Gaps between hardwood planks are usually caused by low humidity or improper acclimation. Most gaps can be fixed without replacing the floor now please.
LVP and hardwood both gap. Identify the cause before repairing, because the fix depends on it.
Why gaps occur
Hardwood is hygroscopic. It absorbs and releases moisture with changes in humidity. In winter, dry air causes the wood to shrink and gaps appear. In summer, humidity swells the wood and gaps close. This seasonal expansion and contraction is normal.
Excessive gaps indicate a problem. Causes include low indoor humidity, improper acclimation before installation, incorrect moisture content at installation, and subfloor moisture. Gaps wider than 1/8 inch (3 mm) are considered excessive.
Seasonal gaps that close in summer are normal. Gaps that persist year-round indicate a problem that needs repair.
Causes of excessive gaps
Low indoor humidity is the most common cause. Heating in winter dries the air below 30 percent, causing the wood to shrink. Improper acclimation means the wood was installed before it reached equilibrium with the room. Incorrect moisture content means the wood was too dry or too wet at installation.
Subfloor moisture can cause the wood to swell and then shrink as it dries. This creates gaps that persist. A subfloor with high moisture content will cause ongoing problems.
Each cause requires a different fix. Low humidity needs a humidifier. Improper acclimation cannot be fixed after installation. Subfloor moisture needs remediation.
Diagnosis procedure
Follow this sequence:
- Measure the gap width with a feeler gauge or ruler.
- Check indoor humidity with a hygrometer.
- Test the moisture content of the floor and subfloor.
- Check the installation history and acclimation period.
- Match the symptom to a cause.
Most gap problems have a single primary cause. Address that cause before repairing the gaps.
Repair: seasonal gaps
Seasonal gaps that close in summer do not need repair. Maintain indoor humidity between 35 and 55 percent to minimize seasonal movement. Use a humidifier in winter and a dehumidifier in summer.
If the gaps are unsightly, a colour-matched wood filler can be used. The filler should be flexible enough to accommodate seasonal movement. Rigid fillers crack and fall out.
Repair: persistent gaps
Persistent gaps require a more permanent repair. Options include:
- Wood filler: colour-matched filler pressed into the gap. Suitable for small gaps up to 3 mm.
- Wood sliver: a thin strip of matching wood glued into the gap. Suitable for larger gaps.
- Rope or jute: traditional filler for wide gaps in old floors. Compressible and durable.
- Re-installation: in severe cases, the floor may need to be re-installed with proper acclimation.
Choose the method based on gap width and flooring type. Consult a professional for gaps wider than 6 mm.
Repair versus replacement
| Condition | Repair | Replace |
|---|---|---|
| Seasonal gaps, sound floor | No repair needed | No |
| Small persistent gaps | Filler or sliver | No |
| Large persistent gaps | Sliver or re-install | Maybe |
| Subfloor moisture damage | Fix subfloor first | Then replace flooring |
| Improper acclimation | Filler only | Re-install if severe |
Repair costs
| Repair type | Cost range (USD) | Time |
|---|---|---|
| Wood filler | 10 to 25 | 1 to 2 hours |
| Wood sliver | 20 to 50 | 2 to 4 hours |
| Re-installation | 3 to 8 per sq ft | 2 to 5 days |
| Professional assessment | 75 to 150 | 1 hour |
Prevention
Prevent gaps by controlling humidity, acclimating the flooring properly, and testing subfloor moisture before installation. NWFA, RFCI, and TCNA all address acclimation. Maintain indoor humidity between 35 and 55 percent. Acclimate solid hardwood for 5 to 7 days and engineered wood for 2 to 3 days.
Test subfloor moisture before installation. Concrete slabs should be below 3 lbs per 1000 sq ft per 24 hours. Wood subfloors should be within 2 to 4 percent of the flooring moisture content.
When to call a professional
Call a professional when the gaps are large, when the subfloor is damaged, or when the cause is unclear. A professional can assess whether the floor can be saved or needs replacement.
Structural issues, such as joist damage or subfloor rot, require a contractor. Do not attempt structural repairs without proper assessment.
Case study: winter gaps
A homeowner notices gaps in January. Humidity is 25 percent. The gaps close in May when humidity rises. A humidifier maintains 40 percent humidity and the gaps stay closed.
Tools and materials
Gap repair requires a hygrometer, moisture meter, wood filler, putty knife, and colour-matched filler. A basic kit costs 20 to 40 USD. A moisture meter costs 30 to 80 USD.
Seasonal movement
Hardwood moves 1/8 inch per 3 feet of width for every 4 percent humidity change. A room 10 feet wide can move 3/8 inch total. This is normal and expected.
Engineered wood moves less than solid wood. Its cross-ply construction resists dimensional change. Engineered wood is more stable in variable humidity.
Acclimation importance
Acclimation allows the wood to reach equilibrium with the room. Solid hardwood needs 5 to 7 days. Engineered wood needs 2 to 3 days. Skipping acclimation causes gaps within months.
Acclimation is not optional. It is a requirement for a stable installation. NWFA, RFCI, and TCNA all specify acclimation periods.
