For tile with at least one edge 15 inches (about 38 cm) or longer, plan for no more than 1/8 inch of variation in 10 feet and no more than 1/16 inch in 24 inches. Check those spans with a straightedge, measuring gaps from the surface’s high points. A floor can meet the flatness requirement while still being intentionally sloped, such as a shower floor.
Those figures are commonly published industry guidance. The controlling requirement for a contract is the current ANSI/TCNA method, the project specification and the tile-system manufacturer’s instructions.
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The flatness requirement by tile size
Large-format tile spans a wider area than small tile, so a hump or depression affects more of the piece. For that reason, installation guidance uses tighter short-span and long-span tolerances once any tile edge reaches the large-format threshold.
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| Tile size category | Commonly published maximum variation | How to check |
|---|---|---|
| One edge 15 inches or longer | 1/8 inch in 10 feet and 1/16 inch in 24 inches | Place a suitable straightedge on the surface and measure gaps from high points |
| All edges shorter than 15 inches | 1/4 inch in 10 feet and 1/16 inch in 12 inches | Use the stated long and short spans; verify the exact requirement in the current standard |
The threshold should be stated as “15 inches or longer,” not “greater than 15 inches.” Some guides describe large format as one side greater than 15 inches, while commonly used tolerance language applies at 15 inches exactly. A 12-by-24-inch tile therefore falls in the first row because one edge is longer than 15 inches, but the installation still has to account for the particular tile, layout and substrate.
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The tolerances describe deviation from the required plane. “Flat” does not mean “level.” A shower floor may slope toward a drain by design; it still needs to be a consistently flat plane relative to that designed slope. A short bubble level cannot check a 10-foot straightness requirement.
Why large tiles expose substrate problems
A small tile can cross a limited area of unevenness, and minor changes can sometimes be distributed over several joints. A large tile has fewer joints and less opportunity to absorb a change in plane. A low spot can leave inadequate mortar support beneath part of the tile, while a high spot can hold the tile up and force an adjacent piece lower. Those conditions can increase the risk of poor adhesion or cracking under a concentrated load; they are installation risks, not a prediction that every uneven floor will fail.
Unevenness also makes lippage more visible. The Tile Council of North America (TCNA) reproduces the ANSI A108.02 definition: “Lippage refers to differences in elevation between edges of adjacent tile modules.” Substrate-plane variation is only one contributor. Tile thickness variation, tile warpage, joint width, the chosen offset pattern, lighting and reflective surfaces all affect what the eye sees.
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Do not add tile warpage twice
Permitted lippage already includes the tile’s actual warpage. You do not take the full permitted warpage allowance and add it again to the lippage allowance. Inspect the actual pieces, keep the substrate within its flatness requirement and choose a layout and joint width that suit the tile’s geometry.
How to check a floor or wall before setting tile
Make the inspection before ordering corrective materials or spreading mortar. Establish the intended plane first: a level plane for most rooms, or a specified slope for a wet area. Confirm whether the substrate is concrete, a wood-framed assembly, an existing tile surface or an underlayment system, because preparation products are not interchangeable.
- Gather the right measuring tools. Use a straightedge long enough to span 10 feet for the long check, plus a straightedge or accurate straightedge segment for the 24-inch check. A laser or level can help establish elevations, but it does not replace a straightedge for finding gaps along the plane.
- Clean and expose the surface. Remove dust, loose material, adhesive ridges and debris that could hold the straightedge off the actual substrate. Do not measure over piles of thinset or construction waste.
- Check the long span. Set the straightedge in multiple directions, including parallel and perpendicular to the room, across doorways and over transitions. Look for daylight or use feeler gauges to measure the largest gap. For tile with an edge of 15 inches or longer, the commonly published limit is 1/8 inch over 10 feet.
- Check the short span. Move the straightedge to representative areas and suspected problem spots. The large-format short-span limit is 1/16 inch over 24 inches. A floor can pass the 10-foot check and still fail this tighter local check around a ridge or patch.
- Mark high and low areas. Circle high spots that will need removal and outline low areas that need filling. Record the depth and whether the correction must preserve a designed slope.
- Recheck after preparation. Once grinding, patching or underlayment work is complete and ready for tile, repeat both span checks. Preparation is part of compliance; it is not enough that the surface looked better immediately after application.
For tile with all edges shorter than 15 inches, commonly published language allows 1/4 inch in 10 feet and 1/16 inch in 12 inches. Do not substitute the large-format spans or assume that a project using mixed sizes has one universal tolerance. Follow the current standard and the project documents for each installation area.
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Ways to correct an out-of-tolerance substrate
TCNA guidance identifies grinding high spots, filling low spots and using a suitable self-leveling underlayment (SLU) as possible approaches. The correct choice depends on the substrate material, the size and depth of the defect, the area involved, product compatibility and whether the target is flat or intentionally sloped.
