How do I repair a chipboard floor that swelled after a leak?
Why swollen chipboard cannot be dried and reused, how much has to come out, and how to support and replace the section so the repair is as strong as the original deck.
- Difficulty
- intermediate
- Time
- 2 hr
- Read
- 3 min
- Safety
- warning
Short answer
It cannot be saved. Chipboard swells because the wood particles absorb water and the resin bond breaks, and drying it leaves a weakened, crumbly board at its swollen thickness. Cut the affected area back to sound material over joists, add noggins or battens to support the new edges, and fit moisture-resistant flooring-grade board.
Do this first
The tempting move is to dry the floor out and sand the swollen joint lines flat. It looks like it works and it does not: the swelling is delamination inside the board, the strength has gone, and the sanded-flat area is now thinner as well as weaker. Anywhere it carries a load, it will fail again.
Safety
Swollen chipboard has lost strength permanently and it does not come back as it dries. Do not put a bath, a washing machine or heavy furniture back onto a swollen deck, and do not simply cover it. Water that reached the deck also reached the joists and often the ceiling below, so check both before rebuilding, and confirm the leak is genuinely fixed first.
What you'll need
- a cable and pipe detector
- a circular saw or multi-tool with depth adjustment
- moisture-resistant flooring-grade chipboard or plywood
- timber for noggins and battens
- coarse-thread flooring screws
- a moisture meter (optional)
Step by step
- Confirm the leak is fixed and the void is dry.Rebuilding onto wet joists just relocates the problem. Get the source repaired, then dry the area properly — a dehumidifier and airflow in the room, and airflow into the void if you can.
- Find the full extent.Press across the area with a thumb and probe with a screwdriver. Swelling spreads further than the visible bulge, usually following the sheet joints where the water sat. Mark the boundary generously.
- Check the joists and the ceiling below.Probe the joist tops under the wet area. If the ceiling below is stained or soft, that has to be dealt with too, and it is easier while the floor is open.
- Plan the cut lines onto joist centres.Cut back to the centreline of a joist where you can, so the old and new decks share a bearing. Where a cut has to fall in a bay, plan for a noggin or a batten.
- Scan and cut with the depth set to the deck thickness.Downlighter cables and pipes lie directly under an upstairs deck. Set the depth precisely to the board thickness — this is a common place for a saw to find a cable.
- Lift the swollen material out and clean the bearings.Scrape the joist tops clean of old adhesive and swollen crumbs so the new board sits flat, and pull or punch down any remaining fixings.
- Add support to every unsupported new edge.Noggins between joists under any cut line that does not fall on a joist, screwed through the joists at each end, top face flush.
- Fit the replacement in the right material.Moisture-resistant flooring-grade chipboard, or plywood of matching thickness. Check the thickness matches the existing deck exactly, or the new panel sits proud or low and telegraphs through the covering.
- Glue and screw it down.A bead of flooring adhesive on every joist top and noggin, then coarse-thread flooring screws at the manufacturer's spacing. This is what the original deck should have had and it is what makes the repair silent.
- Check the level across the joint before covering.A long straight edge across the repair. Any step at the joint will show through vinyl, will rock a floating floor, and will wear a carpet at that line.
Common mistakes
- Sanding the swollen joints flat and laying the floor back — The board is weaker as well as thinner at that point. Under a bath or a washing machine it is a failure waiting to happen, and under a floating floor it creaks.
- Patching with ordinary chipboard from a sheet rack — Flooring chipboard is a specific structural, moisture-resistant grade with a machined edge. General-purpose chipboard has neither the strength nor the moisture resistance and will swell again from ordinary humidity.
- Rebuilding before the joists are dry — Sealing damp timber under a new deck and a new covering is how a repaired leak becomes a rot problem two years later.