Each foundation system has many variations, and it is important to select the one best suited to the climate, the soil type, the site, and the building program. With all foundations, you should investigate the local soil type. Soil types, along with their bearing capacities, are often described in local soil profiles based on information from the U. S. Geological Survey (USGS). If there is any question about matching a foundation system to the soil or to the topography of the site, consult a soil or structural engineer before construction begins. This small investment may save thousands of dollars in future repair bills.
design checklist
Because the foundation is so important to the longevity of the building and because it is so difficult to repair, it is wise to be conservative in its design and construction. Make the foundation a little stronger than you think you need to. As a minimum, even if not required by code, it is recommended that you follow this rule-of — thumb checklist:
1. Place the bottom of the footing below the frost line on solid, undisturbed soil that is free of organic material. (Local codes will prescribe frost-line depth.)
2. Use continuous horizontal rebar in the footing and at the top of foundation walls (joint reinforcing may be allowable in concrete-block walls). Tie the footing and wall together with vertical rebar.
3. Tie wood members to the foundation with bolts or straps embedded in the foundation. Anchoring requirements in hurricane and severe earthquake zones are shown in the following chapters, but specific requirements should be verified with local codes.
4. Provide adequate drainage around the foundation. Slope backfill away from the building and keep soil
6 in. below all wood.
Many codes and many site conditions require measures beyond these minimum specifications. In addition, there are several other considerations important
to a permanent foundation system, and these are discussed in this chapter. They include support of loads that do not fall at the perimeter wall, such as footings for point loads within the structure and at porches and decks; insulation and moisture barriers; waterproofing and drainage; protection against termites, other insects and wood-decaying organisms; and precautions against radon gas.
5. Get the details right. Use pressure-treated or other decay-resistant wood in contact with concrete. Straps, hangers, and fasteners in contact with pressure-treated lumber should be hot-dip galvanized to protect against degradation from the preservative chemicals. Use a moisture barrier between all concrete and untreated wood.
other foundation systems
The permanent wood foundation (PWF), developed in the 1970s, now accounts for about 5% of foundations in the United States and 20% in Canada. Made of pressure-treated framing, the crawl-space or basement walls sit on a bed of compacted gravel rather than a concrete footing. The same framing crew that constructs the structure above can build the foundation walls; and when insulation, wiring, and other utilities are required, they can be located in wall cavities between studs as they are in the rest of the building.
Insulating concrete formwork (ICF) may be used in place of wooden formwork for the walls of a basement or heated crawl space. The insulation stays in place after the concrete walls have been poured and provides thermal separation for the space within. ICF walls must be protected on the exterior, and wiring and other utilities must be either integrated or carved into the interior insulation surface.
