Prepping a Wood Subfloor

Intro

After you have removed or repaired the existing surface of the floor, turn your attention to the subfloor. Ceramics and stone must be kept from cracking and all subfloors should not squeak or sag. Inspect the subfloor and make repairs that will assure it provides a solid, stable bed.

Dimensional lumber -- 1x4 or 2x6 planking -- is not suitable as a bed for any tile. Planks expand and contract with changes in temperature and humidity, as does tile, but at different rates. The result is cracked tile, broken grout joints, or split seams. Install plywood or backerboard on plank. If the resulting floor will be too high for smooth transitions to adjacent floors, tear up the planking and install 3/4-inch exterior-grade plywood, followed by backerboard for ceramics.

Prestart Checklist

Time
About 30 minutes to check defects in an average-size room. Repair time will vary with size and condition of floor; could average 45 minutes per square yard.

Tools
Repair subfloor: 4-foot level, cordless drill/bits, hammer, circular saw Repair surface: mason's trowel, belt sander, nail set Installing membrane: roller, trowel

Skills
Driving nails with hammer; removing fasteners with cordless drill; sawing with circular saw; troweling with a belt sander

Prep
Remove or repair finished flooring

Materials
Subfloor: 2x4 lumber, 2-1/2-inch coated screws, 8d nails, wood shims Surface: thinset mortar Installing membrane: membrane, adhesive

Previous | Next

Trowel thinset into depressions, Sand high spots

Low Spots, High Spots: Level the subfloor

Vacuum the floor thoroughly. Trowel thinset into depressions and chips with a mason's trowel or margin trowel. Feather the edges of the repair so it is level with the floor. After it dries, sand the edges of the repair if necessary.

Before you level any high spots on the floor, make sure the heads of all nails and fasteners are set below the surface. Level high spots on the floor with a belt sander. Keep the sander moving when it is in contact with the floor.

Step 1

Divide the floor into imaginary 6-foot sections and within each section rotate a 4-foot level. Use a carpenter's pencil or chalk marker to outline sags, low spots, high spots, and other defects. Then walk the floor to test it for squeaks and weak spots. Mark these areas.

Previous | Next

Support the stone

Because stone tiles are more brittle than ceramic tile, they won't forgive unstable subfloors or surfaces that are not flat. Stone installations require subfloor support that is solid and free from deflection. Substrates also must be smooth and level.

When preparing a subfloor for stone tile, be thorough and precise when marking defects such as high spots or depressions. Finish the edges of thinset repairs with care and make sure you have removed all structural defects in the joists and subfloor.

Stone is substantially heavier than tile, and current flooring, especially in large rooms, may not be strong enough to support it. Check with a structural engineer if you are in doubt.

Step 2

If the entire subfloor is weak, cut 2x4 bridges to fit between the floor joists. Measure the joist spacing across the floor and, if the dimensions are equal, cut all the bridges at one time. If the spacing varies, cut the pieces to fit. Nail the bridges in place, offsetting each one by 24 inches.

Previous | Next

Waterproofing membrane

Although many tiles and setting materials are impervious to water, virtually no tile installation is completely waterproof without a waterproofing membrane.

Water that penetrates a tile bed weakens the adhesive, promotes rot, and nourishes organisms destructive to the wood subfloor. Bathrooms, kitchens, and surfaces that require frequent cleaning are especially vulnerable.

One of the easiest membranes to apply utilizes an adhesive that spreads with a roller. To install it, start at a wall opposite a doorway and apply the adhesive in sections with a roller. Let the adhesive cure, spread the fiber membrane over it, and trowel the membrane into the adhesive (bottom).

Step 3

Shore up broken or sagging joists by nailing a 2x4 cleat against the subfloor. Force the cleat snugly against the subfloor with a 2x4 prop, nail the cleat in place with 8d nails, and knock the prop out to remove it.

Previous | Next

Step 4

Fill minor sags and separations between the subfloor and joists by driving shims or shingles into the gap. Tap the shim gently until it's snug -- forcing it may cause the flooring above to bow.

Previous | Next

Step 5

Securly fasten loose subflooring material by screwing it to the joists. Drive screws into any repairs you have made with shims. You can use ringshank or spiral shank nails as an alternative, setting the nailhead below the surface with a hammer and nail set.

Previous | Next

Comments