How to Check a Floor for Flatness and Map It

Why this matters

Almost every hard-surface failure that is not moisture is flatness. A click-lock joint that sits over a dip flexes every time someone steps on it until the locking profile breaks; tile set over a hump rocks, cracks, and shows lippage; glue-down plank over a low bridges and sounds hollow. The flooring maker states a flatness requirement, and a floor that does not meet it voids the warranty whether or not anyone measured. Prep is where flatness is fixed, and you can only fix what you have found and written down.

This card is the procedure for finding it: a grid of readings, marks on the floor, and a correction plan with a fix assigned to every out-of-tolerance area. The tolerance numbers by floor type are owned by 'Floor Flatness Tolerances by Floor Type'; correcting the highs and lows is 'How to Grind Highs and Patch Lows Before a New Floor'.

Flat is not level

A floor can be out of level by half an inch across a room and still be flat, and a perfectly level floor can have a dip that breaks a click joint. Flatness tolerances are stated as a maximum deviation over a span (for example, a gap under a straightedge of a stated length), because a plank or tile only feels the floor under it and beside it. So the tool that matters is the straightedge, or a laser used to compare points a known distance apart, not a bubble level.

When a maker states two spans (a long span and a short one, as many tile and plank instructions do), check both. A floor can pass over 10 feet and fail over 2 feet where a sharp ridge sits between two flat areas.

Before you start

• Tools. A straightedge at least as long as the maker's long span (a 10 ft aluminum straightedge for a 10 ft tolerance, plus a 2 or 6 ft one for short spans), a taper gauge or a set of shims of known thickness, a lumber crayon or carpenter's pencil, a tape, a floor plan sketch, and optionally a self-leveling laser with a grade rod or target.
• The target. The flooring maker's instructions for the product being installed, open to the flatness requirement. Write the target at the top of the sketch before you take a reading.
• The floor. Bare and swept. Any existing resilient floor or black adhesive of unknown age is presumed to contain asbestos until tested, so do not scrape or grind it to get a straightedge down; see 'Asbestos in Old Resilient Floors and Adhesive'.

If you use a laser, keep the beam out of anyone's eyes and set it per its label class.

The procedure, run on one room

The room for this whole walkthrough: a 16 ft by 20 ft family room on a slab, getting a click-lock rigid core plank. The plank maker's instructions, read for this product, state 3/16 in over 10 ft. The slab has a sawcut control joint running east-west across the room 6 ft in from the south (patio door) wall.

Step 1: Sweep, then look at the floor low and long

Sweep and vacuum so a grain of grit cannot hold the straightedge up. Then put your eye near the floor with a light source low across it (a work light on the floor, or the low sun through the patio door). Ridges, curled joint edges and trowel lines show as shadows.

In this room: the low light shows a ridge along the control joint and a shallow dish near the northeast corner. Neither is measured yet; they tell you where to look harder.

Step 2: Run the straightedge on a grid, both ways and diagonally

Lay the long straightedge in lines about 2 ft apart, first north-south, then east-west, then on both diagonals, overlapping each placement by about half its length as you move it along a line. At each placement, watch for two things:

• It bridges (touches at both ends, gap in the middle): that is a low. Slide the taper gauge or shims under at the deepest point and read the gap.
• It rocks (pivots on a point): that is a high. Balance it on the high so the gaps at the two ends are equal and read them; that end gap is how far the high stands above the floor over that span.

Use the short straightedge across any ridge or edge the long one flags.

In this room: 16 ft of width at about 2 ft spacing is about 8 lines one way, and 20 ft at the same spacing is about 10 lines the other way, plus the two diagonals. That is around twenty lines, and on a clean slab it takes one person under an hour.

If you use a laser instead. A laser gives elevations, not flatness, so it takes one more line of arithmetic. Read the rod at points along a line, then for any span the maker states, compare the middle point to the straight line between the two ends: the deviation is the middle reading minus the average of the end readings. Say the rod reads (distance from beam down to floor, so a bigger number is a lower floor) 3.50 in at 0 ft, 3.78 in at 5 ft and 3.56 in at 10 ft. The ends average (3.50 + 3.56) / 2 = 3.53 in, and 3.78 - 3.53 = 0.25 in, so the floor at 5 ft sits 1/4 in below the line between the ends, which is what a 10 ft straightedge laid there would show as a gap. A laser earns its setup time on big open floors and on slabs headed for a full pour, where the same grid also gives the pour depths. On a single room, the straightedge is faster and reads the span directly.

