How to Frame an Elevated Floor

Foundation

• Elevated floor frames rest on either a foundation wall, also known as a stemwall, around the perimeter of the structure or on a system of concrete piers that support the floor at multiple points and transmit the weight load of the building to the ground. Pier foundations are required in some flood-prone locations because the absence of foundation walls allows flood water to pass beneath the building without causing structural damage. Pier foundations also allow better ventilation beneath the building, resulting in fewer problems with excess moisture in the crawl space.

Sill Plate and Sill Beam

• In stemwall foundation designs, the floor frame is attached to the foundation via sill plates made from 2-by-4-inch or 2-by-6-inch lumber bolted directly to the concrete foundation wall. In areas prone to earthquakes or hurricanes, additional metal strapping may be required to anchor the sill plate to the foundation. In pier foundation designs, sill beams of solid-sawn lumber or laminated construction span the spaces between piers and provide support for the floor framing. The sill beams are attached to the piers with anchor bolts.

Joists

• Floor joists span the foundation and rest on the sill plates or sill beams. Joists are fashioned from either solid-sawn lumber, engineered wood beams or engineered floor trusses. They are typically attached to the sill plates or sill beams with nails by toenailing through the end of the joist. Joists should overlap sill plates or beams by a minimum of 1 1/2 inches. A band joist, also called a rim joist, rests on the sill plate perpendicular to the floor joists and is nailed to the ends of the floor joists.

Subfloor

• Subflooring material forms the floor deck and is the base upon which the finished floor materials are laid. The subfloor is usually constructed from 4-by-8-foot sheets of plywood or oriented strand board nailed directly to the floor joists, with small gaps left between the sheets to allow for expansion caused by humidity or temperature changes. In earthquake-prone or hurricane-prone areas, the subfloor helps transmit lateral forces caused by wind or earth movement from the walls to the foundation.