What self-leveling underlayment does
Self-leveling underlayment (SLU), or self-leveling compound, is a cement-based mix that flows across a floor and settles into a flat, smooth surface. It fills dips and low spots before you install tile, vinyl plank, laminate or hardwood, which need a flat base to avoid cracking, gaps and squeaks.
How much to buy
Manufacturers state the yield of a bag, the volume of mixed compound it makes, usually about 0.45 to 0.5 cubic foot per 50 lb bag.
- Volume = floor area × average thickness (in feet)
- Bags = volume × (1 + waste allowance) ÷ yield per bag, rounded up
- Coverage per bag = yield ÷ thickness
Worked example
You need to level a 200 sq ft floor with an average thickness of 1/4 inch, using bags that yield 0.45 cu ft at $45 each, plus 10% for waste.
- Volume: 200 × 0.25 ÷ 12 = 4.17 cu ft.
- With waste: 4.58 cu ft.
- Bags: 4.58 ÷ 0.45 = 10.2, so 11 bags, about $495.
- Each bag covers 21.6 sq ft at 1/4 inch.
- Total weight: 550 lb of powder to carry in.
Coverage by thickness (0.45 cu ft bag)
| Thickness | Coverage per bag |
|---|---|
| 1/8 in | 43 sq ft |
| 1/4 in | 22 sq ft |
| 3/8 in | 14 sq ft |
| 1/2 in | 11 sq ft |
| 1 in | 5.4 sq ft |
Finding the average thickness
Lay a long straight edge or a laser level across the floor and measure the gaps at many points. Use the average depth, not the deepest spot. For very uneven floors, map the low areas and calculate them separately: a deep corner can use more compound than the rest of the room.
Steps for a good pour
- Clean the floor of dust, grease, paint and loose material. Grind high spots.
- Seal gaps at walls, doorways and floor drains with foam or tape so the compound cannot escape.
- Prime the floor with the manufacturer’s primer and let it dry as directed.
- Mix precisely: measure the water for every bag, since extra water weakens the surface. Use a drill and paddle for about 2 minutes.
- Pour in ribbons and work quickly, spreading with a gauge rake. Most products stay workable for only 10 to 20 minutes.
- Use a spiked roller to release air bubbles and blend the edges between batches.
On wood subfloors, most products require metal lath and a minimum thickness. Check the manufacturer’s instructions for the subfloor, thickness limits and drying time before flooring goes down.