Ultimate Guide to DIY House-building -- Foundation Construction

Once you decide on the type of foundation that you think best fits your requirements or is intrinsically suited to the house you are planning, you then must decide how that foundation should be constructed. Three options are concrete block, poured concrete, and precast concrete walls. All can be used to provide crawl space foundations (usually built up from a footer having walls 18 to 24” above ground level), or basement foundations that are typically 7’ high on the inside.

Ill. 12 Concrete blocks.

Concrete Block Construction

You probably already know what concrete blocks are. They're rather rough-feeling, heavy, gray, and have several rectangular holes running through their insides, vertically (Ill. 12). When installed, they're laid one course or row on top of another, staggered so their vertical joints don't coincide with each other. The staggering of joints makes for a stronger interlocking bond. At the same time, the center rectangular holes are large enough to overlap so the courses can be tied together with reinforcement rods that are inserted vertically and then filled with wet concrete.

On a foundation, the concrete blocks begin at the footer and are laid to form the desired height of the house foundation or basement. On houses using brick exterior finishes, the brick may also start with the footer and follow the house blocks right up. This will help support the brick veneer and strengthen the entire foundation.

An alternative for brick exteriors is to go with a wider foundation block that provides a ledge at the outer top of the foundation walls on which the brick can be conveniently laid or begun from.

Advantages

1. Concrete block walls permit a stop-and-go schedule during construction.

2. Concrete block walls are easier to repair than poured concrete walls.

3. Concrete block construction is often preferred by builders costing out the foundation job because block construction eliminates the need for concrete forms.

4. Concrete block walls absorb sound better than solid poured walls.

Disadvantages

1. Concrete block walls are strong, but not as strong nor as impenetrable as solid poured concrete walls.

2. A concrete block wall is more likely to develop small cracks that can allow air infiltration, moisture, and even insects inside the foundation.

Solid Poured Concrete Construction

Solid poured concrete foundations, even though not as common as block foundations, are desirable in a number of situations.

Advantages

1. They have a slight advantage in strength and load-carrying capacity over block wall construction.

2. They offer the best protection against air infiltration, moisture, and insects. They're also less likely to result in wet basements.

This can be a major advantage where rainy weather is common.

3. They can often be cast integrally with the footer, at a substantial time and cost savings.

Disadvantages

1. They can be more expensive than block construction.

2. It's somewhat difficult to be sure that you'll get as good a mix of cement as specified. There's an element of risk involved. It means, again, dealing only with reputable contractors for the concrete.

Established builders will do so, but vanishing or marginal builders might not. A defective mix might not be detected until the house is up and the contractors are long gone. By then it will be a night mare to correct major flaws or problems.

Precast Concrete Construction

Precast concrete stud wall construction provides an interesting alternative to concrete-block masonry construction. Precast walls are strong and capable of being efficiently installed as both foundation walls and walls above grade.

Precast wall panels are built with steel-reinforced concrete studs (typically about 7 1/2 by 2”), 1” rigid insulation board, rebar reinforced top and bottom bond (footer) beams, along with a concrete facing about 2” thick. The bond beams and concrete facing are cast in one continuous pour. Studs are connected by encapsulating vertical rebars plus galvanized hooks and pins that protrude from the top, bottom, and back of each stud. The galvanized pins protruding from the studs secure an approximately 2”-thick face pour. Pressure-treated furring strips are pre-attached to the inner face of each stud. This construction provides a base for the homeowner to finish the basement without having to add any additional studding. Holes are cast into each stud to accommodate wiring and small-diameter plumbing (Ill. 13).

Ill. 13 Precast wall panel.

Pre-cast Construction Points

+ Custom-made panels are made to support door and window openings.

+ All precast panels, and each individual concrete stud, should include vertical rebar reinforcement. This gives considerable tensile strength to the entire structure.

+ The rebar-reinforced top bond beam typically has a perforation about every 24” to enable the secure bolting of a sill plate to the top of the wall. The beam also provides strength and helps to equally distribute the weight of the home.

+ Insulation built into the wall supplies an insulating value of about R-5. The wall's cavity, about 7 1/2” deep, permits a thick blanket of insulation to be applied. Combined with the 1” thick polystyrene panels between the concrete studs and the concrete skin, those components can yield an effective cumulative rating approaching R-24.

