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Concrete Flatness and Level

Problem

Cause

Cure

ANSI A108 flatness standards for • Large format tile (any one side greater than 23 in. [0.58 m]) require no greater than 1/8 in. in 10 ft. (3 mm in 3 m) (ACI Class A) and 1/16 in. in 2 ft. (2 mm in 610 mm); rough correlation FF equal to 50 per ASTM E1155 • Less than 15 in. no greater than 1/4 in. in 10 ft. (6 mm in 3 m) (ACI Class B) and 1/16 in. in 1 ft. (1.6 mm in 305 mm); ACI 117-10 concrete industry standards • Allow 1/4 in. in 10 ft. (6 mm in 3 m) (ACI Class B); local deviation of 3/8 in. (10 mm) allowed; rough equivalent of overall FF equal to 25- 35 • Disparity exists between tile and concrete industries’ maximum allowable substrate flatness tolerances ACI 117 concrete industry standards Allow plus or minus 0.3% deviation from level elevation in 10 ft. (3 m) (3/8 in. [10 mm]) Concrete slab flatness deviations caused by: • ASTM E1155 flatness testing not required across control or column isolation joints; curling of slab on grade (especially at control joints) • ASTM E1155 flatness testing conducted within 72 hours before all shrinkage/ curling; slab flatness can change over time despite passing test; Examples – dead load (weight) deflection of elevated concrete slab or continued shrinkage/curling of slab

• Recommend specification/installation of flowable/self-leveling underlayment or bonded/unbonded mortar bed to achieve required flatness for large format tile. See note below. Note: Thinset or Large and Heavy Tile (LHT mortar) may not be used for truing or leveling.

Acceptable concrete substrate flatness tolerance

Concrete slab flatness improved by: • Specification and quality control testing of slab per ASTM E1155 after concrete placement • Informal laser level/dipstick testing • Re-check for changes prior to tile installation • FF 25-35 correlates with 1/4 in. in 10 ft. (6 mm in 3 m) (standard tile flatness tolerance) • FF 50-60 correlates with 1/8 in. in 10 ft. (3 mm in 3 m) (large format tile flatness tolerance) Concrete slab flatness corrected by: • Grinding/scarification • Mortar bed, patching or self-leveling underlayment • FF 50-60 or 1/8 in. in 10 ft. (3 mm in 3 m) (large format tile flatness tolerance) will require mortar bed or flowable/self-leveling underlayment prior to tile installation

Excessive concrete substrate flatness deviations

• Improper placement or finishing of concrete surface; surface defects

Tile edge lippage caused by: • Excessive concrete flatness deviations can cause excessive lippage (greater than or equal to 1/8 in. (3 mm) joint)

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NTCA Reference Manual | 2024 / 2025

Chapter 2 | Substrates