Subfloor repair: what to know

What the job involves, what changes the price, and what to ask a pro.

What subfloor repair is

Subfloor repair cuts out soft, rotted or damaged subfloor, repairs the joists if needed, then installs new plywood and fastens it so the flooring above sits on a firm, level base. People need it when a floor feels soft, sags, squeaks badly or shows water damage, and it has to be fixed before any new floor goes down.

Signs you need it

  • The floor feels spongy or bounces near the toilet, the tub, the sink or an exterior wall
  • The floor has dark staining, a musty smell or a soft spot that gives under a light push
  • The floor squeaks or moves in a pattern across a room, which can mean loose or split subfloor
  • The new flooring you want to install shows a dip or a ridge that the subfloor is causing
  • The floor has sagged between joists or beams, and doors in the room no longer close well

What the pro does

  • Find the extent of the damage by probing the subfloor, checking the joists and the adjacent wall plates, and opening a test area
  • Find and fix the source of any water, such as a leaking supply line, a failed drain or a flashing gap, before any repair
  • Cut out the damaged subfloor back to sound framing, and check the joists for rot, notches and splits
  • Repair or sister the joists and add blocking as the framing needs, using the lumber and fasteners the structure calls for
  • Install new plywood or subfloor panels, staggering the joints and fastening them to the joists at the spacing required
  • Check the floor for flatness and squeaks, then fill and level the surface for the new flooring

What changes the price

Every job is different, so we do not publish prices. These are the things that move the quote up or down.

  • Area of the damaged subfloor and how many joists or beams need repair
  • Whether the rot is limited to the subfloor or extends into the joists and the plates
  • The source of the water, and whether a plumbing or drainage repair is needed first
  • Access from below, such as an open crawl space or a basement ceiling that must be opened
  • Thickness and type of subfloor panel needed to match the existing floor height
  • Removal and disposal of the old flooring and the old subfloor

What a written quote should spell out

  • Area of subfloor replaced, with the panel type and thickness named
  • Joist repair scope, including the lumber size and the sister or blocking method
  • Water source repair listed as a separate line, with the plumber or the trade named
  • Fastener type and spacing for the new subfloor, and the leveling products used
  • Permit, inspection and any engineering review needed for structural repair

Questions to ask before you hire

  • How do you find out how far the rot goes before you start cutting?
  • What do you do about the joists if they are damaged, and do you need an engineer's review?
  • Where does the water come from, and who fixes that source before the new subfloor goes in?
  • What panel and thickness do you use, and does it match the floor height at the doorways?
  • Do you need a permit for the structural repair, and who pulls it?
  • Can you show me a subfloor repair you did on a similar floor and framing?

Red flags

  • The pro lays new plywood over rotted subfloor without cutting out the damage
  • The pro does not check the joists below and says the framing is fine
  • The pro does not find or stop the water source before the repair
  • The pro says a joist can be repaired with no engineering review on a large or load-bearing span

What you can do yourself

A homeowner can clear the room, remove loose flooring and identify the soft area by probing it, and can call the plumber about any leak they find. Cutting into a subfloor with unknown framing, sistering joists, and any repair in a bathroom, a load-bearing area or a crawl space should be done by a licensed carpenter or contractor, because a mistake in the framing can make the floor fail under load.

Licenses and permits

Subfloor repair that touches joists or structural framing often needs a building permit, and the work may require an engineer's review for larger spans. Check with your state or local licensing agency and your local building department before any joist repair.

Subfloor repair questions

Can I just screw down a soft floor?

Screwing down a soft floor can hide the problem for a while, but the rot or the broken joist keeps growing. Ask the pro to find the cause and repair the damaged section before the floor is fastened.

How do I know if the joists are damaged?

Soft wood, dark staining, sagging and visible splits are the common signs. A professional should check the joists from below, since surface damage often hides deeper rot.

Will the repair need to go through the wall?

Sometimes, if the damage runs to the wall plate or the sill. Ask the pro whether the wall or the sill plate is affected and how it is repaired.

About this guide

This national guide is written and checked by infinibell. Rules and conditions differ by place, so local pros are credited on guides for the cities they serve, not on national ones.

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