths of about 3 to 5 meters). The required number is calculated by dividing the total length of the reinforcement plan by the standard length, and adding the amount for cutting and overlap joints. Some home centers also sell short lengths.
For cutting and bending, use a rebar cutter and bender, or hire a contractor. D13 cannot be bent by hand.
Concrete material: Dispense up to the number of bags
- For on-site training: Instant cement (25kg bag). The required number of bags is "required volume ÷ kneaded volume of 1 bag". In the previous example, about 17 bags
- For ready-mixed concrete: Order a 3-piece set of "nominal strength - slump - coarse aggregate" (example: 18-15-20)
- Crushed stone and gravel are calculated based on the volume equivalent to the thickness of the crushed stone land of 60 mm.
Bags are heavy, so be sure to factor in loading them into your car and transporting them to your home. 17 25kg bags alone weighs 425kg.
Hammer drill: Select by shaft standard
Hammer drills have different diameters that can be drilled with SDS-plus and SDS-max.
The SDS-plus shaft has a diameter of 10 mm, and the approximate drilling capacity is up to approximately 30 mm. The SDS-max shaft has a diameter of 18 mm and can accommodate up to approximately 52 mm (Buildy Magazine, n.d.).
For detached house inserts and post-installation anchors, SDS-plus is sufficient. Consider SDS-max only when drilling diameters exceeding 30 mm are planned.
There are three numbers to look at when choosing. Shaft standard (SDS-plus), concrete drilling capacity (mm), and mass. The lighter the weight, the easier it will be, but if it's too light, the hole won't advance. When choosing, keep in mind that you will be using it in a narrow position under the floor.
Rebar detector: 100mm cover is the dividing point
Electromagnetic induction type rebar detector can detect reinforcing bars with a cover of up to around 200mm. However, when the cover exceeds 100 mm, the accuracy decreases rapidly (Nippon Steel Technology, n.d.).
The foundation of a detached house has a thin cover of about 6 cm, so it is within the practical range. Multi-stage reinforcement or magnetic materials other than reinforcing bars may cause false judgments.
This is a tool where choosing not to buy is the most dangerous. When drilling a hole in an existing foundation, cutting the reinforcing steel inside will weaken the reinforcement.
Formwork materials/others
- Plywood for formwork (control panel): Thickness 12mm. The finished surface is plywood for concrete formwork (JAS compliant product) with surface treatment. As long as the rising part becomes invisible, there is no practical problem even if ordinary structural plywood is used.
- crosspiece: Square timber that supports the formwork from the back. Close the gap according to the height of the rise
- separator: A metal fitting that connects opposing formwork and secures it so that it does not open due to lateral pressure.
- concrete shooter: A transportation aid that guides ready-mixed concrete long distances, such as under the floor where it is difficult to reach with a unicycle.
- anchor bolt: M12 or more, total length that can secure embedment of 250mm
- Adhesive anchor: Capsule/injection resin that matches the perforation diameter. For parts of the structure that receive loads, select specialized products that have undergone performance evaluation.
- Brush for cleaning inside the hole: A diameter that matches the drilling diameter. Cleaning efficiency will decrease if the diameters do not match.
- Protective equipment: Dust mask and safety glasses. Concrete dust can get into your eyes and lungs.
Can it be installed over tiles?
We often receive the question, "Can I just hit on top of the current tile?"As a general rule, peel it off before installing..
When you put new concrete on top of tile or old mortar, the bottom and top layers don't blend together. Lifting and peeling will occur after some time. As a general rule, public works specifications also require removing existing finishing materials from surfaces and roughening them before construction (Ministry of Land, Infrastructure, Transport and Tourism, 2006). A similar idea is also shown in general construction on concrete bases (Ministry of Land, Infrastructure, Transport and Tourism, Minister's Secretariat, Construction and Maintenance Department, 2025).
Roughing is the process of slightly scraping the concrete surface to make it rough.
