How to Level Floor Tiles with Shims
Shims cannot make an uneven floor suitable for tile. First, check the structure, moisture, and substrate flatness, then correct any problems with a compatible repair or underlayment system. Tile-leveling clips and wedges can help align neighboring tile edges, but they cannot fix a floor that moves or dips.
Using the right repair method can save tile, mortar, and time. It can also prevent waste: fixing the substrate before setting tile is better than removing and replacing tile that fails later. The key is to tell the difference between a floor that is not level and one that is not flat. Tile needs a suitably flat, stable base; it does not always need to be perfectly level.
Inspect Subfloor Flatness, Deflection, and Moisture
A substrate is the surface beneath your tile, such as concrete or a wood underlayment assembly. Before setting tile, check that the full floor system is stable, dry enough for its specified products, and within the flatness limits for your tile size. Shims cannot correct movement, moisture problems, or dips in the substrate.
Check flatness and structure
Use a long, straightedge to check the floor in several directions. Mark high and low spots, and measure gaps beneath the straightedge. A floor can be flat but slope slightly; flatness means the surface does not have unacceptable bumps or dips.
ANSI A108.02 gives these substrate flatness limits:
- For tile with all edges under 15 inches: no more than 1/4 inch variation in 10 feet and 1/16 inch in 1 foot.
- For tile with any edge 15 inches or longer: no more than 1/8 inch variation in 10 feet and 1/16 inch in 2 feet.
These are flatness tolerances, not a requirement that the floor be level. Large-format tile has tighter tolerances because a small dip can leave one tile sitting higher than its neighbor.
On wood floors, check for loose panels, damaged areas, and movement. Confirm the subfloor and underlayment meet the selected tile system’s thickness, fastening, and installation requirements. The whole assembly matters, not just the joists. Under applicable design loads, the floor system must meet at least L/360 deflection for ceramic or porcelain tile, or L/720 for natural stone. If you cannot verify the assembly, get qualified help before tiling.
Concrete must be sound and sufficiently cured for the products you plan to use. Test moisture using ASTM F2170 in-situ relative humidity or ASTM F1869 moisture vapor emission rate, as required by the specified system. There is no universal acceptable RH or MVER limit. Follow the strictest applicable limit for the membrane, mortar, and other products.
| What you find | What it means | Appropriate next step |
|---|---|---|
| A high spot or dip beyond the tile-size tolerance | The surface is not flat enough | Grind or fill only with an approved system |
| Loose or moving subfloor | The assembly is unstable | Repair or stiffen it before tiling |
| Concrete moisture above a product limit | The planned system may not be suitable | Choose a compatible specified system or resolve the moisture issue |
| A small edge-height mismatch between tiles | Possible tile lippage | Check the substrate and setting method; clips may help align edges |
Do not rely on a leveling clip to hide a dip, a loose panel, or a soft floor. Excess movement can crack tile or grout, or loosen the installation, even if you use an uncoupling membrane.
Prepare the Substrate and Set Tile with Precision
Preparation means correcting the floor before tile goes down, using products approved for the substrate and tile system. Once the surface meets the required flatness and stability, set tile with the specified mortar and proper coverage. Clips or wedges may assist with edge alignment, but they are not a substitute for substrate repair.
Correct the floor before setting tile
For a wood floor, repair loose or damaged areas and follow the chosen system’s directions for subfloor thickness, fastening, and underlayment. Do not assume that adding a layer of material will fix a weak or moving floor. Use only compatible, manufacturer-approved underlayments, membranes, primers, mortars, and fasteners.
For concrete, remove contamination that could affect bonding, and repair cracks or low spots with products approved for that use. Address moisture only with a specified system that permits the measured conditions. A patch or leveling product must suit both the substrate and the tile installation; follow its instructions for preparation, thickness, and cure time.
Set tile and use clips as alignment aids
Mix and spread mortar according to its instructions. Choose a trowel notch suited to the tile format, the substrate, and the product directions. Comb the mortar ridges consistently, then move the tile across the ridges to collapse them. Do not spot-bond tile with isolated blobs of mortar. Those gaps can leave tile edges and corners without support, raising the risk of cracks, debonding, and hollow spots.
Check coverage by lifting a tile now and then. The target is at least 80% contact in dry areas and 95% in wet areas or exterior installations, with full support at edges and corners. If coverage is low, adjust your mortar, trowel, or setting method in line with the product instructions.
Tile-leveling clips and wedges can help bring neighboring edges into alignment while mortar is workable. Use them only as their manufacturer directs, and do not tighten them so hard that they force tile over a substrate defect. Remove clips at the specified time and in the specified way, before grouting as directed. Keep the joints between tiles clear and consistent.
Detail Transitions, Movement Joints, and Safety
Movement joints give a tile assembly room to respond to changes in temperature, moisture, and building movement. They must remain free of grout and mortar. Plan them along with transitions and perimeter gaps, following TCNA EJ171 and the project conditions rather than filling every joint with grout.
