When building a house, we tend to focus only on the visible parts, but in fact, it is the invisible parts that greatly affect the safety and comfort of a house.
Among these, the "foundation" is an important structural part that holds the fate of the house in its hands. In recent years, the "solid foundation" has been attracting attention especially from the viewpoint of earthquake resistance and ground protection.
Solid foundations are used in many houses as a construction method that supports the entire building with a plane surface. This article explains the basic structure of a solid foundation, the difference from a cloth foundation, its merits and demerits, and its cost, as well as other information necessary for those who are considering building a house in an easy-to-understand manner.
As the first step in building a house, the choice of foundation construction method is very important. Please read to the end and make use of this information in your home building.
A solid foundation is a foundation construction method in which the entire bottom of the house is covered with reinforced concrete. The name "solid" comes from the fact that the concrete is laid flat (sticky). In this construction method, the entire bottom plate takes the load of the building and the structure is supported by the surface.
The greatest feature of the solid foundation is that reinforced concrete is cast under the entire floor of the building to form an integrated and solid foundation. This structure distributes the load on the building evenly and is expected to reduce ground subsidence and building sway during earthquakes.
The Building Standard Law clearly stipulates the structure of a solid foundation. The thickness of the rising portion (the wall portion of the foundation) must be at least 12 cm, the thickness of the base must be at least 12 cm, and the height of the rising portion must be at least 30 cm above the ground level. These regulations are the minimum standards to ensure the safety of the building.
The structure of a solid foundation consists of two major parts: the base and the risers. The bottom portion is a flat concrete plate that supports the entire building, and the rising portion is a wall-like portion built around the bottom portion. Together, these two parts form a solid foundation structure.
During construction, the ground is first excavated and leveled, crushed stone is spread, and a moisture-proof sheet is laid. Then, reinforcing bars are assembled, formwork is installed, and concrete is poured. After the concrete has hardened, waterproofing is applied to complete the foundation work.
| Item | Regulation value | Remarks |
|---|---|---|
| Thickness of rising part | 12 cm or more | According to Article 38 of the Building Standard Law Enforcement Ordinance |
| Thickness of bottom plate | 12cm or more | According to Article 38 of the Building Standard Law Enforcement Ordinance |
| Height of rising part | 30 cm or more from the ground | Important for moisture-proofing and termite-proofing |
| Concrete used | Design standard strength of 18N/mm² or higher | 21N/mm² is common for general housing |
| Rebar thickness (main reinforcement) | D13 (13mm) or more | Standard for general housing |
| Rebar thickness (reinforcing bars) | D10 (10 mm) or more | Standard for general housing |
| Pitch of rebar | 20cm or less | Guideline for spacing of reinforcement bars |
There are two main types of foundation construction methods for houses: "solid foundations" and "fabric foundations. Understanding the difference between the two is very important in order to select the appropriate foundation method for housing construction.
Cloth foundation is a construction method in which a concrete foundation is provided only around the perimeter of the building and under the interior load-bearing walls. The building is supported by lines, and the subfloor remains soil. In contrast, a solid foundation, as mentioned above, covers the entire bottom of the building with concrete and supports the building on the surface.
The most significant difference between the two is the way in which the building's load is supported. Since solid foundations support the entire building on a plane, the load is easily dispersed, making them suitable for locations where the ground is weak or where unequal settlement (the ground does not settle evenly) is a concern. On the other hand, fabric foundations support the load of the building with lines and are suitable for locations where the ground is stable.
There are also differences in construction methods. Solid foundations tend to require more construction time and cost than fabric foundations because the entire subfloor is excavated and reinforced concrete is poured. Cloth foundations require less material and a shorter construction period because the foundation is installed only around the perimeter of the building and under the load-bearing walls.
The characteristics of the underfloor space are also different. With a cloth foundation, the subfloor is still soil, which can easily become damp and increase the risk of termite damage. On the other hand, with a solid foundation, the entire subfloor is covered with concrete, making it easier to prevent moisture from rising and termites from entering.
