Understanding Home Construction: The Foundation

Home | Fire Safety | Skyscrapers

Home Emergencies | Glossary

Home builders have an old saying that, if you start out with a level, square foundation, then almost everything after that is cosmetic. Section 1 discussed planning and preparing the site for the foundation. Now Section 2 will explain the actual design and construction of the foundation. Although the discussion will focus on masonry foundations, it will also touch on pressure-treated wood foundations. The first step in building the foundation is constructing the footings.

Constructing the Footings

The builder begins by excavating for the foundation and marking the location of the footing trenches. (See drawing 2.1.) The footings form a continuous concrete pad on which the foundation walls rest. The builder usually digs the footing trenches around the perimeter of the house and extends them several inches or more on either side of the location of the future foundation walls. If the site has a slope, then the builder must construct stepped footings to follow the grade of the slope. Under some conditions, the builder may use lumber forms to construct the footings on the ground. After the concrete sets, the builder then removes the forms.

2.1 Excavation of House Corners and Footing Trenches

The builder may construct a rough frame of batter-boards at the corners for the house. (See drawing 2.1.) The builder then marks the location for the exterior foundation walls with heavy twine between the batterboards. Using this temporary life-size pattern, the builder makes sure that the house layout is square and the cleared footprint of the house is level.

Brick and stone masons will use the lines marking the shape of the house to build the foundation. If the foundation is even slightly out-of-square, then everything above it will also be out-of-square. This is no small matter, so the builder takes great care to ensure the foundation is square. Only after the builder and often a local building inspector are satisfied with this layout can the workers begin excavating the foundation and digging the trenches or building the forms for the footings. (For a picture of footings under construction, see photo 2.2.)

2.2 Construction of Footings

The builder has other trenches dug at this time as well, if the house has a crawl space or a basement foundation, then the interior pillars or columns require footings. For these the builder digs square or rectangular holes, somewhat larger than the proposed pillars. The builder, along with the designer or engineer, determines the proper size and depth of all trenches based on several factors, including local soil conditions and building codes. Again, a local inspector may be required to certify that the trenches or forms are properly prepared to receive the concrete footings.

If a home has a slab-on-grade foundation, which is a layer of concrete poured directly on prepared soil, then the builder digs the trenches for the plumbing and electrical lines at this time. Before the slab is poured, the builder calls in the plumber and electrician to install the rough plumbing lines and the conduit for the electrical lines. Some slabs are poured at the same time as the footings and some are poured separately. A variety of climate and site conditions, building techniques, and local codes can influence when footings are poured.

Local codes and soil conditions may require the builder to reinforce the footings with steel bars, often called reinforcing bars or rebars. These bars are usually laid horizontally, running the entire perimeter of the footing. The builder often raises these bars up a few inches off the bottom of the footing trench and rests them on metal or masonry supports. In areas of high wind or seismic stress, local codes may also require that a vertical rebar connect the footing with the foundation. Otherwise, the foundation or slab simply rests on the footing.

Once the concrete footing is in place, it needs to cure or harden for a short period before construction can continue. Usually a day or two is sufficient for the concrete to cure, but the curing process may take longer in colder weather. In fact, in very cold weather, the builder needs to take special pre cautions to keep the ground and concrete from freezing.

After the footings have cured, the builder can then move on to the next steps in constructing the foundation. The type, materials, and design of the foundation will affect how it's constructed.

Types of Foundations

Houses usually have slab, crawl-space, or basement foundations. Several factors determine which type of foundation is selected for a particular house. Some of these factors relate to site conditions, including the slope of the lot, the location of the water table, or the condition of the soil. In addition, local building codes, the design of the house, buyers’ preferences, and even local customs may determine the type of foundation selected. Regardless of which foundation is chosen, builders categorize slab, crawl-space, and basement foundations by the type and amount of space between the floor and underlying soil.

Slab Foundation. For a slab foundation, the underlying soil supports a flat layer of concrete, usually 4 inches thick. Drawing 2.3 shows a slab floor surrounded by a concrete block foundation and a perimeter footing. The foundation wall can also be made of concrete or pressure-treated wood. A monolithic slab, on the other hand, is surrounded by an integral footing poured along with the slab. With this approach, the slab is both the foundation and the first floor of the house. As Section 1 notes, a slab home typically requires a nearly flat lot.

2.3 Slab Foundation

Crawl-Space Foundation. A crawl-space home typically has a perimeter foundation wall of brick, block, stone, poured concrete, or occasionally pressure-treated wood. Interior pillars arranged within the foundation area support the main floor of the house, which is usually made of wood.

The wood floor is located at least 2 feet above the underlying soil, and as Section 1 described, the space between the floor and soil is called the crawl space. Builders typically use this crawl space for insulating the floor, running heating and cooling ducts, and installing electrical and plumbing lines. Foundation vents provide air flow to the crawl space and one or more access doors allow for easy maintenance. The builder can build a crawl-space home on a flat lot or one with a moderate slope.

