Step 18Subfloor

How to Repair Damaged Subfloor

Subfloor repairs aren't glamorous, but they're essential. Replacing rotted sections, securing squeaky areas, and filling gaps creates a solid foundation for beautiful new flooring that lasts decades instead of years.

Quick Summary

Time needed

4-8 hours

Cost

$100-300 (materials)

Difficulty

Medium

Common Subfloor Repairs

1. Replace Rotted or Water-Damaged Sections

Rotted wood has no structural integrity. It must be completely removed and replaced with new plywood.

Step-by-Step Process:

  1. 1. Mark the damaged area extending to center of nearest joists
  2. 2. Set circular saw to subfloor thickness (usually 5/8 or 3/4 inch)
  3. 3. Cut along marked lines - stop 1 inch from corners to avoid overcutting
  4. 4. Finish corners with jigsaw or handsaw
  5. 5. Pry out damaged section carefully
  6. 6. Inspect joists underneath - replace if rotted
  7. 7. Add 2x4 blocking between joists where patch edges don't land on joists
  8. 8. Cut new plywood patch to fit opening exactly
  9. 9. Apply construction adhesive to joists and blocking
  10. 10. Screw patch down with 3-inch screws every 6 inches

2. Fix Squeaky and Loose Areas

Squeaks mean the subfloor isn't properly attached to joists. This is an easy fix that prevents future flooring problems.

Squeak Repair:

  1. 1. Locate floor joists (usually 16 inches on center)
  2. 2. Drive 3-inch deck screws through subfloor into joists
  3. 3. Space screws 8-12 inches apart along squeak areas
  4. 4. Countersink screw heads slightly below surface
  5. 5. Walk area again to verify squeak is eliminated

Pro Tip: Use screws, not nails. Nails work loose over time. Screws pull subfloor tight and stay tight.

3. Fill Gaps and Holes

Small defects need filling to create a smooth, continuous surface for new flooring.

Filling Methods:

Gaps between sheets (1/8-1/4 inch):

Fill with construction adhesive or flexible floor patch compound. Smooth with putty knife.

Small holes and gouges:

Use wood filler or floor patch compound. Overfill slightly, let dry, sand smooth.

Large gaps (over 1/4 inch):

Cut thin wood strips to fit, glue in place with construction adhesive, then sand flush.

4. Remove Protruding Fasteners

Old nails, staples, and screw heads that stick up will damage new flooring or prevent proper installation.

  • • Pull out or pound down old carpet tack strip nails
  • • Remove all staples with pliers or flat bar
  • • Countersink raised screw heads
  • • Grind down or pull protruding nails
  • • Run your hand across floor - should feel completely smooth

Materials and Tools You'll Need

Materials:

  • ✓ Exterior-grade CDX plywood (match thickness)
  • ✓ 3-inch deck screws (1 lb box)
  • ✓ Construction adhesive (2-3 tubes)
  • ✓ 2x4 lumber for blocking
  • ✓ Wood filler or floor patch compound

Tools:

  • ✓ Circular saw or jigsaw
  • ✓ Drill/driver with bits
  • ✓ Pry bar
  • ✓ Hammer
  • ✓ Level and measuring tape
  • ✓ Safety glasses and dust mask

Pro Tips for Subfloor Repairs

Match Plywood Thickness Exactly: If existing subfloor is 5/8 inch, use 5/8 inch patches. Mismatched thickness creates bumps. Measure with calipers if unsure - old subfloor may be different than modern standard sizes.

Oversize Your Cuts: When removing damaged sections, cut at least 6 inches beyond visible damage. Rot often extends further than you can see on the surface.

Use Exterior-Grade Plywood: Always use exterior CDX plywood for subfloor repairs, even in dry areas. It handles occasional moisture better. Never use OSB for patches - it doesn't hold up.

Frequently Asked Questions

Can I patch small areas of subfloor damage or do I need to replace entire sheets?

You can patch small damaged areas (under 2 feet square). Cut out the damaged section to the nearest joists, add blocking for support, and screw in a patch. For damage affecting more than 30% of a sheet or multiple areas, replacing the entire sheet is often easier and stronger.

What type of plywood should I use for subfloor repairs?

Use exterior-grade CDX plywood, same thickness as existing subfloor (typically 5/8 or 3/4 inch). 'Exterior grade' handles moisture better than interior. Match the existing thickness exactly - mismatched heights create bumps in your new flooring. For bathrooms and kitchens, consider tongue-and-groove AdvanTech subfloor for extra moisture resistance.

Ready for the Next Step?

With subfloor repairs complete, you now need to level any remaining high or low spots.

Back to ChecklistNext: Level Subfloor