However, the weight of judgment changes depending on the location.
| place | Involvement of waterproofing | judgment |
|---|---|---|
| Entrance dirt floor/porch | small | If there is no lifting or cracking, and if laitance can be removed and roughened carefully, it can be considered with some conditions. |
| Bathroom/washroom/back door | big | In principle, not allowed. Only if a specialist can confirm that there is no deterioration of the waterproof layer. |
| outdoor dog run | Moderate (rainy) | If expansion/contraction adjustment joints can be installed at the edge of the frame, there is an option to add more. |
| Indoor dirt floor/tile floor | small | It is assumed that the additional thickness will not interfere with the fitting of adjacent floors and fittings. |
The bathroom area is a special case. If you pour concrete from above without checking the condition of the waterproofing layer, you will end up pouring new concrete while covering up the existing waterproofing defects. The waterproofing guidelines for wooden buildings stipulate that the slope of the waterproof base should be between 1/100 and 1/50 (Japan Waterproofing Materials Association, n.d.). Although these guidelines directly target flat roofs and balconies, the idea of preventing water from stagnating also applies to the foundations around bathrooms.
Reference video: [Site report] Earthen floor concrete construction site! I will show you everything from 1 to completion! [Doma Kon](AG Exterior Channel) You can check the flow from construction start to completion.
Parts used and alternative candidates (compatible with genuine)
*This is based on the author's verification and is not an official compatibility certification from the manufacturer.
| parts | Genuine/standard | Alternative candidate | compromise | DIY judgment |
|---|---|---|---|---|
| Adhesive anchor capsule/injection resin | Dedicated capsule with performance evaluation obtained by anchor manufacturer | General purpose injection resin | Some products do not have a performance evaluation report from a third-party organization. | For parts of the structure that bear loads, use genuine parts with evaluation documents. General-purpose products can also be considered for non-structural purposes such as temporary fixing. |
| Deformed reinforcing bars for insert bars | Standard product compatible with structural calculations (JIS G 3112 compliant). Main reinforcement is D13, reinforcing reinforcement is D10 | General-purpose products of the same standard | The manufacturer's own certificate may not be attached. | Substitutions may be made if conformity to standards is confirmed. Keep the delivery note and mill sheet. |
| Plywood for formwork | Plywood for concrete formwork (with surface treatment, thickness 12mm) | Repurposing general structural plywood | Poor removability and surface tends to be rough when demolding | Any rising part that becomes invisible is acceptable. Special plywood is recommended for the finished surface of the dirt floor. |
| Formwork fixing fittings (separator) | Separator exclusively for formwork (bolt with spacer function) | Self-made fixing with general-purpose fully threaded bolts and nuts | There is no water stop or spacer function on the formwork surface, and separate measures are required. | For small-scale startups, self-made fixing can be used. Check frequently as the risk of infection is high. |
| Brush and blower for cleaning inside the hole | Dedicated size for the drilling diameter specified by the anchor manufacturer | General-purpose size brush/air duster | Cleaning efficiency will decrease if the diameter of the hole is not matched exactly. | As long as you choose a diameter that matches the hole diameter, there are no practical problems even with a general-purpose product. |
| fresh concrete | Ready-mix concrete from JIS A 5308 certified factory | On-site mixing of instant cement and dry mortar | You need to manage variations in strength using your own kneading method. | DIY mixing is practical in areas where the amount of pouring is small and ordering is inefficient. Prefer ready-mixed concrete if the structure will receive a large load |
| Ready-mixed concrete transportation aid (concrete shooter) | Dedicated shooter tailored to site conditions | Homemade simple shooter made from PVC pipe and corrugated sheet | Strength, durability, and airtightness of connections are inferior to dedicated products. | For one-time, small-scale construction, it is possible to make it yourself. If you use it frequently, consider purchasing a dedicated product. |
| anchor bolt | Structural anchor bolts with diameter M12 and embedded length 250mm or more | Short and small diameter general-purpose bolts | The embedding length and placement rules at both ends of the load-bearing wall may not be met. | Use structural materials that meet standards for foundation bonding. Be sure to check specifications with the designer/specification sheet |
| Unicycle for transportation (cat cart) | deep unicycle for construction | Shallow and small model for gardening | The load capacity is small and the number of round trips increases. | If the passage width can be secured, a deep type is advantageous. Switch to buckets or neribune in places where you cannot pass, such as under the floor. |
Checklist of things to consider
- [ ] Have you determined the location of the load-bearing column next to the new opening?
