How to Attach Underlayment to a Subfloor

"Underlayments are crucial to the success of the flooring on top of it," according to Tim Carter of Ask the Builder.

An underlayment is a layer of wood or board between the subfloor and the finished flooring. The ideal underlayment is a cement board or gypsum board rated for moisture protection. Plywood is sometimes used, but it should not be used over crawl spaces or in moist environments. Properly mounting the underlayment to the subfloor requires proper measurement and tapered-head screws.

Measure the thickness of the subfloor and the underlayment. The tapered-head screws need to be long enough to penetrate the underlayment but not so long that they penetrate through the subfloor. A 3/4-inch screw is ideal for 1/2-inch subfloor with 1/2-inch underlayment.

Measure and cut each piece of underlayment. Plywood requires that you cut the material with a jigsaw. Cut cement "backer board" or gypsum underlayment material to size using a utility knife.

Place the underlayment onto the subfloor in position. Drive a proper-size tapered-head screw through the underlayment into the subfloor while sinking the tapered head to just below the surface of the underlayment. Space the screws approximately 8 inches apart and 1/2 to 1 inch from the edges.

Stagger the seams in the underlayment panels so that the material follows a brick wall pattern. Staggering the seams provides a sturdier base for the finished flooring.

Fill in gaps and screw holes with mortar for cement backer board or with plaster for gypsum. Cover the seams with fiberglass joint tape covered in mortar or plaster. Fill screw holes and seams in plywood underlayment with wood putty. Smooth all mortar, plaster or putty with a putty knife.

Things You Will Need

  • Drill
  • Screwdriver drill bit
  • Measuring tape
  • Jigsaw
  • Utility knife
  • Wood screws with tapered heads
  • Mortar
  • Plaster
  • Wood putty
  • Fiberglass joint tape
  • Putty knife

Tip

  • Sand any lips or rises between boards to level the surface of the underlayment before applying seam fillers. Proper application and staggered joints helps to avoid lips.