To level a floor for tile, find the high and low spots with a long straightedge, grind down the highs, fill the lows with patch or self-leveling underlayment, and check again before you set a single tile. Large format tile is the least forgiving, so the bigger the tile, the flatter the floor needs to be.
How flat is flat enough?
For most tile, the floor should be within 1/4" over 10 feet. For large format tile (any side 15" or longer), the tolerance tightens to about 1/8" over 10 feet, with no more than about 1/16" over 24". Thinset is not a leveling product. If you try to fix a dip with extra mortar, you get hollow spots, lippage and cracked tile.
Step 1: Find the high and low spots
Lay a long straightedge or level across the floor in several directions. Mark low spots where you can slide a shim under the edge, and high spots where the straightedge rocks. A laser is faster on big rooms.
Step 2: Fix the substrate
- Concrete: grind high spots with a grinder and diamond cup wheel. Fill small dips and cracks with a patch like Laticrete NXT Patch or Maxxon Skim Fill.
- Wood subfloors: re-screw loose sheets, plane down high seams, and make sure the floor is stiff enough for tile. Bounce is a bigger problem than flatness. Add a layer of plywood if needed.
Step 3: Pour self-leveling underlayment for big dips
When a room is out of flat across a wide area, self-leveling underlayment (SLU) is the fastest fix.
- Clean and prime. Vacuum the floor, then apply the primer specified by the SLU maker, such as Mapei Primer T. Primer keeps the SLU from drying out too fast and pinholing.
- Dam the edges. Seal gaps at doorways, floor drains and wall plates with foam or tape so the pour stays put.
- Mix right. Use the exact amount of water on the bag and a mixing drill with a paddle. Too much water weakens the surface.
- Pour and spread. Work fast with a gauge rake and smoother. A kit like the Ardex Self-Leveling Tool Kit or Kraft Tool kit has what you need. Wear spiked shoes to walk in the wet pour.
- Let it cure per the manufacturer before tiling. Many SLUs accept tile the next day, but always check.
Step 4: Control lippage while you set
Even on a flat floor, tile itself is rarely perfectly flat. A tile leveling system holds the edges of neighboring tiles in plane while the mortar cures. Clips and wedges or caps, like Raimondi clips, are standard for large format.
The real installer debate: leveling clips or a flat floor?
The clip camp says leveling systems are cheap insurance. They take care of tile warpage and small variations, speed up the job, and keep lippage callbacks off your phone.
The flat-floor camp says clips can't fix a bad substrate. Clamp tile edges together over a dip and the middle of the tile is still hollow. Spend the time on floor prep and clips become a nice extra, not a crutch.
Where it lands: both. Get the floor flat first, then use clips on large format and plank tile to manage tile warpage. Clips control edges. Floor prep controls everything else.
Frequently asked questions
Can I use thinset to level a floor? Only for very minor variations. Thinset shrinks as it cures and isn't designed for thick build-up. Use patch or self-leveler for anything more.
How thick can self-leveler go? It depends on the product. Some go from feather edge to over an inch in one pour. Check the bag and add aggregate if the product allows it for deep fills.
Do I need to prime before self-leveling? Yes, almost always. Primer is what makes the SLU bond and flow correctly.
Shop self-levelers, patches and primers, tile leveling systems and tile tools at Contractors Direct. Pro pricing, fast shipping, and a team that preps floors for tile and can help you pick the right products.