Quick answer
Self-leveling underlayment (SLU) is a pourable mix, usually based on cement or gypsum, that is spread over a primed concrete or approved wood subfloor and flows out under its own weight to form a flat, smooth surface ready for tile, vinyl, laminate or wood flooring.
Remodelers use it to fix dips, humps and old patches before a new floor goes down. It is a flattening layer, not a structural repair, a waterproofing layer or a moisture barrier.
Quick facts
The key facts about Self-Leveling Underlayment, with sources:
- Common binders
- Gypsum, or blends of portland and calcium aluminate cements 1
- Product standard
- ANSI A118.16 (cementitious SLU) 5
- Resilient flooring flatness (ASTM F710)
- No more than 3/16 in in 10 ft and 1/32 in in 12 in 1
- Large-format tile flatness (ANSI)
- No more than 1/8 in in 10 ft and 1/16 in in 24 in 1
- Minimum temperature (one product)
- 50°F substrate and air during install and cure 2
Key takeaways
- Self-leveling underlayment is a fluid cement or gypsum mix that flows out flat over a floor before new flooring.
- It exists because modern floors are unforgiving: resilient flooring and large tile need tight flatness.
- Primer, a clean sound substrate and the right temperature decide whether it bonds.
- Cement-based SLUs cure fast; gypsum-based ones dry slowly and stay out of wet areas.
- Every number that matters, from thickness to cure time, comes from the product’s data sheet.
Self-leveling underlayment in plain terms
Picture pouring pancake batter into a pan. It spreads until it finds its own level. Self-leveling underlayment works the same way, but it sets hard.
Installers mix the powder with water, pour it over a primed floor, and help it spread with a gauge rake and a smoother. Within hours it is a flat, hard surface.
Why bother? Because new floors show every flaw below them. A Custom Building Products white paper notes that floor flatness matters more than ever with luxury vinyl planks and large-format tiles, and that most floor covering makers require a high degree of flatness 1.
A dip under a long vinyl plank can make a joint open. A hump under a big tile can crack it or leave edges uneven.
SLU is one tool among several for flattening a floor. Others include grinding high spots, patching low spots and, on wood floors, adding new plywood. For the layers under a finished floor, see our subfloor and floor underlayment pages.
How a self-leveling pour works
The sequence below follows manufacturer guidance; each product’s data sheet sets the exact steps 1 2.
- Pick the product for the depth needed, whether a feather edge is required, the floor load and whether the substrate flexes 1.
- Prepare the surface. It must be sound, clean, dry and free of anything that blocks bond 1 2.
- Prime. Primers help bond, reduce bubbles and pinholes and keep the mix from losing water to the slab 1.
- Dam and seal. Seal holes and block off edges, doorways and drains so the liquid stays put 1.
- Mix and pour. Mix fully per the instructions and keep a wet edge through the whole pour 1.
- Spread and smooth. A gauge rake sets depth; a smoother evens the surface and releases trapped air 1.
- Cure. Wait for walk-on time, then for the flooring install time the data sheet lists 2.
- AssessMeasure dips and humps; choose product
- PrepClean, sound, dry substrate
- PrimeMaker’s primer for that substrate
- PourMix fully; keep a wet edge
- CureWalk-on, then flooring install time
Steps drawn from a CustomTech SLU white paper and an ARDEX technical data sheet. Source: [1] [2]
Types of self-leveling underlayment
| Type | Strengths | Watch-outs |
|---|---|---|
| Calcium aluminate / portland blend | Rapid curing and drying; usable in all interior locations 1 | Most products need a primer and a minimum temperature 2 |
| Gypsum-based | Fire ratings in many designs; little floor prep 1 | Slow to dry; dry areas only; less resistant to trade traffic 1 |
| Portland-only | Simple chemistry | Less common; less dimensionally stable than aluminate blends 1 |
| Deep-pour formulas | Some reach several inches in one lift 1 | Check the maximum depth and any aggregate rule |
| Fiber-reinforced, for wood subfloors | Formulated to flex with wood without metal lath 1 | Wood must meet the maker’s plywood rules 4 |
| Feather-finish patches (trowel-applied) | Thin transitions and small areas | Not a self-leveler; used where a true feather edge is needed 2 |
Example: reading one data sheet before a bathroom floor
You plan to level a concrete bathroom floor before porcelain tile, using ARDEX V 1200 as the example product. Here is what its data sheet says, and what each line means for your schedule 2.
