How to Put in a Plywood Underlay

Written by jackie lohrey
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Plywood underlay sits between the structural subfloor and flooring material of a building and provides a solid, smooth base to help the floor look better and last longer. It creates the foundation for the flooring material. While other materials are available for use as underlay, plywood is a common choice because of its strength, durability, and versatility with many different flooring types. It is not especially difficult to install.

Skill level:

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Things you need

  • Plywood underlay sheets. ¼-inch
  • Drill
  • Screw-type drill bit
  • Circular saw
  • Deck screws, 1 inch
  • Wallboard knife
  • Floor patching compound
  • Power sander

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  1. 1

    Lay a full sheet of plywood underlay, smooth side up, alongside the longest wall in the room, leaving a ¼-inch gap between the wall and the plywood underlay sheet. Make sure the seams of the plywood underlay do not run alongside the seams of the subfloor.

  2. 2

    Fasten the plywood sheet by inserting screws to a depth just below the surface of the plywood underlay, at 6-inch intervals along the edges of the sheet, and at eight-inch intervals across the width of the sheet. Use a drill with a screwdriver bit and 1-inch deck screws. Continue laying sheets of plywood underlay, leaving a ¼-inch gap between the plywood sheets.

  3. 3

    Create notches in plywood underlay sheets to fit into doorways, using a circular saw, and then fasten to the floor using the drill.

  4. 4

    Spread a thin layer of floor patching compound over all seams and screws, using a wallboard knife, and allow a few hours for the compound to dry.

  5. 5

    Sand areas where you applied the floor patching using a power sander until the area is smooth. Vacuum and wipe the floor with a damp cloth to remove all traces of dust.

Tips and warnings

  • Lay the plywood underlay up against a wall in the room where you will install and allow the plywood to sit for a few days before installing. This is necessary to ensure it has time to adjust to room conditions, such as moisture and temperature.
  • Ensure no weak areas develop in the floor by installing plywood underlay so underlay seams never align with subfloor seams.

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