Hardwood Floor Gaps in Winter and Cupping in Summer: What to Do

If you have hardwood floors in Ontario, you've probably noticed them change with the seasons. Thin gaps appear between boards in January, then disappear by summer. Or the edges of the boards rise up and the floor looks wavy after a humid August. Both are caused by humidity, and both can be managed.

Why Do Hardwood Floors Get Gaps in Winter?

Wood takes on and gives off moisture to match the air around it. When the furnace runs all winter, indoor humidity in Ontario homes often drops below 30%. The boards dry out and shrink, and small gaps open between them. Gaps usually peak in late winter and close again as humidity rises in spring.

Thin, even seasonal gaps in solid hardwood are normal and are not an installation defect. Don't fill them with wood filler: when the boards swell back in summer, the filler gets pushed out or causes the boards to crush against each other.

Why Are My Hardwood Floors Cupping?

Cupping is when the edges of each board sit higher than the centre. It's the opposite problem: too much moisture, usually from below. Common causes:

• High summer humidity with no air conditioning or dehumidifier.

• Moisture coming up from a basement, crawlspace or concrete slab.

• A plumbing, dishwasher or fridge leak.

• Wood installed without being acclimated or moisture-tested.

Mild cupping often flattens once the moisture source is fixed and the floor dries out. Don't sand a cupped floor too soon: if you sand it while it's still wet, the boards can "crown" (centres higher than the edges) once they dry.

What Is the Ideal Humidity for Hardwood Floors?

Most manufacturers recommend keeping indoor humidity roughly between 35% and 55% year-round. Check your flooring's warranty for its exact range. To stay in that window:

• Buy an inexpensive hygrometer to track humidity.

• Run a humidifier (or a whole-home furnace humidifier) in winter.

• Use air conditioning or a dehumidifier in humid summer months.

Is Engineered Hardwood Better for Humidity?

Yes. Engineered hardwood has a real wood top layer over a cross-layered plywood core, so it moves far less than solid wood. That's why it's become the go-to choice for GTA condos, basements, homes with radiant heating, and today's popular wide planks, which show seasonal movement more than narrow boards do.

How Does Proper Installation Prevent These Problems?

• Moisture testing of both the wood and the subfloor before installation.

• Acclimation so the wood matches your home's conditions.

• Vapour barriers over concrete and above damp crawlspaces.

• Correct expansion space around the room.

• The right product for the room: engineered, not solid, below grade or over radiant heat.

A professional hardwood flooring installation covers every one of these steps. If your floors are worn rather than damaged, floor refinishing can bring them back to life.

Hardwood Floor FAQs

When should I worry about gaps in my hardwood floor?

Call a professional if gaps are wider than a dime, stay open all summer, appear in only one area (a possible sign of a leak or subfloor issue), or if boards are cracking.

Can a cupped hardwood floor be saved?

Often, yes. Once the moisture source is fixed and the floor has dried, many cupped floors flatten out or can be sanded and refinished. Severe cases may need some boards replaced.

Need a Flooring Expert to Take a Look?

Worried about gaps or cupping in your hardwood floors? Alpine Flooring has installed and repaired floors across Toronto and the GTA for more than 20 years. We're licensed and insured, rated 4.9 on Google, and you deal directly with Sam from your first call to the final walkthrough. Request your free quote and we'll come see the floor in person, often within the same week.

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