How to Level an Uneven Subfloor
A level subfloor is crucial for flooring success. Even small variations cause planks to rock, tiles to crack, and installations to fail. Learn professional techniques for grinding high spots and filling low spots with self-leveling compound.
Quick Summary
Time needed
4-6 hours + cure time
Cost
$100-250 (compound + tools)
Difficulty
Medium
Step-by-Step Leveling Process
1. Map Out High and Low Spots
Use a 6-foot or longer level to find variations across the entire floor. Mark problem areas clearly.
- • Place level in multiple directions across room
- • Shine flashlight under level to see gaps (low spots)
- • Mark high spots with red tape, low spots with blue
- • Measure gap sizes to estimate compound needed
- • Most flooring needs within 3/16" over 10 feet
2. Grind Down High Spots
High spots must be sanded or ground flush. This is dusty work - wear a mask and seal off the room.
Methods:
Belt Sander (small areas):
Use 36-grit belt. Move continuously to avoid gouging. Check progress frequently with level.
Floor Grinder (large areas):
Rent for $70/day. Much faster than sanding. Includes dust collection. Worth it for big jobs.
Safety: This creates massive dust. Wear N95 mask, safety glasses, ear protection. Seal doorways with plastic.
3. Clean and Prime the Floor
Self-leveling compound won't stick to dusty or oily surfaces. Thorough cleaning and priming is essential.
- 1. Vacuum entire floor thoroughly
- 2. Wipe down with damp cloth to remove fine dust
- 3. Let dry completely (very important!)
- 4. Apply primer per manufacturer directions
- 5. Let primer dry per package (usually 1-4 hours)
4. Mix and Pour Self-Leveling Compound
This is time-sensitive - compound starts setting in 15-20 minutes. Work quickly and have help ready.
Application Process:
- 1. Mix compound with water per package (use drill mixer)
- 2. Mix to pancake batter consistency - no lumps
- 3. Pour into low spots starting at far corner
- 4. Spread gently with trowel or push broom
- 5. Compound will self-level in 10-15 minutes
- 6. Don't walk on it for 24 hours minimum
- 7. Wait 48 hours before installing flooring
- 8. Check final level after curing
Pro Tip: Work in sections if area is large. One 50 lb bag covers about 50 sq ft at 1/4 inch thickness. Calculate needs carefully and have extra bags on hand.
Common Leveling Mistakes to Avoid
Skipping primer: Compound won't bond and will crack or pop loose. Always prime.
Installing flooring too soon: Compound needs 48 hours to cure fully. Installing early causes cracking.
Mixing too thick: Thick compound doesn't self-level properly. Follow package water ratios exactly.
Frequently Asked Questions
How level does subfloor need to be?
Most flooring requires within 3/16 inch variation over 10 feet, or 1/8 inch over 6 feet. Check your specific flooring manufacturer specs. Luxury vinyl is more forgiving, while hardwood and tile are stricter. Use a long level to check - gaps under the level indicate low spots.
What is self-leveling compound and how does it work?
Self-leveling compound is a cement-based product that flows like pancake batter and seeks its own level by gravity. You mix with water, pour into low spots, and it spreads and levels itself within 15 minutes. Cures in 24-48 hours. Brands like Henry or LevelQuik work well for subfloors. Must prime the surface first for proper adhesion.
Ready for the Next Step?
With your subfloor level and smooth, it's time to test for moisture issues that could damage new flooring.