Keep movement gaps open
Leave perimeter and field movement joints where required by the tile layout, room conditions, and installation system. Fill them with the specified sealant or joint system, not grout. Keep mortar and grout out of the gaps during installation; material that bridges a movement joint can stop it from working as intended.
At doorways and floor transitions, use a detail suitable for the tile assembly and the adjoining floor. A transition should not pinch the tile or block a required movement gap. If a height change is involved, choose a transition that fits the finished floor heights rather than using tile wedges to force one edge into place.
DCOF, or dynamic coefficient of friction, measures a surface’s resistance to slipping under specified test conditions. ANSI A326.3’s DCOF value of 0.42 is a reference for level interior areas expected to be walked on when wet. It is not a guarantee of safety and is not a universal requirement for every setting. Consider the tile’s intended use and follow the relevant product and project requirements.
Field Examples: Choose the Fix That Matches the Problem
These examples describe common installation situations, not a substitute for checking your own floor. The right response depends on the substrate, tile size, product instructions, and measured conditions. In my work, separating an edge-alignment issue from a floor-flatness or structure issue is often the first step toward avoiding a costly mistake.
A common case is two tiles with a small height difference at their shared edge. If the substrate is flat and stable, careful mortar placement and a compatible clip system may help align the edges. Lift a tile to check mortar coverage, and do not use the clip to pull it down over a dip.
Another frequent problem is a long tile rocking over a hollow or uneven area. A clip can hold edges in line, but it cannot support the tile’s full underside. Stop and correct the substrate with a compatible product before continuing. If the floor flexes underfoot or panels move, investigate the assembly rather than trying to compensate with mortar or wedges.
| Situation | Use clips or wedges? | Repair first? |
|---|---|---|
| Neighboring edges need alignment on a flat, sound base | Yes, as directed by the clip maker | No, if checks confirm the base is suitable |
| Tile rocks because the substrate has a dip | No, not as a fix | Yes |
| Floor moves or deflects | No | Yes, assess the full floor assembly |
| Floor is flat but has a slight slope | Possibly for edge alignment | Not necessarily; flatness and level are different |
Protect, Maintain, and Seal the Finished Floor
Protection begins after setting: let the mortar cure for the time stated by its manufacturer before grouting, walking on the floor, or adding loads. Keep joints and edges clean, and use only specified grout, sealants, and care products. Sealing is product-dependent; do not assume every tile or grout needs it.
After the mortar has cured as directed, grout the tile joints while keeping movement joints clear. Fill movement joints with the specified sealant or joint system. Follow the grout and tile manufacturers’ instructions about sealing. Some grout products may call for sealer, while others may not; tile surfaces also differ. Avoid applying a sealer simply because the floor is new.
Protect the floor during later work so tools, grit, and heavy loads do not damage the tile or grout. For routine care, use products approved for your tile and grout. If a tile becomes loose, grout cracks repeatedly, or the floor develops new movement, look for the cause rather than covering the symptom. A surface repair will not fix an underlying structural or moisture problem.
Conclusion
Use shims or leveling clips only to help align neighboring tile edges on a floor that is already sound and flat enough. Check flatness with a straightedge, confirm the assembly’s stability and moisture limits, and correct defects with compatible products before setting tile. Then verify mortar coverage, keep movement joints open, and follow product instructions through cure and finish.
Frequently asked questions
Can shims level a tile floor?
No. Shims or tile-leveling wedges can help align adjacent tile edges, but they cannot make an uneven or moving substrate suitable for tile.
What is the allowed floor variation for tile?
For tile with all edges under 15 inches, the limit is 1/4 inch in 10 feet and 1/16 inch in 1 foot. If any tile edge is 15 inches or longer, it is 1/8 inch in 10 feet and 1/16 inch in 2 feet.
Does the floor have to be level?
Not always. Tile substrate requirements focus on flatness. A floor can be flat while sloping, though the design and use of the room may require a level surface.
Do tile-leveling clips prevent lippage?
They can help align tile edges, but they do not guarantee a lippage-free floor. Tile quality, substrate flatness, mortar coverage, and setting technique also matter.
Can I use clips to pull tile over a low spot?
No. Correct the low spot with a compatible product before setting tile. Forcing tile over an uneven area can leave it poorly supported.
How much mortar coverage should tile have?
Aim for at least 80% contact in dry areas and 95% in wet or exterior areas, with full support at edges and corners. Lift tiles periodically to check.
Can I put tile over a wood subfloor?
Only if the complete floor assembly meets the tile system’s structural, subfloor, fastening, and underlayment requirements. Confirm the system details before installation.
How do I know whether concrete is dry enough?
Test it by ASTM F2170 or ASTM F1869 as required by the chosen system. Compare the result with the strictest applicable product limit; there is no single limit for all installations.
Should movement joints be filled with grout?
No. Keep required perimeter and field movement joints free of grout and mortar. Use the specified sealant or joint system.
Does every tile floor need sealer?
No. Sealing depends on the tile and grout products. Follow their instructions rather than applying sealer by default.
(This article was written by one of our staff writers, David Chen. Visit our Meet the Team page.)
