How to Level a Concrete Floor
To level a concrete floor, clean and prime the slab, then pour self leveling concrete: a cement-based underlayment you mix thin, pour out and let flow flat. Depending on the product, one pour runs from a feather edge up to between 5/8 inch and 3 inches deep. For tile the floor only has to be flat, within 1/8 to 1/4 inch over 10 feet, not level, and knowing that can cut the bag count by more than half.
Self-leveler, patch or lift?
The shape of the problem picks the product. Check it with a straightedge before you buy anything.
A few divots or cracks
Patch, don't pour. A trowel-applied floor patch fixes local dips for a couple of bags. Concrete floor repair covers cracks and spalls.
A wavy, dished or sloped floor
This is the self-leveler job: a pour that covers the whole area and sets as one flat plane, ready for tile, vinyl or laminate.
A slab that has sunk or still moves
Stop. A leveler bonded to a moving slab cracks with it, and it can't raise a slab that settled. That's slab lifting, a different trade.
Self-leveling underlayment is a smoothing layer for flooring to go over. It isn't structural, it isn't a wear surface unless the data sheet says so, and it isn't a moisture barrier. Need more than about 2 inches across a garage? Price a bonded concrete topping with the slab calculator instead.
Flat vs level: tile only needs flat
The Ceramic Tile Education Foundation separates the two. Flat means "the surface does not have any significant high or low spots." Level means the surface is "parallel to its perfectly horizontal plane." A floor can be one without the other, and the tile standard asks for flat.
That matters because a liquid always finds horizontal. Pour enough self-leveler and you get a level floor, and filling a sloped room to level takes a wedge of material.
Example: a 10 by 14 foot room (140 sq ft) sits 1 inch lower at one end. A bag rated 21 sq ft at 1/4 inch yields 21 × 0.25 ÷ 12 = 0.4375 cubic feet.
- Fill it level, 1/8 inch at the high end to 1-1/8 inch at the low end, an average of 5/8 inch: 140 × 0.625 ÷ 12 = 7.3 cubic feet, about 17 bags.
- Fill it flat at an average of about 3/16 inch: 140 × 0.1875 ÷ 12 = 2.2 cubic feet, about 5 bags.
The exception is drainage. A floor that has to drain needs a deliberate fall, which a self-leveler can't produce. The concrete slope calculator handles that.
How flat a floor must be for tile
ANSI A108.02 sets two tolerances, and your tile size decides which one applies. The Ceramic Tile Education Foundation quotes both, and its large format tile page repeats the stricter one.
| Tile | Over 10 feet | Shorter span |
|---|---|---|
| All edges under 15 in | 1/4 in | 1/16 in in 1 ft |
| Any edge 15 in or longer | 1/8 in | 1/16 in in 2 ft |
The standard measures "from the high points in the surface." Rest a 10 foot straightedge on the two highest points it touches and measure the biggest gap underneath. A 4 foot level won't find a dip that spreads over 10 feet.
Check the floor first
- Mark the high spots. Walk a 10 foot straightedge across the floor in a grid, both directions, and mark where it rocks.
- Measure the worst gap under the straightedge and compare it with the table above. Inside tolerance? You may not need to pour at all.
- Read the pattern. Scattered small gaps mean divots. One end deep means slope. The middle deep everywhere means a dished slab.
- Check for moisture. Tape an 18 by 18 inch sheet of plastic to the slab, leave it at least a day, and look for darkening or condensation underneath. Coatings maker Tnemec describes this ASTM D4263 test and notes it only shows moisture near the surface and doesn't measure how much.
- Measure door thresholds. The finished floor has to clear every door that swings into the room.
How to use self-leveling concrete, step by step
- Clean the slab. Sweep and vacuum, and remove grease, paint and loose material. The leveler bonds to clean, sound concrete, and contamination causes it to let go.
- Repair cracks and holes. Fill open cracks and holes and seal gaps at walls and penetrations, because the liquid will find every opening.
- Build dams and prime. Dam doorways and edges. Quikrete's Fast-Setting data sheet says dams must hold the material "at a height greater than the finished surface elevation." Then roll on the primer your product names (see below).
- Mix. Water into the bucket first, then the powder: about 5 quarts per 50 lb bag for Quikrete Fast-Setting, mixed at least 4 minutes with a heavy-duty drill and paddle "capable of 650 RPM or greater." Mix only what you can place before it stiffens: Quikrete Fast-Setting stays fluid about 20 minutes, and Ardex lists a 10 minute flow time for K 15.
- Pour and spread. Start at the far end, work back to the door, and spread with a gauge rake. Keep each batch flowing into the last one before either sets.
- Break the bubbles. Roll the wet surface with a spiked roller. CTS says its spiked roller "minimizes pinholes by releasing trapped air."
- Cure, then wait for flooring. Keep traffic off until the walk-on time, then wait the separate time your product gives before flooring goes down (table below).
Don't skip the primer
On bare concrete, primer stops the slab from pulling water out of the mix. Laticrete's primer guide (TDS 230) spells out the cost of getting it wrong: "insufficient drying or poor film formation will result in pinholes and poor bond strength and may cause self-leveling products to debond." It also says to prime again if the primer dries within 30 minutes or sits longer than 24 hours.
- Use the maker's primer. Laticrete's NXT Level sheet names four primers and says "only." Quikrete primes with its Concrete Bonding Adhesive cut 1 part to 2 parts water, two coats on very porous concrete.
- Over wood, full strength. Ardex says to prime plywood with P 51 "at full strength (do not dilute)."
- The exception: Ardex K 60 ARDITEX needs "no priming" except over non-Ardex epoxy.