Advantages

1. The panels come with ready-to-finish interior walls. A treated wood nailer is factory installed to the inside of the reinforced concrete studs, ready for the application of a vapor barrier and wall board.

2. The panels resist moisture, mold, and mildew well.

3. The insulation panels also provide a thermal break and vapor barrier.

4. Precast panels can be installed in almost any weather. There's no danger of the concrete freezing before it cures or curing too quickly from excessive heat.

5. These panels save time when being finished in a basement, because there's no need for additional framing to support the insulation batts, vapor barrier, and drywall which can be attached directly to the furring strips with no loss of space.

Foundation Construction Points

Here's a collection of various construction specs and procedures used in proper concrete block, poured concrete, and some precast concrete foundations:

1. The minimum thickness for any home in the most ideal situations requiring minimum loadings is 8”. However, 10” is preferable and safer. The minimum thickness should be increased to 10” if the walls will be subjected to any lateral pressures such as large snowdrifts, if the walls are more than 7’ below grade, if the walls are longer than 20’, or if the house is going to carry a heavier than normal load. This can be the case if you elect a two-story home or if you plan on having considerably heavy furniture and items such as a grand piano, pool table, waterbeds, or exercise equipment. For extra-deep basements, use 12” concrete block. As the block size increases, so should the footer.

2. Before the foundation floor is concreted, all necessary plumbing and sewer pipes should be installed. Make sure all pipe cleanouts are present in the foundation walls and floor. Once the rough (underground) plumbing is situated in the foundation, the plumbing inspector may wish to check that the drainage system, when full of water, will hold up without leaking; check for proper pipe slope or fall; check the cleanouts; inspect the piping for proper sizing.

3. Once the foundation walls are up, the floor of the crawl space, basement, or garage should be filled with 6 to 12” of 3/4” stone, with 3- to 4” drainpipes running through the stone. The stone (or gravel) should be compacted as it's put down. Over the drainpipe and stone, a plastic vapor barrier should be placed to seal out dampness. This is especially important if the foundation is a crawl space. A concrete floor at least 4” thick will hold the water and dampness down into the ground, and the drainpipe will direct the water to a sump hole. Should the sump hole ever fill up, a sump pump can be installed to pump out the water and pipe it away from the house.

4. If concrete block is used, the first layer of blocks against the footer should be special drain blocks with grooved or weep holes along their bottoms. This will prevent a buildup of water pressure against the walls. Instead the water will flow into the 6” gravel bed beneath the concrete floor. The drainpipe running through the gravel will convey the water to the sump hole.

5. Along the outside of the foundation, 4” rigid plastic pipe having openings or perforations along the top should be placed end to-end on a gravel base along the footer, pitched toward the spot where the water will be piped away from the house. This pipe should be lower than the basement floor and not simply resting on the footer. With this setup, water flowing against the wall of a home has a place to go. These drainage pipes need to be continuously sloped toward the discharge end; otherwise, sediment might build up at a low point and completely block the line. If the soil is full of clay or silt, consider installing a soil-filtering fabric to protect the stone and the drain piping from becoming plugged with sediment washed down from the backfill. The water table should be maintained no higher than the elevation of the pipe under the entire basement or crawl space so water pressure is held at a minimum. This perimeter drainage pipe system should be covered with approximately 12” of 3/4” stone.

6. On some lots there is no place to discharge the house drain pipe from the foundation to daylight. In this situation, footer drains can be run inside to the basement sump hole.

7. A sump pump should be installed in the foundation floor when necessary (Ill. 14). Some foundations, either crawl space or basement types, might never need one, while others may have to employ one year-round. Many building codes have been making it mandatory to include a sump hole so a pump can be added later, if needed. For the most part, sump pumps are set up so that if water backs up to a certain level and threatens to flood the cellar, the pump will automatically kick in and pump out the water into an outside drainage line that will carry it away from the house.

The sump hole itself should be 24” in diameter or 20” square and should extend at least 30” below the bottom of the basement or crawl space floor. The sump hole should be covered to prevent people and pets from stepping into it and moisture from escaping it into the basement.

