Floor Flatness Tolerances by Floor Type

Why this matters

Most hollow spots, lippage complaints, peaked click joints and telegraphed humps trace back to a floor that was never measured for flatness, or was measured against the wrong number. Each floor type has its own tolerance, and the strictest one in the job governs. A kitchen that is perfectly acceptable for 12 inch tile can fail outright for 12 by 24 porcelain, and the only way to know before the tile is on the floor is a straightedge and the right figure.

Flat is not level

Level is a floor's relation to gravity. Flat is a floor's relation to itself: how far the surface departs from a straight line laid across it. A floor can fall 1/2 inch across a 14 foot room and be perfectly flat, and a dead-level floor can carry a dip you could lose a pencil in.

Every tolerance on this card is a flatness tolerance. None of them say anything about slope. That matters in two places:

• The customer conversation. A homeowner who says "the floor isn't level" usually means a door swings shut or a ball rolls. Flattening the floor for tile does not change that, and pouring a self-leveling underlayment to true level over a sloped floor is a different scope with a different material quantity. Say which you are pricing.
• Transitions. A floor that is flat but sloped still has to meet the adjoining room. Where the slope runs toward a doorway, the height change at the threshold is a transition problem, not a flatness one. See related: Transition Profiles and the Accessible Level Change.

How the tolerances are stated

A flatness tolerance is two numbers: a gap and the length of straightedge it is measured under. "1/4 inch in 10 feet" means a 10 foot straightedge laid on the floor, bearing on its high points, shows no gap underneath larger than 1/4 inch. The length matters as much as the gap. A long, gentle dip that is fine under 10 feet can still be a short, sharp hump that a 2 foot straightedge catches, and a large tile spans the short hump, not the long dip.

A) DIP: 10 FT EDGE BEARING ON TWO HIGH POINTS

 high point                         high point
 v                                           v
 =============================================
  \__                                     __/
     \___                             ___/
         \___________________________/
                      |
           gap at deepest point: 3/16 in

B) RIDGE: 2 FT EDGE ROCKING OVER A HUMP

              ===================
  ___________/         ^         \__________
                crown of the ridge
           gap at each end of edge: 1/8 in

Measure at the deepest point of the gap, not just the middle of the straightedge, and slide it in at least two directions across the room. A dip found only east to west is often a trough you can step across north to south.

The tolerances by floor type

Tile (ANSI A108.02). The installation standard states, in substance, that for tile with all sides shorter than 15 inches the substrate may vary no more than 1/4 inch in 10 feet from the required plane and no more than 1/16 inch in 12 inches measured from the high points. For tile with any side 15 inches or longer, the limit tightens to 1/8 inch in 10 feet and no more than 1/16 inch in 24 inches, measured from the high points. The standard is a consensus document; it binds you where your contract, the tile maker's warranty or the project specification adopts it, and most tile and mortar makers write it into their instructions. Check the current edition.

Why large tile is stricter: a 24 inch tile cannot follow a curve the way a 12 inch tile can. Over a dip it bridges and leaves a void under its center; over a hump it rocks, and the edges of neighbors sit at different heights (lippage). Thinset is not the fix. Large-format mortar can build some thickness, but only within the maker's stated maximum, and troweling extra mortar to fill a dip leaves soft, shrinking areas under the tile.

Floating floors (laminate, rigid-core plank). Per the maker. A common figure in laminate and rigid-core instructions is 3/16 inch in 10 feet, and some add a shorter-span figure. What fails when you ignore it: the plank bridges a dip, flexes under foot traffic, and the click joint works itself open or peaks where two planks rise together. See related: Click-Lock Flooring Joints: Angle, Drop and Fold.

Glue-down resilient (LVT, sheet vinyl). Per the maker, and read the smoothness requirement as well as the flatness one. Thin resilient material shows every ridge, trowel line and nail head underneath it, so a floor can be flat within the long-span figure and still telegraph a 1/32 inch ridge as a visible line in raking light.

Hardwood. Per the maker, and per the NWFA guidelines where the maker defers to them. Nail-down solid is usually allowed the loosest figure, because the fasteners pull each board to the subfloor; glue-down and floated engineered are held tighter, because the adhesive or the joints carry the floor over any void.