How much you need and how deep you can pour
A 50 lb bag covers about 21 to 32.5 sq ft at 1/4 inch, and coverage halves every time the depth doubles. In a 200 sq ft room at 1/4 inch that's 200 ÷ 21 = 9.5, so 10 bags of Quikrete No. 1249, or 200 ÷ 32.5 = 6.2, so 7 bags of Ardex Liquid BackerBoard, before waste. Measure the average depth, not the deepest point, or you'll over-order.
| Product | Max one pour | Walk on | Flooring |
|---|---|---|---|
| Quikrete No. 1249 | 5/8 in | Not published | Not published |
| Quikrete No. 1249 Fast-Setting | 1-1/2 in | 6 hr | 24 to 48 hr |
| Ardex Liquid BackerBoard | 1-1/4 in | 2 to 3 hr | 6 hr tile, others after a mat test at 24 hr |
| Ardex K 15 | 1-1/2 in, 5 in with aggregate | 2 to 3 hr | 6 hr tile, 16 hr other |
| Laticrete NXT Level Plus | 2 in | 2 to 3 hr | From 12 hr |
| Laticrete NXT Level | 3 in | 2 to 3 hr | See data sheet |
Watch the Quikrete suffix. Standard No. 1249 caps at 5/8 inch per layer; the Fast-Setting version under the same number goes to 1-1/2 inches. Deeper than your product's limit means pouring in layers, priming between them. The self-leveling concrete calculator has coverage, bag size and maximum pour for 16 products and tells you how many bags and pours your floor takes.
Most products need at least 1/8 inch over the high point. Quikrete No. 1249 goes down to a feather edge.
Self-leveling underlayment over plywood
Many levelers allow wood subfloors, but always with conditions. Read the substrate list on your product's data sheet before you buy.
- Stiffness
- Ardex wants a plywood subfloor that is "rigid, well supported, properly anchored and free of undue flex and vibration," with open joints filled first.
- Lath
- Quikrete requires expanded metal lath over wood. Laticrete NXT Level wants diamond lath; NXT Level Plus goes over plywood or OSB "without the use of metal lath."
- Thickness
- Quikrete Fast-Setting needs at least 5/8 inch over lath. NXT Level Plus starts at 1/4 inch on wood.
Leveling a basement floor
Two checks come before you mix anything.
- Moisture. A leveler won't stop vapor coming up through the slab, and flooring that fails from slab moisture will fail over a fresh pour too. If the slab tests damp, deal with the moisture first. See basement concrete floors for testing.
- The drain. If the floor slopes to a floor drain, that slope is on purpose. Level it out and water from a burst pipe has nowhere to go.
Which self-leveler to buy
Most floor per bag Ardex Liquid BackerBoard
32.5 sq ft
per 50 lb bag at 1/4 in
- Max pour
- 1-1/4 in
Deepest pour Laticrete NXT Level
3 in
in a single lift
- Coverage
- 22 sq ft at 1/4 in
Over plywood Laticrete NXT Level Plus
No lath
over plywood or OSB
- Max pour
- 2 in
Easy to find Quikrete Fast-Setting 1249
1-1/2 in
in one pour
- Coverage
- 21 sq ft at 1/4 in
Ardex vs Mapei or any other brand? Compare price per square foot, not per bag: coverage at 1/4 inch ranges from 21 to 32.5 sq ft per 50 lb bag, so the cheaper bag isn't always the cheaper floor. Then check the depth limit and the substrate list for your floor. For a cracked or worn surface that's already flat, resurfacing is the cheaper fix. Costs are in the concrete floor cost guide.
Frequently asked questions
Do you need primer under self-leveling concrete?
Almost always. On bare concrete the primer stops the slab from pulling water out of the mix. Laticrete warns that poor priming "will result in pinholes and poor bond strength and may cause self-leveling products to debond." Use the primer your product names, full strength over wood. Ardex K 60 ARDITEX is the common exception: it needs no primer except over non-Ardex epoxy.
How long before you can walk on it and lay flooring?
Most products take foot traffic in 2 to 3 hours. Quikrete Fast-Setting No. 1249 needs 6 hours. Flooring waits longer and depends on the covering: Ardex K 15 allows tile at 6 hours and other coverings at 16, Ardex Liquid BackerBoard allows tile at 6 hours, and Quikrete Fast-Setting asks 24 to 48 hours before finished flooring.
How thick can you pour self-leveling concrete?
It depends on the product: 5/8 inch per pour for standard Quikrete No. 1249, 1-1/4 inches for Ardex Liquid BackerBoard, 1-1/2 inches for Quikrete Fast-Setting and 3 inches for Laticrete NXT Level. Some, like Ardex K 15, reach 5 inches only with added aggregate. Deeper than the limit means pouring in layers and priming between them. Most products also need at least 1/8 inch.
How flat does a concrete floor need to be for tile?
ANSI A108.02 allows 1/4 inch in 10 feet and 1/16 inch in 1 foot when every tile edge is under 15 inches. With any edge 15 inches or longer it tightens to 1/8 inch in 10 feet and 1/16 inch in 2 feet, measured from the high points of the floor with a straightedge.
Does a floor have to be level for tile?
No, it has to be flat. Flat means no humps or dips; level means horizontal. Self-leveler naturally fills to level, and on a sloped floor that wedge can take more than three times the material of a flat skim, so decide which you need before you order.
Can you pour self-leveling underlayment over plywood?
Often, with conditions. Quikrete requires expanded metal lath over wood, Laticrete NXT Level wants diamond lath, and Laticrete NXT Level Plus goes over plywood or OSB without metal lath. Ardex Liquid BackerBoard allows untreated plywood that is rigid and free of undue flex, primed with full-strength P 51. Check your product's substrate list first.