Prepping a Wood Subfloor

This story shows how to prevent tile floors from cracking by properly preparing a wood subfloor.


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

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.

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

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

Remove or repair finished flooring

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

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.

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.

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.

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.

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.

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