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| Condition | Possible approach | Decisions and cautions |
|---|---|---|
| Local high spot, ridge or concrete hump | Grind or otherwise remove the high material using a method allowed for the substrate | Control dust, avoid damaging reinforcement or membranes, and verify that the remaining surface is sound and clean |
| Small or moderate low area | Use a compatible patch or fill material | Check maximum lift, primer requirements, cure time and whether the product is approved over the existing substrate |
| Broad or variable low areas | Use a compatible self-leveling underlayment system | Confirm minimum and maximum thickness, perimeter isolation, primer, mixing, working time, cure and readiness for tile; “self-leveling” does not mean it can be poured without planning the finished plane |
| Wet-area floor that must drain | Form or preserve the specified slope with an appropriate patch, mortar-bed or underlayment method | Do not flatten away the drain slope; evaluate the complete waterproofing and tile-setting system |
There is no universal product, thickness or price-performance ranking that works for every substrate. A cement-based SLU suitable for concrete may not be approved over a wood assembly without reinforcement and a specified underlayment. A patch compound may be appropriate for a shallow repair but unsuitable for a deep fill. Read the product data sheet and installation instructions, and follow the tile-system manufacturer’s requirements for primer, movement joints and substrate moisture.
Why extra mortar is not a correction method
Adding more setting mortar to compensate for a wavy substrate can produce variable thickness, shrinkage, poor coverage and loss of support. Mortar is selected and applied within the limits of its installation method; it is not a substitute for bringing the substrate into tolerance.
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Why leveling clips do not make a substrate flat
Lippage-control clips or wedges can help hold adjacent tile edges in alignment while mortar cures. They may reduce movement during setting, but they cannot remove a high spot or fill a low spot. Use them, if permitted by the tile and mortar systems, only after the substrate passes the required flatness checks.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Flatness, levelness and designed slopes
Specify the plane before measuring. In a living area, the plane may be level or may follow an existing structural slope. In a shower, balcony or other drainage area, the plane is intentionally sloped to the outlet. The straightedge test asks whether the surface follows that intended plane without abrupt deviations; it does not ask whether every point has the same elevation.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteWhen correcting a sloped area, mark reference elevations and use a method that maintains drainage. A self-leveling product intended for horizontal, level applications may destroy the slope or be unsuitable near a drain. Waterproofing, movement accommodation and drain-flange details must remain compatible with the correction system.
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Substrate flatness versus finished lippage
Passing a flatness check does not guarantee a perfectly flush finished surface. Finished lippage is influenced by the tile itself and by installation choices.
- Tile warpage and thickness: Large porcelain pieces can have a crowned or twisted shape within their product tolerance. Sort and orient pieces according to the manufacturer’s instructions.
- Joint width: Narrow joints make small changes in height more noticeable. LATICRETE’s guidance specifically warns that narrow joints can emphasize substrate variation and tile warpage.
- Offset pattern: A running-bond layout can place the crown of one tile beside the end of another. The tile manufacturer may limit the offset percentage for a particular format.
- Lighting and reflection: Grazing light and glossy surfaces reveal edge differences that may be difficult to see under diffuse light.
- Drain slopes: LATICRETE’s table lists 1/32 inch allowable lippage for gauged porcelain tile panels or slabs, but that table explicitly does not apply to floors sloping to drains. Do not use that finished-lippage figure as a substrate-flatness tolerance.
Keep the two measurements separate: substrate flatness is checked before installation with a straightedge; finished lippage is evaluated at adjacent tile edges after considering the actual tile, joint and setting method.
Which standards and documents control?
TCNA’s current standards listing identifies ANSI A108.01, General Requirements: Structures, Substrates, and Preparation of Tile, as the 2026 publication; ANSI A108.02, General Requirements: Workmanship for Tile Installation, as current 2023; and ANSI A108.21, covering interior flowable hydraulic-cement underlayment and self-leveling underlayment, as current 2021.
The status listing does not reproduce the complete standard text. For contractual compliance, obtain the current standard, read the project specification and follow the tile, mortar, underlayment and waterproofing manufacturers’ instructions. A 2019 large-format installation guide from The Tile Shop states the 1/8-inch-in-10-feet and 1/16-inch-in-24-inches figures and calls for a straightedge, but it also cautions that its training aid does not supersede ANSI A108 or the TCNA Installation Handbook.
Jobsite checklist before spreading mortar
- Confirm whether any tile edge is 15 inches or longer.
- Identify the required plane, including any drainage slope.
- Check the long and short spans with a straightedge; do not rely on a short level alone.
- Measure from high points and record the largest gaps.
- Identify whether each defect is a high spot, a low spot or a change in slope.
- Verify that the proposed grinder, patch, SLU or other preparation material is compatible with the substrate and tile system.
- Complete correction, allow the required cure or readiness time and repeat the measurements.
- Confirm mortar coverage, movement joints, waterproofing and layout requirements before setting tile.
- Plan joint width and offset with the tile’s actual warpage and the lighting conditions in mind.
For a 12-by-24-inch or larger installation, the practical decision is simple: inspect with the large-format spans, correct the substrate before tile setting and treat lippage-control accessories as setting aids rather than repairs. If the project specification gives a tighter requirement, the tighter requirement governs.
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