Ultimate Guide to DIY House-building -- Foundations

To put it in simple terms, the foundation of a house is what sets directly above the concrete footer and below the wood-frame living levels. Another way of describing most foundations is to call them basement walls. In houses without basements, a slab foundation also incorporates the dwelling's footer in one continuous piece of concrete.

The foundation must be strong enough, whatever its construction, to support the house sills (heavy horizontal timbers or planks attached to the upper part of the foundation to serve as a starting point for the house walls) and other related members of the house structure, as required.

From a structural standpoint, the foundation performs several key functions:

+ It supports the weight of the house and any other vertical loads, such as snow.

+ It stabilizes the house against horizontal forces such as wind.

+ It acts as a retaining wall against the earth fill around the house.

+ In some cases, a basement might be needed to act as a barrier to moisture or heat loss.

No matter which foundation type you must have, or elect to have, some general points apply. All foundations, whether slab type, crawl spaces, basements, or any others, should extend above the final grade enough so that wood members of the house are some distance above the soil. That distance might be regulated by local building codes.

However, keep in mind as the specs of your foundation are laid out that if local building codes are being used as a guide, they'll help, but they could still fall short of what many people would consider optimum construction. Local specs lay down minimum rules for safety and health, but that's about all. In other words, you might want to go a few steps beyond what they recommend. In this case, it's better to position the top of your foundation slightly higher, away from the ground level, for added protection against moisture and insects.

When excavating is necessary-and that's likely for any of the foundation types you will consider-have the valuable topsoil scraped off and saved. It should be pushed into a pile and kept out of the way until the house is completed and the landscaping is roughly finished, so you can spread the topsoil around to provide a fertile base in which to plant grass seed or to lay sod. Don't let the topsoil become "lost" amidst the rest of the soil that's excavated and used to fill in around the foundation.

Ill. 7 Slab foundations.

Ill. 8 A floating slab foundation.

Slab Foundations

The slab foundation, as mentioned earlier, generally involves a combination of footer and foundation into a single slab of concrete (Ill. 7). They're popular wherever basements are impractical or impossible to have, in certain parts of the Southwest and South for instance, or in areas with high groundwater levels.

The "floating" slab is unique in that the finished concrete floor, foundation walls, and footers are reinforced together with steel mesh and metal rods and poured as one integral mass over a bed of gravel (Ill. 8). The entire slab thus floats on top of the ground while functioning as the floor of the house. However, the depth of the concrete should not be equal throughout its overall area: the slab must be thicker beneath support walls if the footer is considered part of the slab.

In cases where the terrain is not relatively flat, a footer must be poured separately from the slab and terraced or stepped down. Then a foundation, usually of concrete block, is constructed to a certain level height so a concrete slab floor can be poured. A slab foundation can be very trouble free and economical when built in adherence to the following construction points:

1. First, the site must be properly graded and compacted, and the footing trenches dug.

2. If the slab will be placed directly on soil, make sure the area is free of biodegradable tree roots and debris.

3. Granular fill makes a better base than soil. Bank or river gravel, crushed stone, or slag can all be used, in sizes from 3/8- to 1” thick. A 6- to 12”-thick granular bed of fill will suffice.

4. At this stage all of the following underground utilities should be installed: plumbing, drains, sewers, heating service lines and ducts, radiant pipes, electrical work, and any other public or private utilities. Steel sleeves with foam insulation inside should be placed over all water and sewer pipes where those pipes protrude from a concrete slab, allowing the pipes to "give" if a slight adjustment is needed when the connection is made. Remember that if your house will have a slab foundation, you're going to end up with one continuous slab of concrete. If anything is done wrong and not corrected after the concrete is poured and before it dries, just think of the trouble and expense you'd have to go through to simply get at the problem, let alone to fix it. Take precautions to see that the utilities are all accounted for and installed in a safe and correct fashion.

When run beneath a concrete floor, water lines should be laid in trenches deep enough to prevent freezing (below the frost line).

Pressure-check the plumbing for leaks before the concrete is poured. The water should be turned on with all faucets and shut offs closed to make sure there are no leaking seams, cracks, or holes through which concrete could seep, to solidify and plug water or drainage lines.

