How do I fix a cupped hardwood floor in my New Brunswick home?

Cupped hardwood floors — where plank edges are higher than the centres, creating a washboard appearance — are almost always caused by excess moisture from below the floor, and the fix must address the moisture source before addressing the wood itself. In New Brunswick's Maritime climate, cupping is one of the most common hardwood floor problems, particularly during summer humidity spikes and in homes with inadequate moisture management.

Before doing anything to the wood, identify and eliminate the moisture source. The most common causes of cupping in NB homes include: summer humidity pushing indoor levels above 60-65% without AC or dehumidification; plumbing leaks beneath the floor (check for dripping pipes, slow supply line leaks, or condensation on cold water pipes); crawl space or basement moisture migrating upward through the subfloor; and inadequate exterior drainage directing rainwater or snowmelt toward the foundation. In coastal NB communities — Saint John, Shediac, and along the Bay of Fundy — persistently higher ambient humidity makes cupping more common and more severe.

Once you have identified the moisture source, give the floor time to recover before sanding. This is critical and the most common mistake NB homeowners make — they see cupped floors in July, panic, and have them sanded flat. But if the wood has not fully dried and returned to equilibrium, sanding removes extra material from the raised edges. When the wood eventually dries and flattens in winter, those over-sanded edges are now lower than the centres, creating crowning — the opposite problem, which is permanent and requires a full refinish to correct.

The proper sequence is: First, reduce and stabilize indoor humidity to 35-50% using a dehumidifier, air conditioning, or both. In NB, this typically means running a dehumidifier from June through September. Second, monitor the floor with a moisture metre over several weeks or months. Third, once moisture readings in the wood stabilize and match the expected equilibrium moisture content for your home (typically 7-9% in NB with proper humidity control), assess whether the cupping has resolved on its own. Mild cupping often flattens completely once the moisture imbalance is corrected.

If cupping persists after the floor has reached moisture equilibrium — which can take 1-3 months or even a full seasonal cycle in NB — then professional sanding and refinishing is appropriate. A professional will verify moisture readings before sanding to ensure the wood is stable. In New Brunswick, hardwood refinishing costs $3-$6 per square foot, with a typical 1,000-square-foot main floor running $3,000-$6,000.

For severe cupping where planks have lifted significantly or subfloor damage is involved, individual planks may need to be removed and replaced. In older NB homes (pre-1970s), the subfloor beneath the hardwood may be original board sheathing that has also absorbed moisture and warped, requiring repair before the hardwood can be addressed.

Cupping repairs should always involve a professional flooring contractor. The diagnostic process — identifying the moisture source, measuring wood moisture content, determining whether the cupping is active or has stabilized — requires experience and proper equipment. Sanding a cupped floor at the wrong time is an expensive mistake that creates a worse problem than the original cupping.

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Looking for experienced contractors? The New Brunswick Construction Network connects homeowners with qualified professionals:

• 3Tone Construction Ltd
• Gionetterenovations
• Thirty Four Renovations
• Forever Epoxy Inc
• The Garbage Guys Ltd