Gaps between hardwood boards in winter are almost always caused by dry indoor air. Wood gives up moisture as the heat runs and shrinks slightly across its width, which opens the seams. Those seams usually close again in humid months, and the humidity inside your house is the main lever that controls how much they move.
What is happening inside the boards
Wood is hygroscopic, which means it trades moisture with the air until the two reach a balance. Each board barely changes along its length, but it changes noticeably across its width. When relative humidity drops, a board sheds moisture and narrows by a small amount. Repeat that across a whole floor and seams that were tight in July can show thin lines by February, with nothing wrong with the boards or the installation.
Some floors show it more than others. Wide planks gain more movement in total. Flat-sawn boards, the ones with the cathedral-shaped grain, usually move more across their width than quarter-sawn boards do. Glossy finishes also make seams easier to spot than matte ones.
Why Maryland winters do this
Cold outside air holds very little moisture. When the furnace warms it, relative humidity falls, sometimes far below what wood prefers, because nothing in the house is adding moisture back. Maryland's heating season runs from late fall into spring, so the dry air isn't a short spell. It lasts for months.
Forced-air heating is the usual culprit, since it moves a lot of air and most homes don't add moisture back unless they have a humidifier. Before the heat comes on each year, it pays to check the humidifier and the rest of the winter setup. The guide to fall cleaning before the heat comes on covers that prep.
The humidity range to aim for
Most flooring makers' care guides point to roughly 35 to 55 percent relative humidity, and that's the target for the whole year. Some guides run a little narrower, so check your floor's care guide if you have the paperwork. Keeping the house in that band does more for a hardwood floor than most other steps.
Measure before you adjust. A basic digital hygrometer does the job. Place it in the main living space, away from the furnace, the kitchen, and exterior doors, and read it at the same time each morning for a week. If winter readings sit well below 35 percent, add moisture. A whole-house humidifier tied to the furnace gives the most even result, and a portable unit can serve one room if you keep it filled and cleaned. Aim for the middle of the range rather than the top, because more moisture isn't always better. Without a meter, watch for static shocks, wood furniture that starts to crack or look dry, and window glass that never fogs in winter. Those signs don't give you a number, but they tell you the air needs attention.
Summer flips the problem. Readings above about 55 percent let moisture soak into the boards, which swells them and can start cupping. Run the air conditioner, and use a dehumidifier in a humid basement or main room if the reading stays high. Condensation on the inside of windows is another sign the house holds too much moisture. For more on that side of the year, see summer humidity and musty smells in Maryland homes.
Normal gaps and gaps to worry about
A seam that opens in February and closes by July is working as designed. To size a gap, slide the edge of a dime or a credit card into it. Gaps that never close, that widen each winter, or that come with other symptoms are different. Use this table to sort out what you're seeing.
| What you see | Likely cause | What to do |
|---|---|---|
| Thin seams in winter that close in summer | Normal seasonal movement | Keep humidity in range and leave the floor alone |
| Gaps that stay open all year | Long dry spells, or a floor installed before it acclimated | Log humidity for a few months, then have a pro look |
| Boards with raised edges beside the gaps | Cupping, often from moisture below | Check for a moisture source, then have it assessed |
| Squeaks or boards that feel loose | Fasteners or subfloor movement | Have the subfloor and fasteners checked |
| Gaps that widen along the walls | Moisture at the perimeter or subfloor movement | Call a pro before the floor shifts further |
Raised edges next to gaps usually mean cupping, which has its own causes and timing. Cupping and crowning in hardwood floors explains how to tell them apart.
What not to do about the gaps
Fixing gaps the wrong way can cause more trouble than the gaps themselves.
- Don't fill them with wood filler or caulk. Filler pops out when the boards expand, and caulk stays rigid while the wood keeps moving, so both tend to crack or look worse.
- Don't wet-mop to close them. Water swells the board edges, which can raise them and make the floor worse.
- Don't sand to close them. Sanding can't close a shrinkage gap, and it thins the boards without touching the cause.
- Don't ignore gaps that come with lifting, cupping, or dark stains. Those point to moisture, and moisture needs attention.
When to call a pro
Seasonal gaps are mostly a humidity problem, and you can manage that at home. Bring in a pro when gaps stay wide all year, when they come with cupping or squeaks, or when you've corrected the humidity and nothing has changed. The cause often sits in the moisture or the subfloor rather than the boards on top, which is why Royal Carpet Cleaners looks at hardwood in person before quoting. If the gaps keep coming back, the hardwood floor cleaning page lays out what's offered, and you can book online once you've decided.
Frequently asked questions
Will the gaps close on their own in spring?
Usually, yes. As humidity rises in late spring and summer, the boards expand and many seams narrow again. If the gaps are still wide once the house has settled into summer humidity, have the floor checked.
Can a humidifier cause problems?
It can, if it pushes indoor humidity above about 55 percent. Excess moisture can swell boards and cup them, and it often shows up as condensation on windows. A hygrometer makes it easy to see where you stand.
Do engineered floors gap the same way?
They usually move less than solid boards because the layered core resists swelling and shrinking, but they still respond to dry air. Very dry conditions can stress the top layer, so the same humidity target applies.
Written by
The Royal Carpet Cleaners Team
Royal Carpet Cleaners is a family-owned, IICRC-certified company based in Bel Air, Maryland. Our technicians clean carpet, rugs, upholstery, tile, and hardwood across Harford, Baltimore, Howard, and Cecil counties.