Make sure copper pipe is wrapped in a rubber or plastic tape wherever it will come in contact with concrete. An undesirable chemical reaction occurs when bare copper meets concrete. It's another simple precaution that can save a lot of time, expense, and inconvenience at some later date.

5. The sub-grade for the slab should be dressed up or smoothed out in preparation for the concrete pour. Whether the subgrade is gravel or slag, it must be thoroughly compacted. It should end up higher than the surrounding grade so water will drain away from the house and so the top of the slab is comfortably higher than ground level.

6. At this stage, a vapor barrier is placed over the sub-base to stop the movement of liquid water and water vapor into the slab. Among materials used as successful vapor barriers are heavy-duty sheets of roofing material, polyethylene plastic, and construction paper.

They act as both an insulation and moisture control, holding dampness in the ground rather than permitting it to penetrate cracks that could form in the slab foundation.

7. The slab should now be reinforced with steel rods. Although the steel reinforcement will not assure the prevention of cracks, it might reduce the magnitude of cracks that would otherwise occur.

For best results, the steel is placed horizontally through the middle of the slab and held in position until the concrete dries. A common practice of placing the reinforcement rods on the sub-base, then pulling it up to the center of the slab with a rake or hook should not be permitted. It's virtually impossible to accurately control the location of the rods or wire fabric with this method.

Instead, steel bridging or "chairs" can be anchored into the sub base. The chairs will stick up to about the midpoint of the slab and will correctly hold the reinforcement when the concrete is poured.

Naturally, the chairs will be concreted right into the slab, along with the reinforcing steel.

8. In addition to steel rods, welded wire fabric or mesh is required for slab foundations. Again, this material makes the concrete less likely to break loose from itself. Wire joints should be overlapped and wired together according to your building code, unless the entire job can be done with a single piece.

9. In an application where a foundation is required to hold up a concrete slab (with the interior filled with bank gravel to support the slab) then the top block-if it's a concrete block foundation-or the top ledge-if it's a poured foundation-should be a header or shoe block form in which a portion of the block/concrete has been cut/left out to provide a base for the concrete slab to be fastened to or sup ported on (Ill. 9). Vertical reinforcement rods should also come up from the foundation walls and be bent into the slab. This will help hold the walls to the floor slab. More about this type of construction can be found in the crawl space/basement section of this section.

10. Before the concrete is poured, a means by which the house sill- the wooden horizontal planks that support the main upper structure of the house-can be secured to the foundation must be arranged. Anchor bolts can be positioned about every 3’ around the perimeter of the slab so they'll be embedded into the slab when the slab dries and cures. Check the local building code for the proper size and spacing of anchors.

11. When the slab is framed and ready to be poured, it should hold the top of the floor about 8” above the ground, and the surrounding grade should be sloped away from the foundation to keep water running away from the house. If an elevated floor slab is used, it should be a minimum of 4” thick at any part.

12. If a smooth finish is desired, specify that the concrete should be troweled by hand or by power-driven machines. A textured finish can be obtained by dragging a broom across the surface before the concrete is fully set.

Ill. 9 A foundation that supports a concrete slab.

Advantages

1. They're very economical to build, especially when compared to crawl space or full basement foundations. Most slab foundations take much less labor and time to construct.

2. They're worm- and rot-proof.

3. They can't catch fire.

4. They're basically wear-proof and are certainly more secure than any other kind except solid rock.

5. They can store heat from the earth and are naturals for use in a passive solar heating system.

6. They require little insulation from the elements.

7. As foundations, slabs are outstanding because they simultaneously act as one gigantic footer. Consequently, they impose the lowest soil loading per square foot of all foundations.

8. They experience less problems from ground moisture . . . there's no leaky basement from a slab.

9. The slab is more adaptable to filled or unstable soils where conventional foundations would settle unevenly and crack.

10. The slab-on-grade foundation eliminates the need to frame a floor on the first level.

11. They are not affected by underneath drafts.

12. Vinyl flooring and wall-to-wall carpeting can be installed directly onto the top surface of a slab.

Disadvantages

1. If a problem occurs with a utility that's concreted into or positioned beneath a slab, it's extremely expensive and troublesome to access the malfunction, to make repairs, and to restore the foundation to the way it was.

