How much movement is normal
Wood takes on and gives off moisture with the air around it. Building scientist Joseph Lstiburek's Moisture Control Handbook, published through the U.S. Department of Energy's Building America program, puts typical indoor humidity at about 30% in winter and 65% in summer in most of the country. Over that range, wood flooring moves between roughly 6% and 12% moisture content by weight. That is the same pattern Philadelphia-area houses see, with dry heated winters and humid summers.
The handbook explains what follows from that swing. Flooring installed very dry, near 6%, can buckle when it reaches 12%. Flooring installed wet, near 12%, shrinks and leaves big gaps when it dries to 6%. That's why it recommends conditioning wood to about 9%, the middle of the range for most of the lower 48 states, and leaving gaps at the room edges. Wide planks show the movement more than narrow strips, because there are fewer joints to share it.
So thin, even gaps that show up every winter and close by late spring are normally just the floor reacting to the seasons. Gaps that keep getting wider year after year, or are much bigger in one room, deserve a closer look.
How to tell normal swelling from a moisture problem
DOE's Building America Solution Center notes that uneven wetting and drying makes wood warp. In a floor, that shows up as cupping when moisture comes from below, from a damp basement, crawlspace, slab or subfloor, or from a leak. Use the pattern to narrow it down:
- Mild, even cupping across the whole house in a humid stretch points toward indoor humidity. Check a humidity gauge before anything else.
- Cupping in one area, next to a dishwasher, refrigerator line, bathroom, radiator or exterior door, points toward a localized leak. A water-meter test for hidden leaks can tell you whether water is running when nothing is on.
- Cupping on a first floor over a damp or musty basement points toward moisture coming up from below.
Stains, a musty smell, soft spots or a known leak mean it's not a seasonal problem. EPA advises drying wet materials within 24 to 48 hours to prevent mold, so don't wait for winter to see if the floor settles down.
What to check yourself
A basic humidity gauge costs $10 to $50 at a hardware store, according to EPA. Put one in the room with the floor and, if you have one, one in the basement under it, and note the readings for a week along with photos of the same boards. EPA's general target is below 60% relative humidity, ideally 30% to 50%. DOE's cold-weather guidance puts the ideal range at 35% to 50%.
Look under sinks and around appliance connections, and check the basement ceiling below the problem area for drips, stains or sweating pipes and ducts. Don't pull up boards, drill holes or crawl into a tight, wet crawlspace to find the source.
Fixing the humidity without making new problems
If summer readings sit above 60%, run bathroom and kitchen exhaust fans, and use a dehumidifier in a damp basement. Why a house stays humid even with the AC running covers the common causes when the air conditioner isn't pulling the moisture out, and sump pumps versus dehumidifiers explains which basement problem each one solves.
In winter, don't crank up a humidifier to close every gap. DOE's guidance says ongoing condensation or frost on windows or cold walls means the humidity is too high for the house and can rot sills and frames. A little winter gapping is a better trade than wet windows.
Who to call, and what to ask
For persistent cupping or gaps, start with a wood-flooring contractor who will take moisture-meter readings, not just quote a sanding. Ask them to:
- Measure moisture content in the damaged boards, in unaffected boards elsewhere and in the subfloor, and write the numbers down.
- Say what moisture content the floor needs to reach before sanding or replacing boards, and how they'll confirm it.
- Name the cause their readings point to.
DOE's Building America guidance warns that installing finish materials over any wet substrate causes problems, so a contractor who wants to sand or refinish right away without readings is a warning sign. If the readings point to a leak, call a plumber. If they point to humidity or a sweating air handler, call an HVAC contractor. If water is coming through the foundation, call a waterproofing contractor. If boards have to be replaced, how to compare floor repair quotes when the flooring is discontinued helps you read the bids.