Why has my wooden floor cupped or crowned?

A timber board absorbs moisture and swells, and it swells most on the face that is wettest. That single fact turns the shape of your floor into a diagnostic instrument pointing at where the water is.

What you'll need

• A straight edge or a steel rule
• A moisture meter
• A hygrometer
• A torch and access to the void, if there is one

Step by step

1. Identify the shape.Lay a short straight edge across a board. Edges high and centre low is cupping. Centre high and edges low is crowning. Both are moisture differentials.
2. For cupping, look below.The underside is wetter than the top. Check for a damp void, a missing damp proof membrane, a leaking pipe, a wet slab, or a bathroom above with a slow leak.
3. Check the ventilation of a suspended floor.Airbricks blocked by soil, plants, decking or insulation are one of the commonest causes. A suspended floor needs cross-ventilation to stay dry.
4. For crowning, look above.A recent spill, a wet cleaning regime, or a floor that was sanded flat while still cupped and then dried out. The last of these is very common and is self-inflicted.
5. Measure the moisture.A pin meter in the boards at several places, and ideally a reading at the underside if you can reach it. The difference between top and bottom is the number that matters.
6. Measure the room humidity.40 to 60 percent relative humidity is the normal target for a timber floor. Very dry heated air in winter shrinks boards; very humid summer air swells them.
7. Fix the source, then wait.Boards equalise slowly — weeks or months rather than days. Most mild cupping recovers substantially on its own once the moisture difference goes.
8. Do not sand until it is flat.This is the critical rule. Sanding a cupped floor removes the raised edges, so when the board dries and flattens, the edges are now too thin and the floor crowns permanently.
9. Help it dry gently.Ventilation, moderate heat and a dehumidifier. Aggressive drying with fan heaters causes splitting and end checking.
10. Assess what is left after equalising.Boards that are still cupped after several months of stable conditions have taken a permanent set and will need sanding or replacing.
11. Check the fixings.Severe cupping can pull nails and split boards along the fixing line. Those boards may need replacing whatever the floor does.

Tips

• Take a photograph with a straight edge across the boards and a note of the date. Comparing it three months later tells you whether it is recovering.
• Clear the airbricks before anything else if you have a suspended floor. It is free, it takes twenty minutes and it is the cause remarkably often.
• Never wet-mop a solid wood floor. Damp-mop with the cloth wrung nearly dry, because standing water at the joints is a direct route into the core.

Common mistakes

• Sanding a cupped floor flat — The board is temporarily deformed. Sanding removes material from the high edges while the board is in that state, so once it equalises and flattens, the edges are thinner than the centre and the floor is permanently crowned — with less thickness left to correct it.
• Drying the floor out fast with heaters — Rapid drying dries the top face far faster than the core, which produces surface checking, splits and end grain damage. Gentle, sustained ventilation and dehumidification is what timber needs.

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