After pouring the basement or crawlspace, the next step is framing the floor. This structure will serve as the support system for the rest of the house, so it’s imperative to use a method whereby the lumber withstands a heavy load without shifting or warping. In addition, you will take steps to prevent damage to the wood from exposure to moisture. With some basic knowledge of carpentry skills and the correct tools, you can frame a floor.
Determine your ground floor layout before framing the floor. This is important because you must provide corresponding support beneath your floor for any future load-bearing walls. If you are unsure about this process, contact a builder or have an engineer look at your floor plan.
Install the sill plate that will form the bottom wood layer between your floor and the concrete foundation. When you poured the foundation, you installed anchor bolts into the concrete in anticipation of the sill plate. Drill holes in the sill plate that correspond with the anchor bolts. Slip the plate into place, attaching nuts to the bolts.
Use only treated lumber for the sill plate. The typical sill plate is 2-by-6 inch dimensional lumber. Any time wood encounters concrete, use only treated lumber. After placing the sill plate, fasten it to the foundation with concrete screws, approximately every 2 feet in addition to the anchor bolts.
Set the rim joist. The rim joist surrounds the inner floor joists and should be of the same dimension. Check your local building code if you’re unsure what size to use, otherwise, 2 by 10 inches is the minimum acceptable size for a rim joist.
Place the rim joist on edge. Flush with the outside of your sill plate, and attach with framing nails at the bottom, inserted at a 45-degree angle through the bottom of the rim joist and into the sill plate. Builders call this “toe-nailing.” There will be 4 inches of empty sill plate inside of the rim joist. This is where the floor joists will rest.
Layout your floor joist pattern, setting joists no more than 16 inches apart. Any span longer than 14 feet below your floor joists must include a supporting wall in the basement. In addition, any future supporting walls on the main floor must have a corresponding wall beneath the floor joists for structural integrity.
Splice very long floor joists on a load-bearing wall beneath the floor. Overlap splices a minimum of 4 inches over the wall beneath. "Toe nail" the spliced floor joists to the supporting wall beneath and to each other.
"Toe nail" your floor joists to the sill plate and to the rim joist securely with framing nails. Cut out the portion of the floor that will serve as the opening to your stairway and cut dimensional lumber to frame the hole.