| Data sheet item | ARDEX V 1200 says | What it means for you |
|---|---|---|
| Minimum thickness | 1/8 in over the highest point; typically averages 1/4 in or more | Plan for at least that much height gain at doors and transitions |
| Maximum thickness | Up to 2 in in most applications | Deeper dips need another product or a patch first |
| Walkable | 2 to 3 hours | Trades can cross it the same day |
| Moisture-insensitive tile and stone | After 6 hours | Tile can start that day in good conditions |
| Most other floor coverings | After 24 hours | Vinyl and similar wait a day |
| Temperature | Substrate and air at least 50°F during install and cure | Heat the room in winter |
| Where it can go | Interior, dry areas only | Not a waterproofing layer under a shower |
Another maker’s product will list different numbers. ARDEX K 15, for example, lists 2 to 3 hours walkable, 6 hours for tile and 16 hours for other floor coverings, with up to 1-1/2 inches neat 3. Always use the sheet for the exact product on site.
Where SLU shows up in your remodel
In the flooring bid. Look for “floor prep,” “leveling” or “SLU” with an area or a depth. If a bid leaves it out, ask how the installer will meet the flooring maker’s flatness rule.
In the flooring warranty. Many flooring makers tie their warranty to their own substrate rules, often stricter than general guidance. Check before you buy.
At transitions. SLU raises the floor. Thresholds, door bottoms and the edge where new floor meets old must be planned. See our exterior door threshold page.
In the schedule. Cure and dry time sit between demolition and flooring. Our timeline tool shows where that fits.
Ask how they will measure flatness
Ask your installer to check the floor with a 10-foot straightedge or a laser before and after the pour, and to compare it with the flooring maker’s written tolerance. That is the number that decides whether your warranty holds.
Benefits and limitations
Benefits
What SLU does not fix
SLU does not stiffen a bouncy floor. If joists or subfloor flex, the leveler can crack and the tile on top can follow. A floor that deflects needs structural attention first; see our floor joist page.
It also does not stop moisture or water. ARDEX states its V 1200 is not a vapor barrier and lets moisture vapor pass, and that water sources must be found and corrected first 2. In a shower, the waterproofing goes on top as a separate membrane.
And it cannot stop cracks from moving joints. ARDEX says to honor all moving joints and cracks up through the underlayment and flooring, and that it knows of no method to prevent telegraphing where movement occurs 2.
What drives the cost of leveling a floor
We do not quote prices. The drivers are the area, the average depth needed, the product type, surface prep (cleaning, grinding or shot-blasting), primer, moisture testing and any moisture control system, damming and transitions, and labor for mixing and pouring. A laser survey sets the depth, and depth drives material.
Over wood, extra prep can apply: MAPEI, for example, calls for exterior-grade plywood meeting APA rules and notes better performance with lath over OSB 4. Properly installed, the leveler lasts as long as the floor system stays still and dry. For flooring budgets, see our flooring calculator.
Who does it, and what to check before the pour
Flooring installers and tile setters usually pour SLU. Check these items with them.
- Which product, and is it rated for this substrate and area?
- How was the floor measured, and what depth is planned? 1
- Has the slab been tested for moisture, where the product requires it? 2
- Which primer, and how long will it dry? 4 2
- How will edges, doorways and floor drains be dammed? 1
- Will the room stay at or above the product’s minimum temperature? 2
- What are the walk-on and flooring install times? 2
SLU work usually does not need a building permit by itself, but the remodel it belongs to may. See our permits guide.
