Creating a Raised Floor Foundation: Essential Steps and Materials
A raised floor foundation elevates a house above the ground using piers or walls, so the structure rests on these supports instead of being directly anchored in the earth like a traditional foundation. This design keeps floor joists away from soil, guarding the wood against moisture and acidic damage, which can extend its service life. While construction depends on various conditions, the fundamental approach is straightforward.
By Rowan Scott
A raised floor foundation refers to building a home atop piers or walls that elevate its floor above the ground. In this setup, the house rests on these supports rather than being fixed directly into the earth as with a standard foundation. This arrangement prevents the wood of the floor joists from touching the soil. Since damp or acidic ground cannot harm the house's wood, the material's longevity is extended. Building such a foundation involves numerous considerations, yet the core method is easy to outline.
Raised floor foundations are some of the simplest to construct.
Phase 1
Erect a standalone concrete stem wall, which serves as the primary foundation component. This wall is positioned on footers set deeply underground, below the winter frost line. For a mid-sized home of 3,000 square feet, these footers typically measure two feet square and around 12 inches in thickness. The wall is commonly made of poured concrete or concrete blocks, and the house will be placed on top. The space between the ground and the floor-supporting joists is usually at least 18 inches. Many building codes specify a minimum stem wall thickness of 8 inches, although a full foot is standard, particularly for larger, multi-story homes.
Phase 2
Factor in the soil type when building the stem wall. If you have a two-story house on weak soil, the footers should be roughly 15 inches wide. On stronger soil, about 12 inches suffices. Strong soil generally means it can handle around 3,000 or more pounds per square foot of pressure from a structure, while weaker soil handles about 1,500 pounds per square foot. A professional soil analysis should be conducted before starting construction.
Phase 3
Create a crawlspace. A raised floor foundation inherently forms a crawlspace through its design. With a stem wall foundation, this space is compact and bordered by the walls, the ground, and the floor joists. The soil must be graded to direct all water away from the building.
Phase 4
Place a 6 millimeter polyethylene liner over the crawlspace floor to maintain dryness. Local codes often mandate one square foot of ventilation for every 150 square feet of crawlspace. This ventilation typically consists of an opening in the stem wall, later fitted with a vent frame to block water from entering.
Phase 5
Position the lumber on the stem wall. Usually, the wall is topped with 2-by-4 or 2-by-6 treated wood pieces, secured using half-inch anchor bolts. These bolts are spaced roughly one foot apart or slightly closer, and they must extend about 7 inches into the concrete.
