How Do I Replace a Damaged Subfloor?

Before you cut anything, find out how far the water damage reaches. Probe the soft spots with a screwdriver and look for discoloration or sagging. Use a moisture meter if you have one to map the wet areas. In a semi-arid climate with cold winters and hot dry summers, drying can happen faster, but trapped moisture still rots wood over time.

Check the joists underneath the subfloor. If they are soft or crumbling, the repair goes beyond a simple subfloor patch. You might need to reinforce or replace joists, which is a bigger job. If the damage spans more than a few square feet, consider hiring local water damage pros to evaluate the structure before you start.

• Look for soft, spongy, or discolored floor areas
• Use a screwdriver to probe for rot near walls and pipes
• Check the crawlspace or basement for standing water
• Inspect joists for sagging or water stains
• Confirm the water source is stopped before any repair

Measure the opening and cut new plywood or OSB to fit. Use the same thickness as the original subfloor, typically 3/4 inch. For a snug fit, cut the panel slightly smaller than the opening, about 1/8 inch gap. Lay the panel over the joists and secure it with exterior-grade screws or nails.

Space fasteners every 6 inches along the edges and every 8 inches in the field. Stagger the seams so they do not align with the old subfloor joints. If you are replacing a large area, install blocking between joists to support panel edges. Check for level and make sure the panel sits flush with the surrounding floor.

• Use plywood or OSB made for exposure to moisture
• Match the thickness of the original subfloor
• Leave a small gap at edges for expansion
• Drive screws or nails at the recommended spacing
• Stagger seams for a stronger floor
• Add blocking between joists if the panel edge is unsupported

This project is doable for a skilled DIYer, but it gets risky when the damage is widespread or the joists are compromised. Cutting into a floor near plumbing or electrical lines can cause more harm. If you find mold, a musty smell, or sagging that extends beyond the subfloor, stop and get professional help.

The pros can handle the full process, from removing wet materials to structural drying and dehumidification. They have the tools to dry hidden cavities and the experience to spot issues you might miss. If the water came from a burst pipe or flood, you may also need to file a claim with your insurance company. Policies vary, and your insurer decides what is covered.