Cupping and Peaking in Timber Floors

Two opposite board distortions, both driven by moisture. One is a dip, the other is a ridge. Knowing which is which -- and why -- determines whether you sand now or wait.

What cupping looks like

Run a straightedge across the boards perpendicular to the grain. If each board has its edges sitting higher than its centre, that is cupping. The face of every board is concave, like a shallow trough. It is most visible under raking light from a window or a torch held low along the floor surface. On a newly laid floor it can be subtle -- a few tenths of a millimetre -- but it gets worse if the moisture source is not addressed.

Cupping is always a sign that the bottom of the board is wetter than the top. The wet side expands more than the dry side, and the board curls upward at the edges. Common causes:

• Subfloor moisture. Concrete slabs without an adequate moisture barrier, leaking pipes under a raised floor, or poor drainage pushing water under stumps.
• Fresh installation. Boards laid over a slab before the slab has dried to specification, or boards that were not acclimatised to the site before installation.
• Seasonal humidity. A floor in a coastal property with no climate control can cup during extended wet weather and flatten again during dry spells.

What peaking looks like

Peaking (sometimes called tenting or ridging) is the opposite shape. The centre of each board sits higher than the edges. If two boards push up together along a join line, that ridge is peaking. It happens when boards expand sideways and have nowhere to go. The expansion pressure pushes the boards upward.

Peaking is caused by:

• Insufficient expansion gaps. If the floor was installed tight against walls, door frames, or fixed cabinetry, there is no room for seasonal expansion.
• Water damage. A flood, burst pipe, or persistent spill that saturates the timber. The boards swell rapidly and buckle upward.
• Over-humidification. Leaving humidifiers running for extended periods in a sealed room can push the moisture content high enough to cause peaking even with adequate gaps.

When to sand and when to wait

This is where most costly mistakes happen. A cupped floor looks bad, and the instinct is to sand it flat immediately. Do not do it. If the boards are still cupped because of an active moisture imbalance, sanding the high edges off removes timber that is needed when the boards eventually flatten. Once the moisture equalises, those sanded boards will crown -- the centre will be higher than the edges, creating the opposite problem with less timber to fix it.

The correct sequence:

• Find and fix the moisture source. Repair the leak, install a moisture barrier, improve ventilation under a raised floor, or get the slab to specification.
• Monitor with a moisture meter. Take readings from the centre and edge of multiple boards every week. Record them. When the difference between top-of-board and bottom-of-board readings drops below 1% MC, the floor is approaching equilibrium.
• Wait for the boards to flatten. Depending on the severity, this can take weeks or months. There is no shortcut. Fans and dehumidifiers speed it up, but the floor sets the timeline.
• Sand only when flat. Once the boards have returned to a flat profile and the MC readings are stable across multiple checks, proceed with the normal grit sequence.

For peaking, the approach is similar: relieve the expansion pressure (cut in expansion gaps at walls if missing, remove the water source), let the boards settle, then sand. If peaking was caused by flood damage and the boards have delaminated or lost structural integrity, replacement is the only real option.

Moisture meter readings: what to target

For most Australian hardwoods in a climate-controlled interior, target an equilibrium moisture content of 9-14% MC depending on species and region. The critical number is not the absolute reading but the difference between the top and bottom of the board, and between adjacent boards. A floor where every board reads 12% is stable. A floor where some boards read 9% and others read 15% is still moving.

Use a pin-type meter for accuracy on hardwoods. Drive the pins in at a slight angle to get a reading from the middle third of the board thickness, not just the surface. Surface readings on their own can be misleading -- a board can feel dry on top and still be holding 18% MC at the core.

For more detail on moisture content targets and concrete slab testing, see the full moisture content guide.

Prevention

• Acclimatise boards before installation. Stack them in the room they are going into, off the floor, with spacers for airflow, for at least two weeks. Longer in humid conditions.
• Leave expansion gaps. 10-15 mm at all fixed edges. Scotia or skirting covers the gap. Never glue or nail tight against a wall.
• Test the slab. Relative humidity testing (in-slab probes) should read below 75% RH before any timber goes down. Calcium chloride surface tests are a secondary check, not a replacement for in-slab probes.
• Seal the system properly. After sanding, prime with Bona Prime Intense or Classic UX depending on species, then topcoat. A sealed floor resists moisture exchange from above much better than a raw one.

Related: Troubleshooting FAQs →