Carpet. Tolerant of gentle dips, intolerant of ridges. A sharp edge or a high seam in the subfloor wears a line through the cushion and the pile.

Commercial new slabs. Specified with F-numbers (FF for flatness, FL for levelness) measured under ASTM E1155 at the time of placement. F-numbers do not convert cleanly to straightedge gaps and describe the slab as poured, not as you find it after trades have worked on it. Measure it with a straightedge anyway before your floor goes down.

Worked example: one kitchen, two tiles

A 14 by 16 foot kitchen on a slab on grade. The customer is choosing between 12 by 12 inch porcelain and 12 by 24 inch porcelain. The installer runs a 10 foot straightedge and a 2 foot straightedge across the floor in both directions and marks the worst spots with crayon.

Reading 1, the dip. Near the center of the room the 10 foot edge bears on two high points and shows a 3/16 inch gap at the deepest point.

Reading 2, the ridge. Along a sawcut control joint the slab edges have curled up. A 2 foot edge centered across the joint rocks, and with it held level on the crown, the gap at each end is 1/8 inch.

Run against 12 by 12 tile. All sides are under 15 inches, so the rule is 1/4 inch in 10 feet. The dip at 3/16 inch is 1/16 inch inside the limit: pass. The short-span rule for this tile size is 1/16 inch in 12 inches; a 1 foot edge centered on the crown shows roughly half the 2 foot edge's gap, about 1/16 inch, right at the limit, and a ridge under a 12 inch tile rocks it, so the joint ridge gets ground regardless. The smaller tile carries one prep item.

Run against 12 by 24 tile. One side is 24 inches, so the stricter rule applies. The dip at 3/16 inch against 1/8 inch in 10 feet is 1/16 inch over: fail. The ridge at 1/8 inch against 1/16 inch in 24 inches is 1/16 inch over: fail.

The work the larger tile buys. Two prep items now go on the ticket: grind the curled joint edges down until the 2 foot edge shows 1/16 inch or less, and patch the center dip until the 10 foot edge shows 1/8 inch or less. Grinding concrete releases respirable crystalline silica; the grinder runs with a shroud and an attached dust collector as 29 CFR 1926.1153 Table 1 describes for the task, or the crew falls back to the standard's alternative exposure control methods. See related: Respirable Crystalline Silica and the Construction Standard. The patch and the grind are their own procedure: see related: How to Grind Highs and Patch Lows Before a New Floor.

The re-check. After the patch cures, the installer re-runs both straightedges over the same crayon marks. The dip now shows a gap under 1/16 inch, and the joint shows under 1/32 inch. Both inside the large-tile limits. That second set of readings goes in the job notes with the first, because together they are the record that the substrate met the standard before tile went down.

What changes the answer

• The tile's longest side, not its area. A 6 by 36 inch plank-look tile has a side over 15 inches and takes the strict rule, even though each tile is smaller in area than a 16 by 16. Long, narrow tile is also the most prone to lippage from its own warpage, which is a separate tolerance the tile maker states.
• Offset pattern. Large rectangular tile set in a half offset exaggerates lippage, because the crowned center of one tile meets the edge of the next. Many makers limit the offset for long tile to about a third. That is the tile's own flatness talking, not the floor's, but the two stack.
• Mixed floors in one room. Where tile meets plank, the stricter tolerance governs where the two meet, and the transition height has to come out right on both sides.

Where measuring goes wrong

The common miss is measuring with a 4 foot level because it is in the truck. A 4 foot edge cannot see a 10 foot dip, and a level's bubble invites the conversation about level instead of flat. A straightedge that has been dropped off a truck is no longer straight either: sight it against a chalk line, or lay it down, mark both ends, flip it end for end and look for a gap that changes. The other miss is measuring once, before demolition, and never again. Old flooring, adhesive and underlayment hide the real substrate; the number that counts is the one taken on the surface the new floor will bond to or rest on. How to grid a room and record the map: see related: How to Check a Floor for Flatness and Map It.

References

• ANSI A108.02, current edition (substrate flatness for ceramic tile)
• ASTM E1155 (F-number flatness and levelness of new slabs)
• 29 CFR 1926.1153 (respirable crystalline silica in construction; Table 1 tasks)
• See related: How to Grind Highs and Patch Lows Before a New Floor
• See related: How to Check a Floor for Flatness and Map It