- [ ] Have you confirmed whether there is an existing foundation right below the pillar?
- [ ] Have you checked with a qualified person whether the wall you plan to remove is a load-bearing wall?
- [ ] Have you confirmed whether or not a confirmation application is necessary depending on the size and region?
- [ ] Have you checked the allowable stress of the ground and confirmed whether a cloth foundation can be selected if it is 30kN/m² or more?
- [ ] Did you check the width of the bottom plate with the table in Notification No. 1347?
- [ ] Are you using plywood for formwork differently depending on whether it has a finished or unfinished surface?
- [ ] Whether the mixer truck can stop by, and if not, who decides how many times it will be transported back and forth?
- [ ] Have you decided where to put the dug soil?
Construction checklist
- [ ] Conducted reinforcing bar exploration and confirmed that there was no reinforcing bar at the drilling location.
- [ ] The excavation depth was calculated backwards from the penetration, crushed stone thickness, and disposal concrete thickness.
- [ ] The finishing material on the existing surface was removed and the surface roughened.
- [ ] Ensured the length of the overlap joint of the insert reinforcement (35 to 45 times the diameter)
- [ ] After securing the dimensions of the edge and edge of the installation anchor
- [ ] After drilling, the chips were thoroughly cleaned to the bottom of the hole.
- [ ] The spacing between the crosspiece and the separator was determined by calculating backwards from the lateral pressure and header.
- [ ] The formwork is fixed with strength to withstand lateral pressure.
- [ ] The pour joint surface was moistened before pouring.
- [ ] The anchor bolt was buried in the correct position before it hardened.
- [ ] After pouring, curing was performed for a number of days or more depending on the temperature.
Materials/tools checklist
- [ ] Ready-mixed concrete (nominal strength, slump, coarse aggregate specified) or instant cement (number of bags already calculated)
- [ ] Deformed reinforcing bar D13 (main bar)/D10 (reinforcement bar)
- [ ] Adhesive/capsule for post-installed anchors
- [ ] Plywood for formwork (control panel) 12mm, crosspiece, separator
- [ ] Structural anchor bolts (M12 or more, embedded length 250mm or more)
- [ ] Hammer drill (SDS-plus, SDS-max for diameters over 30 mm)
- [ ] Rebar detector
- [ ] Brush and blower for cleaning inside the hole
- [ ] Diamond cup grinder for roughening
- [ ] Rod vibrator (concrete vibrator)
- [ ] Unicycle or parade boat/bucket (consider using a concrete shooter for narrow floors)
- [ ] Inking tools such as level and inkpot
- [ ] Dust mask/safety glasses
Summary
Whether it is necessary to add a foundationIs there an existing foundation directly beneath the load-bearing columns next to the new opening?It is determined by
The Ministry of Construction Notification No. 1347 of 2000 stipulates the minimum standards for dimensions and reinforcement when necessary. Rooting 24cm, rising height 30cm, thickness 12cm, base plate thickness 15cm. The main reinforcement is D13 and the reinforcement reinforcement is D10. This number is your starting point.
The finish of the formwork is determined by the type of plywood (finished or unfinished surface) and the strength of the fixation with the crosspieces and separators.
The key point on that day was not the dimensions or the reinforcement arrangement.transportationis. Even a 2m section weighs over 400kg. Decide who will be making the many trips and then decide whether to order ready-mixed concrete.
The necessity of removing load-bearing walls and applying for confirmation is not an area that should be left to amateur judgment. Be sure to check these two points with a qualified person such as an architect.
If you're wondering "How much renovation or remodeling can I actually do on my own home?" or "Where should I even start?", please visit this page. If you're looking to learn more about specific renovation processes or construction details, don't miss this page as well!
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Please note that this document is not necessarily accurate, and should be used as reference information only.
Well, then!
