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Install a floating subfloor over concrete slab

Because they interlock and sit directly on the slab, floating subfloor panels are ideal for concrete surfaces that are level and clear of obstructions. Available in many models, materials and prices, they provide an effective moisture barrier and create a solid base for a finished floor. Browse our wide selection of basement and garage flooring options to create just the right space.

Why a subfloor?

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.

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).
Calculate the surface area and determine the layout

Calculate the materials required

  • 1.1 - Measure the length and width of the room
  • 1.2 - Multiply the length of the room by the width to determine the surface area (square footage) to be covered.
  • 1.3 - Divide the total surface area by the surface of one panel. Most floating subfloor panels measure 2' x 2' but check the precise measurements with the manufacturer. The result of your calculation will provide you with the exact number of panels needed.
  • 1.4 - Add 10% to the total in case of error or defective pieces.
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Cut door casings flush with the new floor

Prepare walls and doors

  • 2.1 - Remove existing baseboards, mouldings and doorway thresholds.
  • 2.2 - Measure the combined thickness of the subfloor and finished floorcovering.
  • 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 - If necessary, remove the 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 and level the slab

Level and clean the slab

  • 3.1 - With a 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 - Clean up construction debris: loose concrete, paint, adhesives, and dust.
  • 3.5 - Fill cracks with concrete filler and smooth out the surface with the flat edge of a trowel.
  • 3.6 - Allow the filler to cure, then seal with water-based polyurethane.
Check the corner for square and plan the layout

Determine the layout

  • 4.1 - Choose the longest straight wall as your starting wall.
  • 4.2 - Check your starting corner for square. If the angle is greater than 90º, trim the edge of the panel to maintain a ¼" gap along the wall.
  • 4.3 - Ensure that the last panel will be at least 3" wide. If necessary, cut the first panel.
Install the first row of floating subfloor panels

Install the first row of panels

  • 5.1 - Beginning in the 'start' corner, place ¼" spacers along the walls.
  • 5.2 - Install the first panel with the tongue-side flat against the spacers.
  • 5.3 - Insert the tongue of the following panel into the groove of the first.
  • 5.4 - Place a block of wood (tapping block) against the edge of the second panel and tap with a rubber mallet to ensure a tight seam.
  • 5.5 - Continue with the other panels in the first row.
  • 5.6 - Ensure the panels are level as you proceed and use levelling shims when necessary.
  • 5.7 - Measure the space for the last panel and cut. Maintain the ¼" space.
  • 5.8 - Use a pull bar to install the panel.
  • 5.9 - Check the first row for level and adjust with shims if necessary.
Install a floating subfloor over concrete slab

Install the remaining rows

  • 6.1 - Stagger the seams for subsequent rows. Cut the first panel, maintaining a minimum 3" width.
  • 6.2 - Maintain the ¼" space along the wall.
  • 6.3 - Cover the entire surface area, staggering the seams in each row.
  • 6.4 - Always check for square and adjust with shims when necessary.
  • 6.5 - Measure and cut to fit the panels for the last row. Maintain the ¼" space against the wall.
  • 6.6 - Use a pull bar to insert panels.
  • 6.7 - Remove the spacers along the walls. The space allows for expansion and contraction and will be concealed by baseboards.

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