Subfloor repair, the line item cheap bids delete

What is subfloor repair?

Subfloor repair means replacing or refastening the structural sheathing layer - usually plywood or OSB - that sits between your joists and your finished floor. It is done when panels have been softened by water, have delaminated, have lost their fastening, or are simply not flat enough for the flooring going over them. It stops at the sheathing: once the joists themselves are damaged, that is structural work.

Signs you need subfloor repair

• A section of floor feels soft, spongy or gives slightly when you step on it
• You can see staining, swelling or crumbling in the panel where old flooring has been lifted
• Nails or staples keep working their way back up through the finished floor in the same area
• There is a persistent squeak in a fixed location, which is usually a panel moving against a fastener
• A toilet, dishwasher, refrigerator line or washing machine has leaked slowly for a while
• The floor slopes or dips noticeably in one part of a room but is level elsewhere
• Your flooring installer has flagged that the panels will not hold fasteners or meet flatness tolerance

Seeing something not on this list? Browse flooring problems by symptom →

How subfloor repair actually works

1. 1

   Exposing and probing the damage

   The finished floor over the suspect area comes up and the panel is probed - a screwdriver or awl into the sheathing tells you quickly where sound wood stops. Damage is almost always larger than the visible stain.

   Watch forA repair scoped from above through the finished floor. Nobody can size this job without seeing the panel, and quotes given before exposure are guesses that turn into change orders.

2. 2

   Finding and stopping the water

   If the cause is a leak, it gets traced and fixed before anything is rebuilt - a supply line, a failed wax ring, a drain, condensation, or moisture rising from a crawl space with no vapour barrier.

   Watch forNew sheathing installed over an unresolved source. It is a genuinely common shortcut and it means paying for the same repair twice.

3. 3

   Cutting out to the joists

   Damaged panel is cut back to the centreline of joists on each side so the new panel has something to land on. Blocking is added between joists along the cut edges that run parallel to the framing.

   Watch forA patch cut to the size of the damage and screwed to nothing along two of its edges. An unsupported panel edge flexes forever and will telegraph through whatever floor goes over it.

4. 4

   Checking the joists while they are visible

   With sheathing off, the framing below is inspected for rot, insect damage, notching from plumbing or electrical, or sagging. This is the only convenient moment to see it.

   Watch forCompromised joists papered over with new sheathing. Rot, deep notching or visible sag is a structural question and needs the right trade, not a thicker panel on top of it.

5. 5

   Installing matched sheathing

   The new panel is matched in thickness to the existing so the surface stays flat, fastened on a proper schedule with screws rather than nails where possible, and gapped slightly at the edges as sheathing is designed to be.

   Watch forA thickness mismatch left to be dealt with by levelling compound. It can be feathered, but it is far better solved with the right panel than with a lump of patch.

6. 6

   Refastening the surrounding area and flatness check

   While the floor is open, adjacent panels are refastened to kill nearby squeaks and the whole area is checked with a straightedge against the tolerance the new flooring requires.

   Watch forThe job ending at the edge of the patch. Refastening the surrounding field is cheap while the floor is open and expensive once the new floor is down.

What does subfloor repair cost?

Subfloor repair is normally priced by the area of sheathing replaced plus access - pulling and reinstating flooring, cabinets or a toilet often costs more than the panel work itself. A small localised patch is a modest job; whole-room replacement is not. The Cost Guide covers ranges.

See the full subfloor repair cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →