Seal existing floor, if concrete, and cover with felt paper. Seal walls with epoxy-based sealer before beginning sub-floor installation.
Measure and cut 2-by-6-inch treated lumber to form a snug frame around the perimeter. Space the sleepers no further apart than 18 inches, splitting the difference on each end. Joist hangers can be used on the ends to hold the beams in place -- check with a level and tape measure constantly to ensure beams are plumb and level.
Cut cross beams if necessary and install at 12- to 18-inch intervals, using galvanized 1 1/2-inch screws and staggering beams to allow room to maneuver. Drive nails in at an angle to secure to end beams. Continue checking for plumb and level until all sleepers are in place.
Install sheets of plywood by screwing flat or nailing to beams. Cut to fit around perimeter, and space fasteners close enough that there will be no possibility of buckling or warping.
Sand any rough edges, and fill gaps with wood putty. Seal with a moisture resistant barrier for extra protection. The sub-floor can now be covered with the desired flooring.
Things You Will Need
- Concrete floor sealer
- Felt paper
- 2-by-6-inch lumber for sleepers and frame
- Circular saw
- Measuring tape
- Joist hangers and fasteners
- 1 1/2-inch galvanized screws
- 2-inch nails
- 1/2- to 3/4-inch plywood
- Wood putty
- If walls are concrete, a ram-set may be needed to attach joist hangers. Alternately, sleepers can be shimmed level and attached to concrete floor with a ram-set.