Frequently Asked Questions (Q&A)
Q. Which reinforcing bar should I buy?
A. The main reinforcement is D13 and the reinforcement reinforcement is D10. These two products are compatible with the public notice's "diameter of 12 mm or more" and "diameter of 9 mm or more." At the sales floor, "D13" and "D10" are used.
Q. Can I use regular control panel instead of plywood for formwork?
A. As long as the rising part is invisible, there is no practical problem in reusing general structural plywood. For areas such as dirt floors that will be shown as a finished surface, choose plywood with a surface treatment specifically designed for concrete formwork.
Q. What should I check to prevent formwork from ``flaring up''?
A. The spacing between the crosspieces and the number of separators. Fill in the formwork at a density that does not deform the formwork, considering the height of the rise and the thickness of the plywood.
Q. What should I do if I can't decide between ordering ready-mixed concrete or mixing it myself?
A. Please decide based on the number of people you can transport. If you are working alone, it is safer to practice on-site so that you can cut the amount. Ready-mixed concrete is delivered all at once, so it requires manpower to cut it off within its pot life.
Q. How much can you put in one unicycle?
A. The guideline is around 50kg, which is equivalent to two 25kg bags. Ready-mixed concrete is much heavier than soil or gravel, so it won't lift when filled to capacity.
Q. How should I decide the nominal strength of ready-mixed concrete to order?
A. The general rule is to follow the values specified in the structural calculations and specifications. As a guideline, numbers 18 to 21 are used for foundation work, but please do not decide on your own as this will vary depending on the ground and design conditions.
Q. What will happen if the length of the lap joint is incorrect?
A. The old and new reinforcing bars may not be able to transfer the force, and the expected strength may not be achieved. Be sure to secure a multiple of the diameter (about 35-45d without hook). For D13, 45d is approximately 585mm.
Q. Which hammer drill should I buy, SDS-plus or SDS-max?
A. SDS-plus is sufficient for insert reinforcements and post-installation anchors for detached houses. The standard drilling capacity is up to approximately 30mm, so consider SDS-max only if you plan to exceed that.
Q. Can we assume that there is no reinforcing steel in a place where there is no response with the rebar detector?
A. We cannot say for sure. Detection may not be possible under conditions such as thick cover, multi-tiered reinforcement, or presence of magnetic materials other than reinforcing bars. If the reaction is weak, move the position or ask a professional to do it.
Q. How can I check whether the anchor bolt is embedded in the length enough?
A. Before pouring concrete, measure the length of the anchor bolt protruding from the outside of the formwork and the total length, and calculate the embedding length backwards. It is difficult to confirm after pouring.
Q. Can I add concrete to the dirt floor without removing the tiles at the entrance?
A. If there is no lifting or cracking of the tiles, and if you can carefully remove laitance, roughen the texture, and moisten the joint surface, there is room for consideration. As a general rule, it is not recommended in areas where waterproof layers are involved, such as around bathrooms.
