FOUNDATIONS
Choosing a foundation
There are variations in each foundation system and it is important to chose the once that will be best for the climate, soil type, the site, and the builiding program. If there is any question about matching a foundation system to the soil or to the topography of the site, consult a soil or structual engineer before construction begins.
Foundation functions
Foundation systems have two functions. First, it supports the building structurally by keeping it level, minimizing settling, preventing uplift from the forces of frost, expansive soils and resisting horizontal forces such as wind and earthquakes. Second, foundation keep the wooden parts of the building above the ground and away from the organisms and moisture in the soil that both eat wood and cause it to decay.
If the foundation does not support the building adequately, cracks and openings will occur over time. No amount of repair on the structure above the foundation will compensate for an inadequate foundation. Once a foundation begins to move and crack, it will continue to move. We now have the knowledge to construct durable foundations, so there is no reason to invest in a building that is not fully supported on a foundation that will endure for the life of the structure.
There are three common foundation types in the United States with three differnet performances, but all rely on a perimeter foundation.
Slab-On-Grade Foundations
Slab-on-grade foundations are mostly used in warm climates where living is close to the ground and the frost line is close to the surface. The footings are usually shallow, and the ground floor is a concrete slab. Many slab-on-grade systems allow the concrete footings, foundation and subloor to be poured at the same time.
Crawl Spaces
Crawl spaces are found in all climates but predominate in temperate regions. In this system, the insulated wooden ground floor is supported above grade on a foundation wall made of concrete or concrete block.The resulting crawl space introduces an accessible zone for ductwork, plumbing and other utilities, and allows for simple remodeling.
Basements
Basements are the dominate foundation system in the coldest parts of the country, where frost lines mandate deep footings. Like crawl spaces, basements are accessible. Basement foundation systems are usually constructed of concrete or concrete block foundation walls. Drainage and waterproofing are particularly critical with basement systems.
Checklist
Because foundation is so difficult to repair, it is recommended that you may want to follow this checklist:
Place the bottom of the footing below the frost line on solid, undisturbed soil that is free of organice material.
Use continuous horizontal rebar in the footing and at the top of foundation walls, (joint reinforcing may be allowed in concrete block walls). Tie the footing and wall together with vertical rebar.
Use pressure-treated or other decay-resistant wood in contact with concrete. Use pressure-treated or other decay resistant wood in contact with concrete. Use a moisture barrier barrier between all concrete and untreated wood.
Tie wood members to the foundation with bolts or straps embedded in the foundation.
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 that are important to a permanent foundation. 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 insects, wood decaying organisms, and precautions against radon gas.