Why Hardwood Floor Gaps Happen | Ground Floor Supply
Why Hardwood Floor Gaps Happen
Hardwood floors don’t fail.
They move.
When moisture leaves wood, it shrinks. When moisture enters, it expands. That movement creates seasonal gaps, compression damage, cupping, and sometimes permanent separation.
Before you reach for filler, you need to understand why the gap exists.
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Wood Moves Across the Grain
Wood movement is dramatically greater side-to-side than along its length.
That’s why:
• Long side gaps are common
• Large butt-end gaps often signal board drift
• Cupping happens during high moisture
• Some gaps never fully close again
When boards swell, they push into each other. That pressure can permanently compress wood fibers — like a hammer dent. When the wood dries again, it may not return fully to its original width.
Small seasonal gaps are normal. Many homeowners even consider them part of the character of a real wood floor.
Loss of Moisture: The Most Common Cause
In winter, especially in Minnesota:
• Heat runs constantly
• Indoor air dries out
• Relative humidity drops
• Floors shrink
In summer, humidity rises and floors expand again.
If humidity swings are moderate, the gaps close.
If swings are extreme, gaps can widen permanently.
This is not product failure.
It’s physics.
Installation: Where Problems Often Begin
Many gap problems start before the first board is fastened.
Flooring must acclimate to:
• The subfloor
• The home’s living conditions
• The regional climate
New construction is notorious for:
• HVAC not running yet
• Radiant slabs still drying
• Subfloors rained on before roofing
• High humidity from curing paint and drywall
There is no universal acclimation timeline. It depends on:
• Starting moisture content
• Species
• Airflow
• Jobsite conditions
If moisture conditions aren’t stabilized before installation, movement problems are almost guaranteed.
Moisture Barriers: Barrier vs Blocker
Important distinction:
A moisture barrier slows exchange.
A moisture blocker stops it.
Over crawl spaces or uncontrolled environments, a barrier slows seasonal shifts and gives the wood time to equalize.
On concrete slabs, proper liquid moisture blocking systems are required to prevent excessive vapor transmission.
Incorrect moisture management is a major cause of abnormal gapping.
Species and Cut Matter
Not all wood behaves the same.
Quarter-sawn lumber moves less than flat-sawn.
Douglas fir behaves differently than white pine.
Wide plank moves more than narrow strip.
Some historic floors show tight stability after 100 years.
Others develop pronounced seasonal gaps that become part of their visual identity.
Understanding species and cut helps determine whether a gap is normal or excessive.
Modern HVAC and Unrealistic Expectations
Modern homes are tightly wrapped and climate-controlled. But extreme winter dryness can still drive significant shrinkage.
Expecting perfectly stable 35–50% relative humidity year-round is not always realistic.
If humidity control isn’t consistent, movement will occur.
When Gaps Are Normal
Do not panic if:
• Gaps appear in winter and close in summer
• The floor is wide plank
• The boards are otherwise flat and secure
• There is no structural movement
Seasonal movement is part of hardwood flooring.
Before choosing a filler, make sure you understand which system fits your floor.
👉 Gap Filler Selection Guide
https://www.groundfloorsupply.com/pages/hardwood-floor-gap-filler-guide
When Gaps May Need Repair
Gap repair makes sense when:
• Gaps remain open year-round
• Boards are permanently separated
• You are sanding and refinishing
• Individual boards are drifting
Before choosing a filler, determine whether the wood is stable or unstable.
That decision determines everything.
Next Step: Choosing the Right Filler
Not all gap fillers are the same. Some are designed for stable wood. Others remain flexible for older, moving floors.
If you’re ready to evaluate repair options, start here:
👉 Gap Filler Selection Guide
https://www.groundfloorsupply.com/pages/hardwood-floor-gap-filler-guide
Hardwood floors are living materials.
The goal isn’t to stop movement.
The goal is to understand it — and work with it