2. Slab foundations can be efficiently used only on relatively flat lots. They require substantial site-work when employed on uneven ground, whereas a crawl space or basement foundation readily adapts to hilly terrain.

3. Floors constructed over a crawl space or basement foundation are easier on the feet and legs.

4. Plastic and other moisture barriers must be punctured for pipes and electrical wires to pass through, thus allowing some under ground dampness to rest against the bottom of the slab.

5. Because the slab is mostly below ground, no ventilation reaches its lower surfaces. The slab tends to adjust to room temperature very slowly and instead follows fluctuations in ground temperatures whenever they occur.

Concrete and Block Wall Foundations

Many houses built today sit on foundation walls that form either a crawl space under the house, a partial basement, or a full basement.

These types of foundations are more common in northern locations where deep frost lines are encountered, but are also found, conditions permitting, in the South. They're constructed of either solid poured concrete or concrete blocks, and if either has the edge over the other, it's concrete blocks. Both foundations will not only support a dwelling, they'll also protect it from water, frost, and insects while providing (when desired) a basement to be used for storage or expanded living space.

If proper foundation construction is not followed, the result will be a below-standard foundation and possibly a house that tilts, floors that sag, walls that crack and leak, doors that won't fit their jambs, and windows that won't open or close-all of the same defects that can also be attributed to a poorly executed footer.

Although crawl spaces are frequently left with soil or gravel floors, both the solid concrete and the block wall foundation floors should be poured with concrete to provide cleanliness, to prevent moisture and insect encroachment, and to supply a useful floor for storage or additional living areas.

The National Building Code requires a foundation to start at least 1 foot below the frost line. Local building codes can tell you how deep the frost line is in your area. Should bedrock (solid rock) be encountered before the prescribed depth is reached, digging can stop because bedrock will not move no matter what happens or how cold it gets.

Even if you build where there's never freezing temperatures, still see that the foundation is situated at least 1 foot below grade (ground level) to assure a firm and level base for the framing structure.

Crawl Space Foundations

These foundations are cheaper to construct than basements, and acceptable when the storage, utility, and living spaces otherwise found in basements are neither needed nor desired (Ill. 10).

Advantages

1. A crawl space foundation is cheaper to construct than a partial or full basement.

2. Because a crawl space is relatively low to the ground, there's not much risk of cracked walls.

3. The crawl space foundation takes considerably less time to build, thus speeding up the overall construction time of the house.

4. A crawl space provides ventilation below the first floor, separating the living areas from contact with the ground and letting the floor follow suit to the temperatures maintained by the living spaces (unlike a slab, which is more affected by ground temperatures).

Ill. 10 Crawl space foundations.

Disadvantages

1. A crawl space foundation is practically useless for storage or living space (for humans).

2. A crawl space can't accommodate large or tall appliances such as water heaters or furnaces.

3. Crawl spaces will attract a variety of small furry creatures (notably rabbits, squirrels, chipmunks, skunks, possums, and mice).

4. Water or sewer lines that run through crawl spaces must be insulated extra well to prevent pipes from freezing.

Partial and Full Basement Foundations

There's a convincing argument that since the house will be placed over a footer anyway, you might as well put the potential space below the regular living level to good use, too (Ill. 11).

Advantages

1. The functional living areas of the house can always be expanded into a basement's lower level. A recreation room can be installed there at a minimal cost.

2. The partial or full basement can easily accommodate a water heater, furnace, and other major appliances such as freezers, washing machines, and clothes dryers.

3. A basement can include a separate entranceway into the house on the lowest level.

4. These foundations provide considerable storage and workbench areas.

5. Like the crawl space, a basement also insulates the main living areas from the ground.

Ill. 11 A basement foundation.

Disadvantages

1. Basement foundations are more expensive to construct.

2. Because of their height, the walls of a basement foundation are more likely to crack and develop problems.

3. A basement foundation can at times be virtually out of the question in areas where the water table is high.

4. Basement foundations take a relatively long time to construct and string out the entire house building process.