Atlanta humidity moves hardwood floors every year. Boards take on moisture in the damp summer and release it in the dry winter, so they swell and tighten in July and shrink and gap in January. That seasonal hardwood floor humidity cycle is normal in small amounts. It becomes a problem only when the swings are large, fast, or fed by a moisture source under the house.
Why Wood Keeps Moving After Installation
Wood is a natural material that never stops trading moisture with the air around it. It absorbs water vapor when the air is humid and releases it when the air is dry, until it matches its surroundings. A board does not stay the size it was on the day it was nailed down.
The movement is lopsided. Boards change a lot across their width and almost nothing along their length. So you see seams open and close, not boards getting longer or shorter.
Width matters too. A wide plank has more wood across its face, so it moves more per board than a narrow strip does. Add that up across a whole room and a wide-plank floor shows larger seasonal gaps.
In most homes, a comfortable indoor range of about 30% to 50% relative humidity keeps solid hardwood at a moisture content of roughly 6% to 9%. The floor is happiest inside that band. Our job is mostly about keeping it there.
Summer: Swelling, Tight Seams and Cupping
North Atlanta summers are sticky. Outdoor humidity often sits at 70% or higher for days. If the air conditioning is running, the house stays drier than outdoors, and the floor stays calm.
Trouble starts when indoor humidity creeps above 55% or 60%. Boards swell, edges press against each other, and seams tighten. Sometimes you hear light creaking as swollen boards rub together.
The bigger summer problem is cupping. That is when the edges of each board rise higher than the center, so the floor looks ridged under low light. It usually means one face of the board is wetter than the other.
Moisture from below is the most common cause. Think of a damp crawlspace, a basement, or a slab that is not fully dry. The underside of the board takes on water, swells, and the board curls upward at the edges.
A slight cup that shows up in August and eases by winter is usually seasonal. A pronounced cup, or one that never eases, points to a moisture source that needs to be found. Do not sand it flat yet. Our guide on when to restore hardwood floors explains why timing matters.
Winter: Gaps Between Boards
The call we get most in winter is some version of “my floor is separating.” Forced-air heat dries the house out. Indoor humidity can fall below 30%, sometimes toward 20% on the coldest weeks.
The boards shrink across their width and the seams open. On standard strip flooring, a seasonal gap is typically about the thickness of a dime or less. Wider planks can open more.
Gaps often show up first near heating vents, in sunny rooms, and by fireplaces. Those spots dry out fastest. You may also notice finish cracking along board edges or a few new squeaks.
We generally do not fill seasonal gaps with rigid filler. The boards need that room when they swell again in summer. Hard filler can get squeezed out, or it can push on the edges and damage the boards.
A better first step is raising the humidity gently. A whole-house or room humidifier that holds the air around 35% to 45% usually slows the shrinking. Go slowly. Large jumps in either direction stress the floor more than a steady condition does.
Spring and Fall: The Shoulder Seasons
These months are easy to overlook. The air conditioner and the heat both cycle on and off, windows are open, and humidity can swing 20 points in a week. Fast swings add stress on top of the average humidity level.
A slow change over several weeks lets the wood adjust evenly. A fast change makes the surface react before the core does, and that is when you see edge cupping and new gaps. Closing windows on muggy spring afternoons and running the AC or a dehumidifier helps.
Homeowners in Alpharetta ask us about this often, especially in houses with finished basements or crawlspaces under a main-level hardwood floor. Those lower spaces hold moisture longer than the living area above.
New Floors: Acclimation and Timing
Humidity matters most at installation, because the boards are installed at whatever moisture content they have that day. If they go down too wet, they shrink and gap hard that winter. If they go down very dry in a humid season, they can swell and push on each other.
That is why new boards are typically given several days to two weeks to adjust inside the house, depending on the product and the conditions. We care less about the calendar than about measurements. The boards and the subfloor should read close to each other, and the HVAC should be running at normal living conditions. Our post on how hardwood floors are installed walks through the full sequence.
Engineered boards have a layered core that resists movement better than solid planks. They still react to extreme humidity, just less. See the options for engineered hardwood floors if stability is a priority. If you are planning a new floor, our hardwood flooring installation team can talk through what suits your house.
What Goes Wrong When Moisture Gets Out of Hand
These are the failure modes we see most. Each one has a different cause, so the fix differs too.
- Cupping that does not ease: usually moisture from below, such as a damp crawlspace, bare soil with no vapor barrier, or a slow plumbing leak.
- Crowning after a refinish: the floor was sanded flat while still cupped. When it dried, the centers rose and the edges stayed low.
- Gaps that stay open after a humid season: debris or filler may be packed in the seams, or the boards may have been installed too wet.
- Buckling: boards lift off the subfloor, often after flooding or a major leak. The water source has to be fixed before anything else.
- Localized movement near one wall or appliance: usually a leak or a vent blowing moist or dry air onto one spot.
A Decision Sequence: Check This First, Then This
When a floor looks different from last season, work through these steps in order. Skipping to the floor itself is the most common mistake.
- Measure the air. Put a hygrometer in the room for a week. Note the range, not just one reading. Anything between 30% and 50% is fine.
- Look at the pattern. If the whole house moves a little, that is probably seasonal. If one room or one wall is worse, suspect a local source.
- Check below the floor. Look at the crawlspace or basement for standing water, wet insulation, or bare soil. Also check around dishwashers, water heaters and exterior doors.
- Fix the source or balance the air. Run a dehumidifier in summer, a humidifier in winter, or repair the leak. Do this before any work on the boards.
- Allow time to settle. Allow several weeks, and in some cases a full season, and confirm with moisture readings before judging the result.
- Then decide on the floor. If it is flat and only worn, a refinish may be enough. If it is damaged or still moving, have a professional look at the boards.
Step 6 is where our hardwood floor restoration and refinishing work comes in. Our overview on how to refinish hardwood floors covers what to expect once the boards are stable.
Keeping the Swing Small Year-Round
You cannot stop the seasons, but you can narrow the range. Keep the HVAC running through summer and set it so the house is never left to sit humid. Use a dehumidifier in basements and closed rooms.
In winter, avoid cranking the heat to the point where the air turns desert-dry. Keep furniture and rugs from trapping moisture against one patch of floor for months. Clean spills quickly, and use a barely damp mop, never a soaked one.
Refinishing timing matters as well. Finish dries more slowly on humid days, and a floor that is still moving will show it. Read our hardwood floor sanding preparation post for what to do before the sanders arrive.
Some seasonal movement is just part of owning real wood. The goal is a floor that moves a little and returns to its original shape, not one that stays distorted.
If your floors are gapping, cupping, or just not looking right, we are glad to take a look. Request a free estimate on our contact page or call 404-578-0926.
Frequently asked questions
Most hardwood floors do best with indoor relative humidity between about 30% and 50%. Georgia summers push the outdoor air well above that, so the air conditioner or a dehumidifier usually does the work in summer. In winter a humidifier may be needed to keep the air from dropping below 30%.
Heated indoor air is dry, and the boards give up moisture and shrink across their width. The gaps are usually the thickness of a dime or less on standard strip flooring and tend to close again when humidity returns in spring. Wider boards show larger gaps.
No. Find and fix the moisture source first, then let the floor settle for several weeks. Sanding a cupped floor flat while it is still wet can leave the board edges lower once it dries, which is called crowning.
Keep reading
- Your Hardwood Floor vs. Your Pets
- Why You Need Hardwood Floor Refinishing
- Why Hire a Professional to Refinish Your Hardwood Floor