What flatness must a screed achieve?

Flatness is the number one cause of disputes on screeding sites, almost always for the same reason: no one specified which requirement applied. However, there is no single standard for flatness; there are different levels of requirement that must be chosen.

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How to measure flatness

The standard method involves placing a 2-meter rigid straightedge on the floor in several directions and at various locations, then measuring the maximum gap between the straightedge and the screed using feeler gauges.

Two other checks complement this one and are often overlooked. The local surface irregularity is measured with a 20 cm straightedge to track down small, localized bumps. The level tolerance, on the other hand, verifies that the floor is at the intended elevation: a screed can be perfectly flat and yet 2 cm too high, which will prevent doors from opening.

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General guidelines

In Belgium, screed flatness classes are defined by Buildwise NIT 189, formerly known as CSTC. The values commonly used for a 2 m straightedge are approximately:

• 5 mm for a standard screed intended for classic-sized tiles;
• 3 mm for higher requirements: glued parquet, large-format tiles, sheet flooring, or resin.

These values are general guidelines: the applicable class and the reference standard must be specified in the specifications or the quote. The final floor covering dictates the requirement, and the larger the format of the materials, the less forgiving the floor will be.

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The habit that prevents disputes

The required flatness is determined before pouring, not after the tiles are laid. If the flooring has not yet been chosen at the time of the quote, the strictest requirement under consideration must be adopted, with the option to relax it later if necessary.

A joint inspection upon acceptance of the screed, using a straightedge and a written report, settles the matter once and for all. And if a discrepancy is found, our sheet on planing or repairing a screed details the possible corrective measures. The thicknesses to be observed, which also influence the final result, are listed in our guide on screed thickness by type and substrate.