Wood Floor Cupping Or Crowning

What's happening?

Cupping and crowning are both moisture gradients through the thickness of a board. When the underside is wetter than the top (from a subfloor that was never dry, a leak below, or high humidity in an unventilated void) the bottom swells more than the top and the edges rise, which is cupping. Crowning is the reverse: the top face has taken up more moisture than the underside, or a cupped floor was sanded flat while still wet and then dried out, leaving the middle proud. Both are symptoms of a moisture imbalance, not of poor timber, and both will keep moving until the moisture is equalised. Sanding a cupped floor before it has stabilised is the single most expensive mistake made with wood floors.

What usually causes it?

• A subfloor above the moisture limit when the floor was laid, still giving up vapour underneath
• A leak below the floor from a pipe, a washing machine or a dishwasher
• An unventilated crawl space or void with blocked airbricks holding damp air under the boards
• Boards fitted without acclimatisation, at a moisture content well away from the room's
• Water spilled and left on the surface, or repeated wet mopping
• Underfloor heating brought up too fast, drying the underside faster than the face
• A cupped floor sanded flat before it had dried and stabilised, producing crowning later

Is it dangerous?

The floor is not, but the cause can be. Cupping that appears suddenly in one area is a strong indicator of a leak underneath, and water sitting under a floor rots joists and grows mould out of sight. A raised board edge is also a trip point in a walkway.

Can I fix it myself?

Find and stop the water. Check appliances, pipe runs, the underside of the floor where you can reach it, and whether external airbricks are blocked. Then let the room stabilise: heat and ventilate normally, over weeks rather than days, and see whether the boards begin to flatten. Many cupped floors recover a long way on their own once the moisture source is gone. Do not sand, do not put a dehumidifier on a hard setting straight onto the boards, and do not screw the edges down.

When should you call a flooring fitter?

Call a flooring fitter once the source is fixed, to measure the boards and the subfloor and to tell you when (or whether) the floor is stable enough to sand and re-finish. Call a plumber first if you suspect a live leak. A fitter will also identify boards that have delaminated or split beyond recovery, which have to be replaced rather than sanded.

What will the flooring fitter actually do?

• Take moisture readings across the boards and the subfloor and compare with unaffected areas
• Identify the moisture path: subfloor, void ventilation, a leak or surface water
• Allow the floor to acclimatise and re-measure over time before any sanding is considered
• Replace boards that have split, delaminated or cupped past recovery
• Sand and re-finish only once the readings have stabilised, so the floor does not crown afterwards

Questions people ask

Sources & References

1. BS 8201: Code of practice for installation of flooring of wood and wood-based panels
2. GOV.UK, Approved Document C: Site preparation and resistance to contaminants and moisture
3. British Woodworking Federation