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How-to install a wood subfloor over concrete

Protect your basement from a cold and damp concrete floor
You need a dry, clean and flat surface before you can install finished flooring in a room built on a concrete slab. One of the simplest options is a sleeper-based subfloor. The result is a subfloor that protects finished flooring from cold and damp concrete-based floors.

Subfloor options

Some flooring products are designed to be installed directly on a concrete surface. Others require some type of subfloor to protect them from the effects of cold, damp concrete. Check the flooring manufacturer's installation instructions before you make your choice. Go to the article: “Is a subfloor necessary in the basement?” It will help you choose the correct subfloor for your needs.

Before you begin

  • Check your local building code, and pay special attention to rules concerning vapor barriers, span ratings (subfloor thickness) and floor drains.
  • Use a digital moisture detector to measure the moisture content (MC) of the concrete slab.
  • Read the flooring manufacturer's guidelines for the product you plan to install, and pay particular attention to requirements as regards the structure and moisture content of the concrete slab.
  • Manufacturers may require sub-floor and flooring materials to be acclimated prior to installation (generally 24-72 hours).
  • Wear eye protection and work gloves, and use an approved dust mask or respirator when cutting pressure-treated wood.

Components of a wood subfloor

Here are the components of a sleeper-and-wood subfloor:
  • Plastic or water membrane
  • Foam insulation
  • 2" x 4" sleepers
  • 3/4" hardwood floor
Calculate the surface and determine the layout
1

Calculate the materials required and establish a plan

  • 1.1 - Measure the length and width of the room and make a layout plan.
  • 1.2 - Calculate the total area of the room (length multiplied by width) which will indicate the quantities of vapour barrier, plywood and insulation you will need.
  • 1.3 - On your plan, draw the location of sleepers around the perimeter of the room and calculate the number required (multiples of 8ꞌ and 12ꞌ)
  • 1.4 - On the plan, draw parallel sleepers located 12" on center and calculate the number required.
  • 1.5 - Add 10% to total quantities of materials in case of error or defective pieces

Make your life easier

Our converters can be a big help in calculating the quantity of materials you need.
Converter
Cut casings to the level of the new floor.
2

Prepare walls and doors

  • 2.1 - 2.1. Remove existing baseboards, moulding and doorway thresholds.
  • 2.2 - 2.2. Measure the combined thickness of the subfloor (sleepers and wood panels) and finished floorcovering.
  • 2.3 - 2.3. With a saw or oscillating tool, cut door casings and jambs to 1/16" above the level of where the finished floor will be.
  • 2.4 - 2.4. If needed, remove doors and use a circular saw to cut 1/8" above the level of where the finished floor will be. (The bottom of a hollow-core door can only be cut to a limited extent. Never cut the top of the door.
Identify high and low spots
3

Check for flatness and clean the slab

  • 3.1 - With a long metal straightedge, identify dips or high spots that exceed recommended tolerances for the floor you are installing (usually within a variance of 3/16" every 10 feet or 1/8" every 6 feet).
  • 3.2 - Use a concrete grinder to level high spots.
  • 3.3 - Fill low spots with self-levelling compound or Portland cement. Follow the compound manufacturer's recommendations regarding surface preparation, mixing and drying time.
  • 3.4 - Sweep, vacuum and remove debris such as drywall compound, paint, dirt, adhesives, tack strips, staples and loose concrete.
  • 3.5 - Fill cracks with concrete filler and smooth out the surface with the flat edge of a trowel.
  • 3.6 - Let the filler cure, then seal with water-based polyurethane.
Install the polyethylene vapour barrier
4

Install the vapour barrier

  • 4.1 - Cover the slab with polyethylene sheeting, overlapping seams by at least 6".
  • 4.2 - Extend approximately 3" of polyethylene up the walls around the entire perimeter of the room.
  • 4.3 - Seal the seams, edges and any tears with blue 60 mm sheating tape.

Pro tip

For additional protection, apply a “skim” coat of asphalt mastic to the slab with the straight edge of a trowel. After about 30 minutes, the polyethylene sheeting can be rolled over the mastic by walking over the film, embedding it in the mastic.
Space 12" sleepers on centre
5

Install the sleepers

  • 5.1 - Lay flat the pressure-treated 2" x 3"s around the entire perimeter of the room, maintaining a ½" expansion gap between each 2" x 3" and the walls.
  • 5.2 - Use shims to level if necessary, and if several thicknesses are required, glue shims together.
  • 5.3 - Screw each sleeper at least 2" into the concrete slab. Space screws 24" apart. Screw heads should lie flush with or just below the surface of the sleepers.
  • 5.4 - Lay 2" x 3"s in parallel rows 12" apart on-center, perpendicular to the direction of your finished flooring. Butt the joints, leaving a ¼" expansion gap.
  • 5.5 - Ensure the sleepers are level, and shim if necessary.
  • 5.6 - Fasten the sleepers to the concrete at 12" intervals.
Add insulation between the sleepers
6

Position insulation panels

  • 6.1 - Measure the spaces to be filled between the sleepers.
  • 6.2 - Mark the cutting lines on the insulation panels
  • 6.3 - Use a utility knife and metal straightedge to score the foam insulation.
  • 6.4 - Turn over the scored foam panel. Press hard against the back (uncut side) of the scored cut for a clean break.
  • 6.5 - Do the same for all panels and place them between the sleepers, on the vapor barrier.
Secure the subfloor to the sleepers
7

Screw in the plywood subflooring

  • 7.1 - Check the plywood panels for chipped tongue-and-groove edges.
  • 7.2 - Measure and cut the plywood panels so interlocking joints meet at the center of the sleepers. Stagger the joints so no joint runs into another joint.
  • 7.3 - Before you place each panel, apply a bead of construction adhesive to the top of the sleepers.
  • 7.4 - Maintain a 1/8" expansion gap around the perimeter.
  • 7.5 - Fasten each plywood panel to the sleepers using flooring screws: every 6" around the edges and every 12" along intermediate sleepers.
  • 7.6 - Clean the plywood subfloor surface before you install the flooring.

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