After the pour
- Keep foot traffic off until walk-on time; keep heavy carts off longer.
- Protect the surface from other trades; SLU is not meant as a long-term wear surface unless the product says so 1.
- Do not force-dry with heaters blowing on the surface unless the data sheet allows it 2.
- Install flooring within the window the data sheet sets so the surface stays clean.
Warning signs and when to call a pro
- Hollow sounds or cracks after curing: poor bond or a moving substrate.
- Pinholes and bubbles: often poor priming or mixing 1.
- A soft, dusty surface: wrong water ratio or poor cure.
- Leveler poured over a bouncy floor: cracks will follow movement.
- Dark adhesive residue on old concrete: possible asbestos-containing cutback; stop and test 4.
Old adhesive and paint can be hazardous
MAPEI warns not to sand or remove cutback adhesive that contains asbestos fibers, and to test cutback for asbestos before removal 4. Grinding or shot-blasting concrete also makes silica dust, a worker-safety hazard. Let qualified crews handle prep on old floors, with dust controls and any required testing.
Standards behind self-leveling underlayment
ANSI A118.16. The American National Standard specification for flowable hydraulic cement underlayment and self-leveling underlayment; TCNA lists the 2021 edition as current 5.
ASTM C1708. Test methods for self-leveling mortars with hydraulic cements, covering flow, setting time, strength and length change 1.
ASTM F710. Practice for preparing concrete floors to receive resilient flooring, including the 3/16 inch in 10 feet flatness limit 1.
ANSI tile flatness and TCNA Handbook. For tile with an edge longer than 15 inches and for stone, substrate variation of no more than 1/8 inch in 10 feet 1. See our tile lippage page for why it matters.
Manufacturer instructions. Primers, thickness, temperature and cure times are product-specific; the data sheet governs 2 4.
SLU vs other floor-prep options
| Option | What it does | Main difference |
|---|---|---|
| Self-leveling underlayment | Flows out flat over a large area | Best for broad dips; adds height |
| Trowel-applied patch or feather finish | Fills small spots and feathers edges | Hand-shaped; not self-leveling 2 |
| Grinding high spots | Removes humps on concrete | Lowers the floor; makes silica dust |
| New plywood underlayment | Adds a flat wood layer on a wood subfloor | Can also stiffen; adds height |
| Waterproofing membrane | Keeps water out of the substrate | Different job; goes over the leveled floor in wet areas |
Self-leveler myths
- Myth It levels itself, so prep does not matter.
- Reality Surfaces must be sound, clean and primed, or it will not bond 1 2.
- Myth It fixes a bouncy floor.
- Reality It flattens; it does not stiffen. Movement cracks it 2.
- Myth It is a moisture barrier.
- Reality One maker states its product lets moisture vapor pass and is not a vapor barrier 2.
- Myth Any self-leveler works over wood.
- Reality Wood needs products and plywood that meet the maker’s rules 4 1.
- Myth Level and flat are the same thing.
- Reality Flatness is about bumps and dips; levelness is about slope. A floor can be flat but not level 1.
Temperature and humidity change the schedule
Climate affects SLU through the jobsite, not the code. Cold rooms slow mixing and curing, delaying walk-on and flooring install, and very dry or windy conditions can dry the surface too fast; very humid conditions can stretch the wait for moisture-sensitive flooring 1.
One product requires 50°F substrate and air during install and cure 2. In winter pours, heat the space ahead of time; in damp basements, test moisture first.
When to plan for a self-leveler
Plan for one if you are installing large-format tile, luxury vinyl plank or another floor with a strict flatness rule over a floor with broad dips.
It matters most in older homes, over slabs with patched trenches, and where several rooms must meet at one height. See our luxury vinyl plank page for that floor’s needs.
