How to Repair Water Damage on a Subfloor

Water damage to the subfloor is an important problem that needs repair. The rotted subfloor will be weak and possibly dangerous. Modern subfloors are built using 3/4-inch plywood or OSB, and when they get wet and rot the layers separate, creating bumps in the floor. The damaged subfloor must be removed and replaced. Use plywood for repairs rather than OSB. It costs slightly more, but the additional strength of the plywood is important since the integrity of the subfloor will be compromised.

Difficulty: Moderate

Instructions

Things You'll Need:

• Framing square
• Tape measure
• Straight edge
• Marking pencil
• Circular saw with carbide framing blade
• Reciprocating saw with demolition blade
• Flat prybar
• 2x6 lumber
• 3-inch framing nails
• Hammer
• 3/4-inch plywood subfloor material
• 2-inch wood screws
• Drill with driver bits
• Safety goggles
• Gloves

Demolition

1. Determine the region of subfloor to be replaced. Plan for a rectangular demolition section. Two sides of the rectangle must be parallel to the floor joists and be on top of the joists. Keep the demolition area square and use a framing square, tape measure and straight edge to mark out the demolition section.

2. Set the circular saw to cut 3/4" deep and plunge cut into the subfloor along the top of a joist, cutting along the line from start to finish. Repeat over the next joist, then cut across the space on both ends between the two cuts. A large area should be removed in square sections.

3. Remove the cut plywood with the flat prybar. Floor nails have ringed shanks, which makes them hard to remove. Any nails that pull through the old subfloor can be hammered down into the joist.

4. Continue cutting and removing the sections of plywood between joists until the entire demolition section is complete.

Construction

5. Nail 2x6 boards onto the joists at the edges of the demolition area. The boards should be flush with the top of the joists, or the floor will not be smooth and level.

6. Cut 2x6 boards to fit between the joists at the ends of the demolition area. Nail them to the 2x6 boards already in place and to the joists. Position the boards so that both the new subfloor and the old subfloor are supported.

7. Cut the plywood subfloor material to fit the demolition area. If the area is large, plan to utilize the tongue and groove on the plywood pieces. If the area is smaller than a sheet of plywood, plan and cut so that the tongue side is removed, not the groove side.

8. Lay the cut plywood in place and keep a 1/4" expansion gap between all boards. On tongue and groove fittings, fit the tongue into the groove, but maintain the 1/4" gap, rather than butting the boards tightly together.

9. Screw the plywood into the joists and 2x6 supports. Use flathead wood screws with a minimum un-threaded shank length of 3/4". Typically, 2-inch screws meet this requirement.

Tips & Warnings

• Mark out the sides of the demolition section by drawing the lines on the inside of the nails that hold the damaged subfloor down. This way, the subfloor not being removed has adequate support. Cutting close to a wall with a circular saw can be almost impossible. Use a reciprocating saw if you need to, but use care not to cut through joists, wires, conduit, water, gas or waste pipe or HVAC duct work.
• Wear safety goggles and gloves while cutting the subfloor during demolition. You WILL hit nails and screws causing sparks and sending metal and wood chips flying. Glasses are not adequate protection in this case; wear goggles. Keep a good grip on the saw during demolition. Don't overextend yourself. The saw may jump or stall when you hit nails or screws.

References

• Subfloor Repair and Replacement Guide

Resources

• Minor Repair Video
• Plywood Subfloor Repair