The house foundation is one of the most important components of a house. It supports1) the house and transfers its loads18) to the ground. The foundation also insulates2) the house from cold temperatures and protects3) it from groundwater and other types of moisture. Plus, it resists the movement of the ground that surrounds the house.
House Foundations are categorized as either shallow or deep. A shallow4) foundation transfers a building's loads approximately three feet below the ground's surface. A deep5) foundation transfers a building's loads to the ground further beneath the surface than a shallow foundation. Common reasons for a deep foundation include inadequate soil conditions at the shallow depth, site limitations like property lines, and very large loads.
This Old House6) reported that the average house weighs 50 tons and the average house foundation weighs 7.5 tons. The foundation accounts for about 8% to 15% of home construction costs. Approximately, 81% of house foundations are poured, 16% are block, and 3% are constructed of other materials. Roughly, 89% of homes in the Northeast and 75% in the Midwest have full basement foundations. Sixty-six percent of homes in the South and 63% in the West have concrete slab foundations.
The type of house foundation on which to build a house depends on the home buyer's space needs, the house's location, and costs. Soil conditions also play an important role in constructing a solid foundation.
Types of Foundations
Three types of foundations are commonly used in home construction: slab, crawl space, and basement.
A concrete slab foundation requires the least amount of site preparation work to construct. Relative to other types of foundations, installation is simple. Concrete footings7) are poured two feet below the expected finish grade. Then, at least two layers of concrete blocks are placed on top of the footings. Next, internal piping is installed. Then, rock filler is added. Lastly, four inches of concrete is poured on top of the filler.
These foundations8) are most common in regions with warmer climates and higher water tables like Florida. They are generally the least expensive to construct and require the least amount of maintenance. However, it can be costly to make plumbing repairs because the concrete slab will need to be broken to reach the pipes. Furthermore, slabs provide minimal protection from inclement weather like storms.
A crawl space9) foundation is typically constructed with cinder blocks and a brick facing. This creates about two or three feet of space between the foundation10) and the house's floor.
Homes in moist climates—where rain tends to accumulate—are built with this type of house foundation because it lifts the house off the ground. Unlike a concrete slab foundation, repairs or upgrades to ductwork, piping, and wiring are easier to make because these systems are installed in the crawl space. They also provide warmer floors because (a) the first floor is built over the crawl space foundation and not directly on concrete and (b) the crawl space can be conditioned11).
Similar to concrete slab foundations, crawl spaces provide very little, if any, protection from harsh weather conditions. In addition, they are prone to moisture (even when vapor barriers are installed) which could cause mold growth.
A basement foundation is constructed by digging an eight feet deep hole. Then, the beams, walls, and slab are poured in this order.
This type of house foundation is most common in colder regions because home builders must install the footings deep enough into the ground to get underneath the frost line. Thus, the structure is already in place to construct the basement. Unlike slab and crawl space foundations, basement foundations add space to homes that can be used for various purposes. It is also easier and less expensive to make repairs to utilities because technicians can work standing up. Additionally, basement foundations provide a "solid anchor" for homes as well as shelter from storms.
They are, however, more expensive to construct than slab and crawl space foundations, particularly if the space is going to be finished. Flooding can be an issue with basements; however, a natural path for drainage or sump pump can alleviate this problem.
Costs to Build a Foundation
According to HomeAdvisor12), it costs on average $8,409 to construct a foundation. The cost typically ranges from $4,006 to $12,381. Costs for slab, crawl space, and basement foundations are listed below.
A concrete slab foundation costs $4,500 to $21,000 to construct, but costs vary by type and location.
A monolithic slab13) costs $4,500 to $12,000 to construct. Both the main slab and shallow footings are formed from a single pour of concrete, so it is less time consuming and expensive to build.
A stem wall, which connects the house foundation to the house's walls, costs $7,000 to $21,000 to construct. It requires more excavation, labor, and materials then a monolithic slab. Three components are used to construct a stem wall: a foundation wall, poured slab, and spread footing.
A crawl space foundation costs approximately $8,000 to $21,000 to build. The insulation and vapor barriers that are installed to prevent moisture in the crawl space cost $1 to $3 per square foot and $0.05 to $1.50 per square foot, respectively.