Basement Foundation. A basement foundation combines the best features of slab and crawl-space foundations. With a basement foundation, the bottom floor is normally a slab. Masonry, poured concrete, or pressure-treated wood walls support the main level of the house. These walls rise 8 or more feet high and allow the basement to serve as a functional storage or living area. The underground slab floor is then placed inside the bottom of the walls to keep the walls from moving inward.

The basement walls not only support the house but also need to withstand a significant difference in pressure between the outside soil and the inside basement living area. So, the builder must follow local building codes or work with a designer or engineer to determine what reinforcement may be required for these walls.

The builder next places perforated plastic or clay drain pipes around the base of the foundation near ground level. (See drawing 2.4.) The pipes slope away from the foundation to a storm drain, to the ground surface, or to a sump pump located below the basement floor.

2.4 Drain Pipe at Base of Outer Foundation Wall

Since basement foundation walls are built underground but above the water table, the builder must thoroughly water proof them and , in some climates, insulate them. The builder should coat the wails with a waterproofing mastic or membrane and then place a porous medium, such as gravel, around the wall extending from the ground surface down to the footing drain.

Foundation Materials

Builders construct foundations from three basic types of materials: poured concrete, concrete block, or pressure- treated wood.

Poured Concrete. Poured concrete is the material of choice for slab-on-grade construction. Drawing 2.5 shows a close-up of a monolithic concrete slab. Here the builder has poured the floor slab and footings in one continuous process. This particular slab has a gravel underlayment for extra sup port, which is optional where the subsoil is sand or gravel and the water table is low.

Before pouring the slab, the builder usually has a licensed pest control company treat the underlying soil for termites. The builder may place some type of vapor barrier—usually a layer of polyethylene plastic—over the soil or gravel. The builder can then add reinforcing rods to strengthen the slab as necessary.

Drawing 2.5 also shows an optional layer of rigid slab insulation that the builder may apply to the exterior of the footing in cold climates. The builder can then erect the main walls of the home directly on the slab. In most areas, the bottom plates of the walls, which are the 2x4s that touch the slab, should be made of pressure-treated material. Workers can then apply finish floorings directly to the slab, including carpet, vinyl, or wood.

Poured concrete is also a common foundation material for crawl-space or basement walls. Drawing 2.6 shows a foundation wall being constructed of poured concrete. Once the footing concrete is poured and before it sets, the workers may form a small keyway around the top of the footing so the wall concrete grips the footing.

2.5 Monolithic Slab Foundation

2.6 Poured-Concrete Foundation Wall

After the footing sets, the workers use the footing as a base to erect temporary forms to hold the wet concrete. The sketch shows timber and plywood forms, which builders sometimes use. However, modern metal forms are rapidly replacing the traditional wood forms. After all of the forms are in place, workers install the reinforcing and pour the concrete from the top between the two sides of the forms. Before this concrete sets, the builder usually inserts anchor bolts into the walls.

Concrete Block. Builders also use concrete block or some times brick for basement or crawl-space foundation walls. Drawing 2.7 shows a section of a concrete-block basement wall. Notice that the builder has inserted a window for light and ventilation in the basement. Below the grade, workers have applied a coating of cement parging, which is mortar cement used for waterproofing, and other waterproofing material.

2.7 Concrete-Block Foundation Wall

In those parts of the country with colder climates, the builder may apply both waterproofing and insulating materials to the basement walls. The sketch shows that the coating is heavier near the bottom and slanted outward to help move moisture away from the base of the foundation after the soil is backfilled against the wall. The top of the wall shows anchor bolts and optional solid cap blocks, which building codes may require in some areas.

Pressure-Treated Wood. Builders sometimes use pressure-treated lumber for basement and crawl-space foundations because wood walls are easier to insulate, wire, and finish than block or concrete walls. For this type of foundation, the wood is treated with a preservative to resist insects and rot.

Drawing 2.8 shows the installation of a typical pressure- treated wood foundation wall. In this situation, workers construct the foundation on a layer of gravel placed below the frost line. This layer of gravel serves as both a footing and a level bed below the basement floor slab. The gravel also assists in carrying moisture away from the floor to any drainage system outside the house.

2.8 Pressure-Treated Wood Foundation Wall

Treated wood foundation walls are often pre-constructed as panels at a factory and shipped to the site for assembly. Generally the panels consist of a frame of treated 2x4s, 2x6s, 2x8s, or 2x10s spaced according to the load they will support. Workers nail a treated plywood facing to the frame, caulk the joints, and wrap the entire exterior with poly ethylene sheeting moisture retarder. Builders have successfully used pressure-treated wood foundation walls for basements in many parts of the country.

Foundation Design

Why would a builder recommend one foundation design or material over another? First, local soil and terrain conditions may require the use of certain foundation designs or materials. As already mentioned, in certain parts of the country, expansive soils make the construction of crawl-space or basement foundations difficult. In such situations, a specially designed basement or slab foundation may be needed. As another example, a basement may be a less expensive approach for a sloped site. On the other hand, construction of a basement on a site with underlying rocks may be more expensive because of the cost of blasting and removing the rock.