There is also a difference in earthquake resistance. Solid foundations have a surface support structure that is highly effective in dispersing seismic shaking, making them highly resistant to earthquakes. Cloth foundations are supported by lines, so their resistance to earthquake shaking tends to be inferior to that of solid foundations.
| Comparison items | Solid foundation | Cloth foundation |
|---|---|---|
| Structure | The entire bottom of the building is covered with concrete | Foundations are provided only around the perimeter of the building and under load-bearing walls |
| Supporting Method | Surface support | Linear support |
| Earthquake resistance | Excellent (load is distributed) | Slightly inferior (load tends to be concentrated) |
| Moisture resistance | Excellent (prevents moisture from rising from the ground) | Slightly inferior (moisture rises easily due to the soil under the floor) |
| Termite control | Excellent (prevents infiltration from the ground) | Slightly inferior (risk of infiltration from under the floor) |
| Anti-settlement measures | Excellent (load is distributed) | Slightly inferior (load tends to be concentrated) |
| Cost | High (about 1.5 to 2.5 million yen/30 tsubo) | Inexpensive (about 1,000,000 to 1,800,000 yen/30 tsubo) |
| Construction period | Long (about 2 to 3 weeks) | Short (about 1.5 to 2 weeks) |
| Suitable ground | Soft ground, ground with risk of unequal settlement | Stable ground, bedrock |
| Plumbing changes | Difficult (entire underfloor is concrete) | Relatively easy (with underfloor space) |
| Suitability in cold climates | Slightly inferior (cold air is easily transmitted) | Excellent (soil has an insulating effect) |
Although solid foundation is a construction method used in many houses, there are advantages and disadvantages as with other foundation construction methods. It is important to fully understand them before making a decision when considering housing construction.
The greatest advantage of solid foundations is their superior earthquake resistance. Because the structure supports the entire building on a plane, it is highly effective in dispersing seismic shaking, making it possible to realize a house that is resistant to earthquakes. This characteristic is especially advantageous in locations where the ground is uneven or soft.
The next advantage is the prevention of moisture and termite damage. Since the entire subfloor is covered with concrete, the rise of moisture from the ground is controlled, preventing wood corrosion and mold. It also reduces the risk of termite damage by blocking the pathway for termites to enter from the subterranean.
Another important benefit is the prevention of unequal settlement. Since the load of the building is distributed over the entire surface, even if a part of the ground settles, the effect on the entire building is minimized. This prevents problems such as building tilt and wall cracking.
On the other hand, solid foundations have the disadvantage of higher costs. Compared to cloth foundations, the amount of reinforcing steel and concrete used is greater, resulting in higher material costs. In addition, since the entire subfloor must be excavated, the cost of disposing of overburden is also added.
Another aspect of this method is that it is not suitable for cold climates. Concrete conducts heat easily, and in cold climates, cold air from under the floor is easily transmitted into the room. Therefore, if concrete is to be used in cold regions, it is necessary to provide sufficient insulation measures.
In addition, construction techniques and quality control are also important. Since concrete is poured over a large area, any construction defects may affect the entire building. The selection of a reliable contractor is extremely important.
Finally, another disadvantage to consider is the difficulty of changing plumbing. Since the entire subfloor is covered with concrete, if plumbing needs to be changed or added in the future, the construction tends to be more complicated and expensive.
| Advantages | Detailed explanation |
|---|---|
| Excellent earthquake resistance | Because the entire building is supported by a plane, it is highly effective in dispersing earthquake shaking. |
| Moisture Prevention | The entire subfloor is covered with concrete, preventing moisture from rising from the ground |
| Prevention of termite damage | Termite infiltration paths from the subterranean are blocked, reducing the risk of damage. |
| Prevention of unequal settlement | Loads are distributed over the entire surface, minimizing the impact of partial subsidence of the ground. |
| Improved building stability | Integrated solid foundation improves the stability of the entire building. |
| Easy installation of under-floor storage | Since the entire underfloor is flat concrete, it is easy to secure storage space. |
| Disadvantages | Detailed explanation |
|---|---|
| High cost | Requires large amounts of reinforcing steel and concrete, and disposal of surplus soil is expensive. |
| Not suitable for cold climates | Concrete conducts heat easily, so cold air from under the floor is easily transmitted to the interior. |
| Construction technique is important | Quality control and technical skills are especially important because of the large area involved. |
| Difficult to change plumbing work | Because the entire underfloor is concrete, it is difficult to change or add piping afterward. |
| Long construction period | Construction period tends to be longer than that of cloth foundation. |
| Large weight | Due to the large weight of the foundation itself, ground improvement is often required on soft ground. |
In residential construction, foundation work is an important part of the overall construction cost, accounting for a large proportion of the total construction cost. It is important to understand the cost and construction period in advance when adopting a solid foundation.
