Basement Subfloor Step by Step

basement subfloor with plywood inIf you plan on installing a ceramic or vinyl tile floor, a basement floor is optional. But if you want laminate, hardwood, parquet, cork, carpeting, or bamboo or flooring in your finished basement, you’ll want to lay down a basement subfloor. This ensures protection of your floors against moisture damage. It also makes for a warmer and more comfortable floor.

A subfloor is the structure below your flooring materials. Most basement flooring materials should have a subfloor that provides moisture protection. Ceramic and vinyl tile have other requirements, such as surface flatness and strength, that are more properly addressed with specialized underlayments like cement backerboard.

The subfloor structure takes the form of a grid of 2 by 3’s (called sleepers) attached to the concrete slab and covered with sheets of 5/8 inch plywood. Sleepers are usually separated from the bare concrete by some type of vapor barrier; either thick felt paper or 6-mil plastic sheeting. Here’s how it’s done:

1. Take Measurements

Sketch out the floor area on a sheet of paper. Figure out how many 2 in. × 3 in. lumber boards you need for the perimeter sleepers and the length each needs to be. Then, establish how many 2 in. × 3 in. lumber boards you need for the lengthwise sleepers; these should be spaced 16 inches center on center. Last, determine the number of 4 ft. × 8 ft. sheets of plywood you need for the subfloor.

2. Lay Vapor Barrier

Check local building codes for requirements on vapor barrier materials. Lay 15 lb. felt paper or 6-mil polyethylene sheeting on the concrete slab. The goal is to prevent any ground moisture infiltrating up to the underneath of the plywood, which can cause mold, fungus, rotting and bad smells, among other things. Allow approximately 4 in. of vapor barrier laying up against each basement wall as well.

3. Frame Subfloor

Begin with the perimeter, laying down 2 in. ×3 in. pieces flat along the walls, immediately on the vapor barrier. Then place the 2 in. × 3 in. sleepers inside this frame. Position the first sleeper 16 in. from the wall and the rest 16 in. apart center to center. These sleepers should be installed parallel to the shortest length of the room.

4. Leveling and Nailing Down Sleepers

With a spirit level, level the 2 x 3 sleepers, placing shims underneath as needed to ensure the plywood sheets will sit flush. After all sleepers are leveled, fasten them to your concrete slab with ramset fasteners. Use these to drive 2 ½ in. concrete nails at a spacing of 24 in. apart. Trim off the excess vapor barrier along the walls with a utility knife. Pencilmark the centers of each sleeper on the basement wall as guidelines for securing the plywood to the sleepers.

5. Screwing Down Plywood Sheets

Apply a thick bead of construction glue on top of the sleepers. Lay and fit 5/8 in. thick Tongue and Groove plywood sheets over the sleepers, parallel to the shorter wall. Using as guide the pencilmarks made on the wall, screw the plywood sheets into the sleepers with 1 ½ in. ring-shank or screw-shank nail or flooring screws every 12 inches. From one row to the next, stagger the joints of the plywood sheets.

Creating a wood basement subfloor on a concrete slab is a DIY project that can be done by most homeowners comfortable with a mid-level difficulty project. If you are going to be installing ceramic tiles on the subfloor, be aware that you will additionally need to install ½ in. thick plywood sheets running in the opposite direction over your subfloor, or cement backerboard underlayment.

See Also: Different Types of Plywood