References
- Akiyama Interior Co., Ltd. (2023). [Housing Renovation ②] How to pour additional foundation concrete? [Video]. YouTube. https://www.youtube.com/watch?v=KTbYKve7s4Y
- CROWN DIY CHANNEL. (2021). Method of pouring and constructing dirt floor concrete that can be made DIY from formwork [Earth floor concrete DIY intermediate edition] [Video]. YouTube. https://www.youtube.com/watch?v=Xqi2zBnWxJY
- Know-it-all eradication channel_1 minute course on architectural terminology. (2021). Control panel (plywood for concrete formwork)_1 minute course [Video]. YouTube. https://www.youtube.com/watch?v=-FUmIBqgu1c
- DIY Forest. (2021). [50 Years Old DIY] #9 Earthen floor (moisture-proof) concrete placement Underfloor damp-proof measures ③ [Video]. YouTube. https://www.youtube.com/watch?v=0zHAWB6-msQ
- Kurimori Assembly Hall DIY Renovation Channel. (2023). Beautiful finish that reflects the surrounding scenery! ? ~ The ability of yellow plywood was real! Old house DIY renovation #71 [Video]. YouTube. https://www.youtube.com/watch?v=sQKPlGO8QGk
- Gardening Channel. (2024). [Formwork Edition] Must watch before pouring concrete on the dirt floor vol.1 [Video]. YouTube. https://www.youtube.com/watch?v=m6rhLTvGWho
- Ministry of Land, Infrastructure, Transport and Tourism. (2000). Establishment of standards for construction methods and structural calculations for building foundations (Ministry of Construction Notification No. 1347, May 23, 2000). https://www.mlit.go.jp/notice/noticedata/pdf/201703/00006441.pdf
- Ministry of Land, Infrastructure, Transport and Tourism. (2006). Post-installed anchor/continuous fiber reinforcement design/construction guidelines Attachment. https://www.mlit.go.jp/jutakukentiku/build/anchor/060707sisin.pdf
- Ministry of Land, Infrastructure, Transport and Tourism, Minister's Secretariat, Maintenance Department. (2025). Standard specifications for public building works (building work edition) 2020 edition. https://www.mlit.go.jp/gobuild/content/001888816.pdf
- Ministry of Land, Infrastructure, Transport and Tourism, Minister's Secretariat, Maintenance Department. (2025). Public building wooden construction standard specifications 2020 edition. https://www.mlit.go.jp/gobuild/content/001888858.pdf
- Ministry of Land, Infrastructure, Transport and Tourism, National Institute for Land and Infrastructure Management. (2017). NILIM Materials No. 884 Chapter 6 Verification of the relationship between defects in post-installed anchors and tensile strength. https://www.nilim.go.jp/lab/bcg/siryou/tnn/tnn0884pdf/ks088411.pdf
- Japan Waterproofing Materials Association (n.d.). Waterproofing construction guidelines for wooden buildings (first edition). https://www.jwma.or.jp/wood/files/木造建築物 防水工事施工指針（初版）.pdf
- Japan Construction Post-installation Anchor Association (n.d.). Post-installation anchor installation procedure. https://www.anchor-jcaa.or.jp/anchor/construction-procedure.html
- Building Standards Law. (n.d.). e-Gov Law Search. https://laws.e-gov.go.jp/law/325AC0000000201
- Building Standards Law Enforcement Order. (n.d.). e-Gov Law Search. https://laws.e-gov.go.jp/law/325CO0000000338
- Housing Bureau of the Ministry of Land, Infrastructure, Transport and Tourism. (2025). Regarding building confirmation procedures for large-scale renovation of wooden detached houses [as of February 21, 2020]. https://www.mlit.go.jp/common/001766698.pdf
- Attached to the Ministry of Land, Infrastructure, Transport and Tourism Housing Bureau Counselor (in charge of architectural planning). (n.d.). A collection of case studies explaining the necessity of building inspection for renovations (wooden detached houses). https://www.mlit.go.jp/common/001853472.pdf
- Housing Finance Agency. (n.d.). Wooden housing construction specifications. https://www.flat35.com/business/shiyou01.html
- Yamaguchi Prefecture Ready Mixed Concrete Industry Association. (2024). JIS A 5308 Ready Mixed Concrete Revision. https://www.pref.yamaguchi.lg.jp/uploaded/attachment/193627.pdf
- Nippon Steel Technology Co., Ltd. (n.d.). Exploration of reinforcing bars in concrete using electromagnetic induction method. https://www.nstec.nipponsteel.com/technology/tr/kk-0040.html
- Buildy Magazine (Buildy Co., Ltd.). (n.d.). List of shaft standards, conversion adapter types, and tools. https://www.bildy.jp/mag/powertool_shaft_compatibility/
- Easy-to-understand structural regulations. (n.d.). What are solid foundations and cloth foundations? Applicable soil, cross section, and reinforcement specifications (Ordinance Article 38/Notification No. 1347). https://kouzou-houki.com/shiyou-kiso-betagumi.html
- blog-architect.me(2024). Unreinforced foundation concrete will be abolished from April 2025. https://blog-architect.me/2024/07/19/unreinforced-concrete-ban-2025/
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