The cost of a solid foundation is approximately 1.5 to 2.5 million yen for a typical wooden house (approximately 30 tsubo). In contrast, the cost of a cloth foundation is approximately 1,000,000 to 1,800,000 yen, with the solid foundation tending to be 20 to 40% more expensive.
The main factors affecting costs include the size of the building (floor area), the condition of the ground, differences in labor costs by region, and the quality of materials used. If the ground is particularly soft, ground improvement work may be required, which will further increase the cost.
As for the construction period, a solid foundation takes slightly longer than a fabric foundation. For a typical wooden house, the construction period for a solid foundation is about 2 to 3 weeks, while that for a cloth foundation is about 1.5 to 2 weeks. However, this may vary depending on weather, ground conditions, and the contractor's system.
The main reasons for the higher cost of a solid foundation are the large amount of reinforcing steel and concrete used, and the cost of disposing of surplus soil due to the need to excavate the entire underfloor. However, in the long run, the benefits of improved seismic resistance and durability, as well as lower maintenance costs, make it a worthwhile initial investment.
To keep costs down, it is important to obtain quotes from multiple builders and compare them. In addition, since foundation construction is a part directly related to the quality of the house, it is recommended to select a contractor based not only on the lowest price, but also on the construction track record and technical capabilities of the contractor.
| Item | Solid foundation | Cloth foundation | Remarks |
|---|---|---|---|
| Cost (30 tsubo wooden house) | Approx. 1.5 to 2.5 million yen | Approximately 1,000,000 to 1,800,000 yen | Variable depending on the region and ground conditions |
| Construction period | About 2 to 3 weeks | About 1.5 to 2 weeks | Varies depending on weather conditions and contractor's system |
| Rebar usage | Much | Less | About 1.5 to 2 times the amount of rebar for a solid foundation |
| Amount of concrete used | Much | Less | About 1.5 to 2 times the amount of concrete for a solid foundation |
| Soil disposal cost | High | Low | Because solid foundations require excavation of the entire subfloor |
| Additional cost if ground improvement is required | Approx. 500,000-1,500,000 yen | Approx. 500,000-1,500,000 yen | Varies greatly depending on ground conditions |
| Long-term maintenance cost | Low | Slightly higher | Difference in moisture and termite control costs |
| Initial investment payback period | Long-term payback | Advantageous in the short term | Consider durability and maintenance costs |
When selecting a foundation method for a house, the condition of the ground is a very important factor in the decision. Understanding the relationship between solid foundations and the ground is the first step in creating a safe and long-lasting home.
Solid foundations are particularly suited for soft ground or ground at risk of unequal subsidence. Because the entire building is supported by a plane, even if a part of the ground settles, the effect on the entire building can be dispersed. This is expected to prevent problems such as building tilting and cracking.
Ground investigation is an essential process in determining the foundation method. The strength and properties of the ground are investigated using methods such as the Swedish sounding test and standard penetration test, and the appropriate foundation method is selected based on the results. If the results of the ground investigation indicate that the bearing capacity is insufficient, a solid foundation is generally adopted after ground improvement work is performed.
Ground improvement methods include surface improvement, columnar improvement, and steel pipe piling. The most appropriate improvement method is selected in consideration of the ground conditions, building size, and budget. Combining ground improvement with a solid foundation can provide a more stable foundation for a house.
In areas with a high groundwater table or poorly drained ground, solid foundations are also effective as a moisture countermeasure. Since the entire subfloor is covered with concrete, this helps control the rise of moisture from the ground and increases the durability of the building.
However, in the case of very hard ground, such as bedrock, a fabric foundation may be more economical than a solid foundation. Depending on the ground conditions, it is important to select a foundation method that balances cost and safety.