Step 3: Mark it where it is, with the number

Circle each high with the crayon and write the measured height inside it. Outline each low at the edge where the gap closes to nothing, and write the deepest gap inside. Mark the straightedge length used if more than one was used.

Wax crayon is a bond breaker under patch and adhesive. Marks inside an area that will be patched or glued get scraped or ground off before any primer goes down, and that note goes on the plan.

In this room:

FAMILY ROOM 16 x 20 ft, slab, rigid core plank
Target 3/16 in in 10 ft (maker). Grid 2 ft + diags.
            N wall
 +--------------------------------------+
 |  (H1 +1/8)              .---------.  |
 |                         | L2      |  |
 |                         | -5/16   |  |
 |                         '---------'  |
 |    .--------------.                  |
 |    | L1 -1/4      |       (H2 +3/32) |
 |    '--------------'                  |
 |=(H3 +5/16 ridge, 9 ft along joint)== | <- sawcut joint
 |                                      |   6 ft from S
 +--------------------------------------+
        S wall, patio door
 ( ) = high, circled    .--. = low, outlined

Step 4: Read the map before writing the plan

Two readings can be one problem. A low beside a high often reads deeper than it is, because the straightedge rests on the high. A high that sits between two lows may be the original floor, with the lows being the real defect. Ask, for every low, what the straightedge was bearing on.

In this room: L1 sits just north of the ridge H3, and the straightedge that read L1's 1/4 in was bearing on H3 at its south end. Part of L1's depth is H3's height. L2 in the northeast is the dish the low light showed, and the straightedge across it bore on flat slab at both ends, so its 5/16 in is real.

Step 5: Write the correction plan

For each mark, write the measured value, compare it to the target, and assign a fix: grind, patch, a full pour, or none. Highs come first in the sequence, because grinding changes what the lows read.

In this room:

Three of five marks need work, and one of the three (L1) may not after H3 is ground. The plan also records: crayon inside L1 and L2 outlines to be scraped before priming; the ridge along the joint is curled slab edge, so the joint itself stays a joint, and grinding takes the edge down without filling the joint (movement and isolation joints in the slab are honored by the flooring maker's rules, not patched over).

Step 6: Decide whether spot fixes are the right fix at all

Count the marks and their area. When lows cover a large share of the floor, or the floor has a general slope with highs and lows everywhere, spot patching becomes more work than a full pour and leaves a floor that is flat in patches. That is the point to switch to 'Self-Leveling Underlayment Application', which owns whole-floor pours.

In this room: L1 and L2 together are under 25 sq ft of a 320 sq ft room, under a tenth of the floor. Spot fixes are right here.

Finish test

Every area of the floor is either within the target with a recorded reading or carries a mark with an assigned fix, and the plan is on the job record with the target and its source written on it. After the fixes, the same straightedge goes back over every mark, and the plan gets its re-check readings written next to the originals.

What changes the procedure

A wood subfloor. The highs are usually crowned joists or a lifted panel edge, and the lows are often between joists or at a sagging joist. Mark the joist lines first so the map reads against the framing; a high that runs exactly along a joist is a joist, and the fix belongs to 'Subfloor Assessment + Repair Reference', not to patch.

Tile, especially large format. Tile makers commonly state a tighter short-span requirement than plank makers, and lippage shows over a short distance. Run the short straightedge across the whole floor, not only across flagged spots.

A slab that will be ground anyway (a polished or coated finish being removed) gets mapped after the grinding, not before, because the removal changes the surface the map describes.

References

• See related: Floor Flatness Tolerances by Floor Type
• See related: How to Grind Highs and Patch Lows Before a New Floor
• See related: Self-Leveling Underlayment Application
• See related: Subfloor Assessment + Repair Reference
• Flooring maker's installation instructions for the flatness requirement and span for the product being installed.