A basement foundation costs $10,000 to $175,000 to construct. Although it can be the most expensive foundation to build, it has the highest return on investment. Unfinished basements cost $10 to $25 per square foot to construct while finished basements cost $30 to $100 per square foot.
Additional Cost Factors
Additional costs associated with constructing a house foundation include building permits, inspections, heating and drainage systems, and concrete sealants.
A building permit must be obtained before the foundation is constructed. The cost varies depending on the house's location and size. On average, a permit costs $1,300, but it can range from $500 to $2,000.
Homeowners can have their foundations inspected to identify issues that could lead to costly problems in the future. Structural engineers have the training to detect potential failures and weaknesses in foundations. Their fees range from $80 to $1,300, with an average fee of $500. Factors that impact costs include the house's size and location as well as the engineer's experience level. More experienced engineers may charge significantly higher fees.
Radiant Heat and Drainage Systems
Installation of radiant heat and drainage features add to the cost of foundation construction. Installing radiant heat in flooring could cost as much as $3 per square foot. On average, homeowners pay between $6,000 and $14,000 for this service.
It costs on average $3,000 to add drainage features like a filtered drainage system. These features are nearly impossible to install after the house is constructed. So this additional expense could be worth it, particularly considering how costly it can be to repair flood damage.
Concrete Slab Sealing
Sealing the concrete slab is important for preventing hot or cold air, moisture, and particulates from entering the home. It also protects the house foundation from water damage and prevents it from weakening over time. It generally costs $0.50 per square foot to seal a concrete slab, with an average total cost of $2,000 to $6,000.
Four Reasons Foundations Fail
According to This Old House, there are four common reasons foundations fail:
Slabs poured over crushed stone that was not firmly tamped lack stability, so they are likely to crack or settle.
Interrupting the Pour
A concrete form14) is like a mold that holds concrete in place or forces it to harden into a particular shape. It should be filled in a single application. If not, the concrete that was previously poured and has started to set and the freshly poured concrete will not intermix. This causes a cold joint15) between the two that will likely cause cracking and leaking.
Rushing the Cure
Concrete must cure (i.e., dry) slowly to reach the appropriate strength (which is typically 3,000 psi). Keeping it damp for at least three days provides sufficient curing.
Soil that has a high concentration of clay or organic matter will retain water. This increases the likelihood that the house foundation will crack when the soil freezes and expands.
Soil Types and Foundations
Clay along with peat, loam, rock, sand/gravel, and silt are six common types of soil16). A soil's properties determine whether it (a) is suitable for building and (b) requires a shallow or deep foundation.
Clay consists of tiny particles that store water like a sponge. Because it holds water so tightly, it expands significantly when moist and shrinks considerably when dry. Moist clay is quite pliable, so it is easy to move and manipulate. Such extreme changes place considerable pressure on a house's foundation, which causes it to move up and down until it cracks.
Peat, which is usually black or dark brown, holds a large amount of water so it is easy to compress. It gets really dry in the summer, which can make it a fire hazard. Similar to clay, peat makes a poor support for foundations because it can shift or change in structure.
Silt, which can be smooth to the touch, has smaller particles that retain water longer. This tendency to hold moisture makes it cold and causes it to drain poorly. This causes it to expand and push against foundations, which weakens them.
Sand/gravel, which is gritty and dry to the touch, has the largest particles of the six soil types. The space between grains of sand is larger, so it drains easily. When moist and compacted, sand holds together rather well. These characteristics make it suitable for house foundations. However, its particles lose their friction when moist, so it can be washed way. This erosion can cause gaps beneath the foundation.
Loam is dark in color and crumbly, dry, and soft to the touch. It is generally a combination of clay, sand, and silt, so it maintains a balanced amount of water. This makes it suitable for foundations as long as other types of soils do not get onto the surface.
Rocks such as bedrock, hard chalk, limestone, sandstone, and shale have high load-bearing capacities. Thus, they have the stability (as well as the depth) to effectively support foundations.
The house foundation is undoubtedly one of the most important components of a house. If it fails, the entire house could suffer structural damage17) like bowing basement walls, cracks in floors and walls, leaning chimneys, and sinking porches. This could lead to costly repairs at best and inhabitability at worst. Therefore, it is important to determine if a foundation is strong enough to withstand a house's loads for its entire life cycle.
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