Regional geological conditions and climate may also influence the design of the foundation. For example, homes built in areas that are subject to earthquakes may require heavily reinforced foundations. As already mentioned, cold weather may require special techniques for pouring and curing concrete. In addition, because workers can't mix and apply mortar to blocks and bricks on extremely cold days, the builder may consider using a wood foundation in colder climates.

The cost and availability of material may also influence the design of the foundation. As one example, clay bricks are only made in certain parts of the country. Shipping them from other areas can be expensive. If a material or method is uncommon to a particular area, the builder may also have difficulty finding a tradesperson who knows how to work with that material, which can again increase construction costs.

The overall design of a home may also dictate the choice of foundation material. Whether using stock plans or plans designed by an architect or designer, the builder must pay careful attention to the details of those plans. If the house plans call for a basement, then building a crawl-space or slab- on-grade foundation can result in a very different style of house. For example, if a builder adds a basement, the plans will have to be modified to accommodate the basement stairs and other changes.

In selecting materials and a design for the foundation, the builder also needs to pay close attention to local customs. If a neighborhood is filled with crawl-space homes, then a builder may prefer to build a home with a crawl space in that area. If no one else in the area has a basement, then there is probably a good reason for that custom. Besides any practical reasons for the construction of particular types of foundations, radical departures from local customs can sometimes adversely affect the resale value of the home.

Constructing the Rest of the Foundation

Even after extensive planning, constructing the rest of the foundation after the footings entails more than just stacking up blocks and mortar. The builder must make sure the structure’s foundation is perfectly level. Since the foundation is built on top of the footings, the masons or concrete formers must correct any inconsistencies in the footings as they construct the rest of the foundation. The masons have to follow the batterboard lines carefully, and a good chief mason will recheck the lines repeatedly to ensure that the foundation is level and square.

At this time, the masons, concrete formers, or other trades people working on the home may also install supporting material and other non-masonry items in the foundation. These items may include reinforcing, as well as—

• crawl space vents and basement windows

• posts, beams, and girders for supporting the main structure

• lintels, which are lengths of angled steel that support any masonry over the top of a window, door, or other opening

• foundation drains and a sump pump if the foundation drains don't drain to the top of the ground

• foundation insulation

• cement parging on concrete block

• damp-proofing with an asphalt coating or polyethylene film

• stucco or other decorative coating above grade

Before a slab or basement floor is poured, a plumber installs any needed water and sewer lines. In some areas, the builder may also need to install a pipe through the slab into the gravel under the slab for venting radon soil gas into the outside air.

As the next step in constructing the foundation, the workers pour the concrete floor slab. They then level it to a rough finish and , when it begins to set, they trowel the concrete into a smooth, hard surface. These two steps are necessary for a strong, lasting concrete slab. However, using too much water can cause the aggregate—the stones or rocks contained in the concrete—to fall to the bottom. This weakens the slab and softens the wearing surface. Here again, the builder can ensure that a trained professional has the experience to deliver a well-built concrete structure.

Building codes require large bolts in the concrete foundation or slab, which extend up so the walls or floors can be bolted down. The builder can install these bolts as the concrete is poured or later by drilling holes at the appropriate places and securing the bolts with anchoring devices. Again, local codes, customs, and traditions will dictate how builders install anchor bolts in the foundation or slab.

In addition to wall thicknesses and construction methods, local building codes dictate the size and number of basement windows or crawl-space vents. The builder will know how to determine the proper number of windows and vents for each home in a particular area based on local codes.

In some parts of the country, basement and foundation insulation is a critical energy-saving feature. The most common method is to install fire-resistant fiberglass insulation on the inside of the basement wall. Another method is to install a rigid insulating board on the outside of the foundation, along with the waterproofing, before the backfill is replaced in the trench around the foundation.

Crawl-space foundation walls are usually not insulated, since in most parts of the country the builder installs insulating batts between the wood floor joists for this type of house.

Building codes require large bolts in the concrete foundation or slab, which extend up so the walls or floors can be bolted down. The builder can install these bolts as the concrete is poured or later by drilling holes at the appropriate places and securing the bolts with anchoring devices. Again, local codes, customs, and traditions will dictate how builders install anchor bolts in the foundation or slab.

In addition to wall thicknesses and construction methods, local building codes dictate the size and number of basement windows or crawl-space vents. The builder will know how to determine the proper number of windows and vents for each home in a particular area based on local codes.

In some parts of the country, basement and foundation insulation is a critical energy-saving feature. The most common method is to install fire-resistant fiberglass insulation on the inside of the basement wall. Another method is to install a rigid insulating board on the outside of the foundation, along with the waterproofing, before the backfill is replaced in the trench around the foundation.

Crawl-space foundation walls are usually not insulated, since in most parts of the country the builder installs insulating batts between the wood floor joists for this type of house.

Prev: Site Preparation
Next: Framing, Roofing, and Siding

top of page    Home