Skip it, or fix something else first, if the floor flexes, if moisture is present, or if the issue is a single hump that grinding would remove. Compare bids for floor prep with our quote comparison tool.
- If Broad dips; large tile or vinyl plank planned
- Then Self-leveling underlayment
- If One or two small high spots
- Then Grind or patch instead
- If Floor bounces underfoot
- Then Fix framing or subfloor first
- If Damp slab or moisture history
- Then Test and treat moisture first
Questions about Self-Leveling Underlayment
What is self-leveling underlayment used for?
It is used to make a floor flat and smooth before new flooring goes down. Installers pour it over a primed concrete or approved wood subfloor, and it flows out to fill dips and bury old patches. It is common before large-format tile, luxury vinyl plank and other floors with strict flatness rules.
How thick can self-leveling underlayment be poured?
It depends on the product. ARDEX V 1200 calls for at least 1/8 inch over the highest point and allows up to 2 inches in most applications. Other products go thinner or deeper, some with aggregate. Check the data sheet for the exact product.
How long before you can walk on self-leveler?
Many modern cement-based levelers can be walked on in about 1 to 2 hours in ordinary conditions, according to a Custom Building Products white paper. ARDEX V 1200 lists 2 to 3 hours. Cold rooms slow this down. Flooring usually waits longer than walk-on.
Do you need primer before self-leveling compound?
Almost always. Primers improve bond, reduce pinholes and keep the slab from pulling water out of the mix. MAPEI requires one of its primers with all its self-levelers. Use the primer the maker lists for your substrate.
Can you use self-leveler on a wood subfloor?
Yes, with products made for it and a subfloor that meets the maker’s rules. MAPEI, for example, calls for exterior-grade plywood meeting APA standards and notes better results with lath over OSB. Fiber-reinforced levelers are designed to flex with wood. A bouncy floor must be stiffened first.
How flat does a floor need to be for vinyl plank or tile?
ASTM F710 calls for no more than 3/16 inch of variation in 10 feet for resilient flooring over concrete. For tile with an edge longer than 15 inches, the ANSI standard calls for no more than 1/8 inch in 10 feet. Your flooring maker may set a stricter number.
Is self-leveling underlayment waterproof?
No. It is a flattening layer. ARDEX states its V 1200 is not a vapor barrier and allows moisture vapor to pass. In showers and wet areas, a separate waterproofing membrane goes over the leveled floor.
What is the difference between floor leveler and floor patch?
A self-leveler is poured and flows out over a large area. A patch or feather finish is troweled by hand into small spots and for thin transitions. Many jobs use both: patch first to fill deep holes, then pour the leveler.
Why did my self-leveling compound crack?
Common causes are a moving substrate, pouring over moving joints or cracks, poor priming, wrong water ratio or cold conditions. ARDEX says moving joints and cracks must be honored up through the flooring, because movement will telegraph through.
Sources
- Custom Building Products, Technical White Paper WP144: Understanding Self-Leveling Underlayment Formulation and Usage (2019), retrieved .
- ARDEX Americas, ARDEX V 1200 Self-Leveling Underlayment technical data sheet, retrieved .
- ARDEX Americas, ARDEX K 15 Premium Self-Leveling Underlayment technical data sheet, retrieved .
- MAPEI, Surface Preparation Requirements for MAPEI Self-Leveling Underlayments, retrieved .
- Tile Council of North America, ANSI Standards (A108, A118, A136) status list, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Binder types, flatness tolerances (ASTM F710, ANSI large-format tile), test standards, prep, priming, mixing and cure behavior are from Custom Building Products white paper WP144.
Product numbers are quoted from the ARDEX V 1200 and K 15 data sheets as product examples, not general rules. Wood-substrate and asbestos-cutback guidance is from MAPEI’s surface-prep guide; the ANSI A118.16 edition from TCNA’s standards list. No prices are given.
Suggest a correction. We fix errors and say what changed.