We have explained the features and selection points of solid foundations, which have been the focus of attention as a foundation construction method for housing. Here, we will reiterate the main points.
A solid foundation is a foundation construction method in which the entire bottom surface of the building is covered with reinforced concrete, and is characterized by a structure that supports the load of the building on its surface. While its superior earthquake resistance and moisture and termite resistance are major advantages, it also has the disadvantage of being more expensive.
In comparison with fabric foundations, solid foundations are suitable for locations with soft ground or a risk of unequal subsidence, and are superior in terms of earthquake resistance and building stability. On the other hand, fabric foundations are suitable for locations where the ground is stable and are advantageous in terms of cost.
In selecting a foundation method, it is important to make a decision based on comprehensive consideration of the ground conditions, building size, budget, and local climatic conditions. In particular, the results of a ground investigation are an indispensable decision-making factor in selecting an appropriate foundation method.
In terms of cost, solid foundations tend to be 20-40% more expensive than fabric foundations, but from a long-term perspective, they offer advantages such as improved earthquake resistance and durability and lower maintenance costs. Since a house is intended to be used over the long term, it is important to consider its value as an initial investment.
Finally, foundation work is an important part of the construction process that directly affects the quality of the house. By selecting a reliable builder and an appropriate foundation construction method, you can take the first step toward creating a safe and comfortable home. We hope this article will be of some help to you in your housing construction planning.
INA&Associates can recommend the most appropriate foundation method for your site conditions and building plan, and introduce you to the contractors who can help you. We provide consistent support from ground investigation to foundation construction and housing construction, so please visit us for a free consultation.
A house is a once-in-a-lifetime purchase. We recommend that you proceed with your plans while incorporating expert advice to ensure that you have no regrets about building your home.
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Yes, a solid foundation is a foundation construction method that is resistant to earthquakes. Since the entire building is supported by a plane structure, it is highly effective in dispersing seismic shaking and is more earthquake resistant than fabric foundations. This characteristic is especially advantageous in locations where the ground is uneven or soft.
However, earthquake resistance is determined not only by the foundation, but also by the overall design of the house, including the strength of the superstructure and joints, so comprehensive earthquake resistance measures are important.
The choice should be based on the following factors If the ground is soft or uneven, if seismic resistance is particularly important, or if moisture and termite control are important, solid foundations are suitable.
On the other hand, if the ground is stable, if cost is a priority, or if the building is to be constructed in a cold climate, a fabric foundation may be suitable. In the final decision, we recommend that you consider the results of the ground investigation, the size of the building, your budget, local climatic conditions, and other factors in a comprehensive manner and make your decision based on professional advice.
The life span of a properly constructed solid foundation is generally considered to be 50-100 years, which is equivalent to the life of the house itself. However, this will vary depending on the quality of the concrete, the condition of the reinforcing bars, the condition of the ground, and the effects of water.
With regular inspection and maintenance, performance can be maintained over a long period of time. In particular, early detection and appropriate repair of cracks (cracks) are important to extend the service life.
For a typical wooden house (about 30 tsubo), the standard construction period for a solid foundation is about 2 to 3 weeks. However, this may vary depending on the weather, ground conditions, and the system of the contractor.
In addition, if ground improvement is required, it may take an additional 1 to 2 weeks. The construction process includes the following steps: ground investigation and soil preparation, laying of crushed stones, laying of moisture-proof sheets, assembly of reinforcing bars, installation of formwork, pouring of concrete, curing, and waterproofing.
Since the entire subfloor is covered with concrete, a solid foundation tends to complicate and increase the cost of remodeling when pipes need to be changed or added in the future. In particular, remodeling that involves large-scale relocation of the water system may be restricted.
On the other hand, the superior under-floor moisture and termite protection means that floors and walls are less likely to deteriorate, making it easier to remodel the superstructure. If remodeling is a prerequisite, it is effective to take measures such as designing with the possibility of future changes and securing plumbing space in advance.
Please use this information as a reference when considering housing construction. In selecting the actual foundation construction method, we recommend that you incorporate the opinions of specialists such as architects and contractors.