8. One aspect of the foundation that deserves special attention is waterproofing. Damages due to water and moisture are among the most serious causes of home deterioration. They cause wood rot, unsightly paint peeling, mildew, rusted appliances, and other maladies. They can even affect the health of the occupants. To prevent damp and wet basements, good waterproofing techniques, proper drainage, vapor barriers, correctly graded lots, appropriate landscaping and positioning of shrubs and trees, generous roof over hangs, and plenty of gutters and downspouts are necessary.

9. If you're going to have a basement, have the septic disposal system line or the sewer line located below the basement floor if possible.

Otherwise, wastewater and solids generated in the basement have to be pumped up to the level of the main lines for disposal. If the disposal lines can't be lowered, the simplest solution is often to completely avoid any sanitary drains in the basement (that means no toilets, wash basins, showers, or laundry equipment). Then the house sewer or septic lines can be suspended beneath the first-floor joists and run through openings cut in the foundation walls. The same thing can be done with a crawl space foundation if necessary, as long as precautions are taken to prevent the lines from freezing.

Ill. 14 A foundation interior with sump pump and drainage.

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WATERPROOFING

Wet and damp basements can be more than just uncomfortable.

Unchecked moisture allows mold and fungi to flourish, which can cause allergic reactions in people and pets and can result in peeling paint; rotting wood; and warped drywall or other wall, floor, and ceiling components.

With either a basement or slab foundation, in addition to applying a 1/2 inch of parging masonry (or cement) on the outside surface of basement walls, followed by two coats of a bituminous tar sealant, make sure good drainage is achieved around the entire dwelling. Grading or sloping the soil away from the home's perimeter is essential to effectively controlling roof and outside wall water runoff. As a rule of thumb, the soil should slope downward at least 6 vertical inches in the first 10 horizontal feet of travel from the foundation wall or outer vertical surface of the slab.

Some of the best drainage systems direct water that comes near the foundation down toward the level of the home's footing, where clay, concrete, or plastic drainage tiles or pipes collect and convey the water away from the dwelling, or collect and discharge the water through a sump pump. For drainage lines to work efficiently, they must be securely installed in beds of porous material-usually washed gravel. To achieve good drainage at window wells, the washed gravel should continue from the bottom of the wells to the foundation drainage tile. The window well should be wider than the window and frame and deep enough for at least two inches of washed gravel to be placed at the bottom of the well- while staying below the lower portion of the window's frame. It's important to prevent any chance of water getting up to any part of the window's frame.

To address the "open" nature of window wells, galvanized steel window wells are available to hold back the earth, and sturdy grating should be fitted at the well's top to prevent someone from stepping into the well. Plastic covers can also be placed over the top of window wells to keep out rain and snow.

Downspouts that carry rainwater away from roof gutters should be connected to a mostly horizontal (slightly sloped) length of pipe that carries the water several feet away from the foundation. If a sidewalk is in the way, drain tile can be used to run the water beneath the walk to open ground or a catch basin, depending on the landscaping serviced. If a downspout empties into a catch basin, then into a drain pipe, the catch basin should be positioned far enough away from the foundation so runoff during a heavy rain will not overshoot it and yet close enough to catch the slower flow during a light rain. The water can also be conveyed to a bubbler or "dry well" where the water is allowed to simply drain off or dissipate into the surrounding ground. Bubblers or dry wells should never be located within the drip line of large trees-where the soil could become saturated for long periods of time and could allow the tree to fall over (roots and all) in high winds.

On flat or level building sites where there's no room for or possibility of establishing surface drainage slopes, a "trunk and arm" drainage system can help handle water runoff (Ill. 15). Such a drainage system consists of a perforated drain tile (usually 6” diameter), which acts as the trunk and runs parallel to the home's foundation, and solid drain tiles (usually 4” diameter), which act as the arms, attached to the trunk with T connectors. The arm tiles run perpendicular to the trunk, spaced about 10’ apart from each other and slope underground to dry wells filled with washed gravel, where the surface runoff water can collect and slowly dissipate into the surrounding earth.

Ill. 15 Trunk and arm drainage system.

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