Subfloor testing
Test subfloor moisture before installation. Concrete slabs should be below 3 lbs per 1000 sq ft per 24 hours. Wood subfloors should be within 2 to 4 percent of the flooring moisture content.
High subfloor moisture causes the wood to swell and then shrink as it dries. This creates persistent gaps that filler cannot fix.
Conditional guidance
If the gap closes in summer, do not fill it. Filling a gap that will close causes the filler to be crushed and the floor to buckle. If the gap persists year-round, fill it with flexible filler.
If the gap is wider than 6 mm, use a wood sliver instead of filler. If the subfloor is wet, fix the moisture before repairing the floor. If the acclimation was skipped, expect ongoing movement.
If the floor is engineered, expect less movement. If the floor is solid, expect more movement. If the humidity is stable, expect minimal movement. If the humidity varies, expect seasonal gaps.
Common mistakes
Using rigid filler on a seasonal gap causes the filler to crack and fall out. Filling a gap that will close causes buckling. Skipping humidity control causes the gaps to return.
Each mistake is avoidable with proper diagnosis. Match the repair to the gap type and the cause.
Insurance and warranty
Gaps from improper installation may be covered by the installer’s warranty. Gaps from humidity changes are not covered. Document the installation and the humidity conditions.
A professional assessment can determine the cause and support a warranty claim. Keep records of the installation and the environment.
Long-term monitoring
After repair, monitor the floor for 30 days. Check that the repair holds. Re-apply filler if needed. Keep a log of observations with dates and photos.
A log helps identify patterns and supports warranty claims. Good records save time on future repairs.
Cost expectations
DIY repair costs 10 to 40 USD in parts and takes 1 to 4 hours. Professional repair costs 150 to 500 USD including parts and labour. Re-installation costs 3 to 8 USD per sq ft.
DIY repair is reasonable for small gaps. Large gaps or subfloor issues warrant professional help.
Safety
Gap repair involves sharp tools and dust. Wear safety glasses and a dust mask. Keep the work area clear of traffic until the repair is complete.
Documentation
Keep records of the repair, including photos and receipts. This documentation supports future warranty claims and helps a professional understand the history if further work is needed.
Good records also help when selling the home. Buyers appreciate documented maintenance history. Keep a maintenance log for future reference.
When to test humidity
Humidity affects hardwood floors. Low humidity causes shrinkage and gaps. High humidity causes swelling. Maintain 35 to 55 percent humidity. A hygrometer costs 10 to 30 USD.
Humidity changes can make gaps more visible by causing the wood to move. Stabilize the environment to reduce movement.
Seasonal considerations
Humidity changes with the seasons. In winter, heating dries the air and floors shrink. In summer, humidity rises and floors swell. Maintaining indoor humidity between 35 and 55 percent year-round prevents both.
A humidifier in winter and a dehumidifier in summer stabilize the environment. This is especially important for solid hardwood.
When to call a professional
Call a professional when the gaps are large, when the subfloor is damaged, or when the cause is unclear. A professional can assess whether the floor can be saved or needs replacement.
Structural issues, such as joist damage or subfloor rot, require a contractor. Do not attempt structural repairs without proper assessment.
Long-term monitoring
After repair, monitor the floor for 30 days. Check that the repair holds. Re-apply filler if needed. Keep a log of observations with dates and photos.
A log helps identify patterns and supports warranty claims. Good records save time on future repairs. Update the log regularly and consistently.
Documentation
Keep records of the repair, including photos and receipts. This documentation supports future warranty claims and helps a professional understand the history if further work is needed.
Good records also help when selling the home. Buyers appreciate documented maintenance history. Keep a maintenance log for future reference.
Common mistakes
Using rigid filler on a seasonal gap causes the filler to crack and fall out. Filling a gap that will close causes buckling. Skipping humidity control causes the gaps to return.
Each mistake is avoidable with proper diagnosis. Match the repair to the gap type and the cause. Keep a maintenance log for future reference.
Cost expectations
DIY repair costs 10 to 40 USD in parts and takes 1 to 4 hours. Professional repair costs 150 to 500 USD including parts and labour. Re-installation costs 3 to 8 USD per sq ft.
DIY repair is reasonable for small gaps. Large gaps or subfloor issues warrant professional help. Keep records of all repairs.
Safety
Gap repair involves sharp tools and dust. Wear safety glasses and a dust mask. Keep the work area clear of traffic until the repair is complete.
Dispose of waste properly. Follow local regulations for disposal of flooring materials.
Insurance and warranty
Gaps from improper installation may be covered by the installer’s warranty. Gaps from humidity changes are not covered. Document the installation and the humidity conditions.
A professional assessment can determine the cause and support a warranty claim. Keep records of the installation and the environment.
Summary
Gaps between hardwood planks are usually caused by low humidity or improper acclimation. Most gaps can be fixed without replacing the floor. Control humidity, acclimate properly, and use the right filler for persistent gaps. For general problems, see flooring problems: causes, diagnosis, and repair. For moisture damage, see how to repair water-damaged wood flooring.
