Subfloor Flatness Standards (ASTM F710)

Flat and level are different things. A floor can slope a full inch across a room and still be flat. Flatness is what matters to flooring. Humps and dips over short distances show through thin products, open the joints in click floors and cause lippage in tile.

Tolerances at a glance

Every flooring type has its own flatness tolerance. Find yours in the table and check the floor against it.

These figures are common industry summaries.^[1] The manufacturer's installation instructions always win. Many rigid-core and large-format products are tighter than the numbers above.

What ASTM F710 covers

ASTM F710 is the standard practice for preparing concrete to receive resilient flooring. Flatness is one line of it. It also covers moisture and alkalinity testing, cracks and joints, and removing curing compounds, sealers and old adhesive.^[2] See Subfloor moisture testing and Concrete pH testing.

How to check flatness

Checking flatness takes a long straightedge and a few shims. The real work is covering the whole room, not just the middle.

1. Sweep the floor. Debris under a straightedge gives false highs.
2. Lay a 10 ft straightedge in several directions across the room, including along walls and across doorways.
3. Slide shims or a tapered gauge under the gaps and note the largest one in each span.
4. Check short spans (12", 24" or 6 ft, depending on the flooring) with a shorter straightedge.
5. Circle highs in one color and lows in another so the fix crew knows which is which.

Fixing it

Once the highs and lows are marked, the fix depends on how big and how widespread they are.

• Highs: grind them down. See Grinding off thinset, mastic and glue residue and silica dust controls.
• Isolated lows: fill with a patching compound. See Patching and skim-coating.
• Widespread lows or waves: a self-leveling underlayment. See Self-leveling underlayment.

Common mistakes

Flatness problems usually get missed because the check was too quick or used the wrong tool.

• Using a 4 ft level. It's too short to read a 10 ft tolerance, and the bubble tells you about level, not flat.
• Checking only the middle of the room. Doorways, walls and old patch are where problems hide.
• Relying on the underlayment pad. Foam underlayment doesn't fix flatness and can make click joints fail faster.
• Skipping the short-span check on large-format tile. Lippage comes from 24" dips, not 10 ft waves.

Tools

A long, rigid straightedge is the core tool for this job.

• Gundlach 144-RS 146.5" Rigid Steel Straight Edge (covers a 10 ft span)
• Gundlach 75-RS 75" Rigid Steel Straight Edge
• Crain 113 6' Journeyman Straight Edge
• Floor patch and levelers and self-leveling tools

References

1. Custom Building Products: Subsurface tolerances and floor flatness requirements
2. ASTM F710: Preparing concrete floors to receive resilient flooring

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