How to Inspect Your Subfloor Thoroughly
This is the most important step in flooring replacement. A thorough subfloor inspection reveals hidden problems that will ruin your beautiful new floors if not addressed. Spend time on this - catching issues now saves thousands later.
Quick Summary
Time needed
1-2 hours
Cost
$30-80 (level, moisture meter)
Difficulty
Easy
Why Subfloor Inspection is Critical
Your new flooring is only as good as what's underneath it. A damaged, uneven, or moisture-compromised subfloor will cause new flooring to fail prematurely - cracking, separating, squeaking, or warping within months.
Common Mistake: Rushing past inspection to start installing new flooring. Hidden subfloor problems always reveal themselves later - usually after you've spent money on materials and labor. Two hours of inspection can prevent a $5,000 mistake.
Think of the subfloor as the foundation. You wouldn't build a house on a cracked foundation. Same principle applies here.
Complete Subfloor Inspection Checklist
1. Check for Water Damage and Rot
Water damage is the number one subfloor killer. It spreads, causes mold, and compromises structural integrity.
What to Look For:
- •Dark stains: Brown or black discoloration indicates past or current water exposure
- •Soft spots: Push on suspicious areas - rotted wood feels spongy, not firm
- •Musty odor: Indicates mold or mildew beneath surface
- •Warped or cupped boards: Water causes wood to swell and deform
- •Visible mold: Black, green, or white growth on wood
Action Required: Any water damage must be investigated and repaired. Find and fix the source (leaky pipe, roof leak, etc.) before replacing subfloor sections.
2. Test for Squeaks and Movement
Squeaky subfloors indicate loose boards. This will telegraph through new flooring and drive you crazy.
Test Method:
- 1. Walk slowly across entire floor in grid pattern
- 2. Listen for squeaks and feel for movement under feet
- 3. Mark squeaky areas with painter's tape
- 4. Bounce lightly in one spot - excessive movement is bad
- 5. Check areas near walls, under where heavy furniture was
Fix: Drive 3-inch screws through subfloor into joists below. Locate joists (usually every 16 inches) and screw down loose sections. This stops squeaks and stabilizes the floor.
3. Check Levelness Across the Room
Flooring manufacturers specify maximum allowable variation. Most require within 3/16 inch over 10 feet.
How to Check Level:
- 1. Use a 6-foot or longer level (8-10 foot is better)
- 2. Place level in multiple locations and directions
- 3. Check for gaps under level - measure with measuring tape
- 4. Shine flashlight under level to see gaps clearly
- 5. Mark high spots and low spots with different colored tape
High Spots:
Sand or grind down with belt sander
Low Spots:
Fill with self-leveling compound
4. Inspect for Structural Damage
Look beyond surface issues to underlying structural problems that need professional attention.
Red Flags That Need a Pro:
- ⚠Sagging or sloping floor (more than 1 inch across room)
- ⚠Visible cracks in concrete slab
- ⚠Extensive rot affecting large areas
- ⚠Subfloor pulling away from joists
- ⚠Active pest infestation (termites, carpenter ants)
5. Look for Gaps, Holes, and Damage
Small defects add up and must be addressed for proper flooring support.
Common Issues to Fix:
- • Gaps between subfloor sheets (fill with construction adhesive)
- • Holes from removed fasteners (fill with wood filler)
- • Gouges or deep scratches (patch and level)
- • Protruding nails or staples (remove or pound flat)
- • Separated seams at sheet edges (screw down and fill)
Document Your Findings
Create a written record of issues found and repairs needed. This helps you track progress and estimate repair costs.
What to Document:
- • Take photos of all problem areas
- • Sketch a floor plan marking issue locations
- • List repairs needed with estimated materials
- • Note any areas needing professional help
- • Measure square footage of areas needing leveling compound
Pro Inspection Tips
Best Lighting for Inspection: Use a bright flashlight or work light at a low angle across the floor. This makes high spots, low spots, and imperfections dramatically more visible than overhead lighting.
Get in the Crawlspace or Basement: If accessible, inspect joists from below. Look for moisture, sagging joists, damaged supports, or pest damage. Issues are often more visible from underneath.
Use a Moisture Meter: $30-50 tool that detects hidden moisture in wood. Essential if you suspect water damage. Wood should read 10-13% moisture. Higher means drying or moisture source issues.
Frequently Asked Questions
What are signs of subfloor water damage?
Dark staining, soft/spongy spots when you push on them, musty odors, visible mold or mildew, warped or cupped boards, and white salt-like deposits (efflorescence on concrete). Water damage must be repaired before installing new flooring or it will continue to spread and damage your new floor.
How level does a subfloor need to be for new flooring?
Most flooring requires the subfloor to be within 3/16 inch over 10 feet, or 1/8 inch over 6 feet. Check your specific flooring manufacturer requirements. Use a long level or straight edge to check. High spots can be sanded, low spots filled with self-leveling compound.
Can I install flooring over a squeaky subfloor?
No, fix squeaks first. Squeaks indicate loose boards that will continue making noise and may cause new flooring to separate or crack. Drive screws through subfloor into joists to secure loose areas. This takes an hour but prevents ongoing problems.
What if I find major subfloor damage?
Address it before proceeding with flooring. Rotted sections must be cut out and replaced with new plywood. Severe structural issues may require hiring a contractor. Installing new flooring over damaged subfloor wastes money - the flooring will fail quickly and need replacement again.
Ready for the Next Step?
With your inspection complete and issues documented, you can now repair any damaged areas of the subfloor.