Assessing the substrate before tiling: flatness, moisture, strength

on Nov 06, 2026

in Installation & DIY Guides, Tiles & Flooring

Assessing the substrate before tiling: flatness, moisture, strength

Short answer: tiles are only as good as the substrate underneath. Check three things before you start. Flatness: lay a 2-metre straightedge in all directions and measure the largest gap; the larger the tile, the smaller that gap may be. Moisture: as a guide value a cement screed must be below about 2 percent residual moisture and an anhydrite screed below about 0.5 percent, measured with the CM method. Strength: the top layer must not be sandy, scratch easily or sound hollow. Only when all three are in order do you prime and start bonding.

Read this article in Dutch as well: Ondergrond beoordelen voor het tegelen: vlakheid, vocht, sterkte.

👷 The guide values in this blog come from practice. The specification, the agreed flatness standard and the instructions of the adhesive and levelling compound manufacturer always take precedence. An experienced tiler or screed layer sees and hears much more than a meter shows. Always consult beforehand when in doubt.

Step 1: measuring flatness

Lay a 200 cm screed straightedge with vials or a 200 cm trapezoid straightedge on the floor, lengthwise, crosswise and diagonally. Measure the largest gap under the straightedge with a tape measure with auto-lock or a measuring wedge. On walls you work with a 120 cm spirit level and also check that the wall is plumb.

Tile size	Guide value under a 2 m straightedge	What if it is more?
Small tiles up to about 30 cm	a few millimetres	often absorbed by the adhesive bed
Medium up to about 60 cm	stricter, preferably under 3 mm	level locally
Large format 60 x 120 cm and larger	as flat as possible, preferably under 2 mm	level before tiling

An adhesive bed is not a levelling compound. Applying adhesive too thick causes shrinkage, hollow spots and height differences between tiles. How to level a floor is explained in our blog on levelling a floor before tiling and our blog on levelling a floor. A reliable spirit level is important here, see our blog on checking a spirit level.

Professional in a helmet holding a spirit level vertically against a plastered wall

Step 2: measuring moisture

Too much residual moisture is one of the best-known causes of loose tiles and discoloured grout, especially with anhydrite and underfloor heating.

Substrate	Guide value residual moisture	Note
Cement-based screed	around 2 percent	often stricter with underfloor heating
Anhydrite screed	around 0.5 percent	with underfloor heating often around 0.3 percent, always prime
Concrete	per adhesive manufacturer	cure for at least several weeks
Plasterboard and gypsum blocks	dry and dust-free	prime, waterproof in wet rooms

With a Hydromette Compact B moisture meter you can quickly find damp spots without drilling. The official measurement for screeds is the CM method, in which a sample is taken from the floor. Have it carried out when in doubt, with anhydrite and with underfloor heating. With an air humidity and temperature meter you also check the temperature and humidity in the room, as these determine how quickly the floor dries.

⚠️ A floor with underfloor heating must be heated up and cooled down according to a heating protocol before tiling. Never skip that, not even under time pressure.

Step 3: strength and adhesion

Scratch test. Scratch a grid into the top layer with a nail or the tip of a multifunctional scraper. If the surface breaks out or loose sand appears, the top layer is too weak.
Tap test. Tap across the floor with a claw hammer. A hollow sound indicates delamination between screed and base. Mark those spots.
Laitance and dirt. A smooth, shiny skin on the screed, old adhesive residue, paint or grease must be removed. Use a long-handled floor scraper and vacuum everything with a construction vacuum cleaner.
Cracks. Small shrinkage cracks can be sealed, but a moving crack or a joint in the screed must be continued in the tiling.

Step 4: priming

An absorbent substrate draws water out of the adhesive. So prime with deep-penetrating primer or use a Primer Uni adhesion primer for mortars on mortars and plasters. Follow the drying time on the label before bonding or levelling. For wall tiles read on in our blog on tiling a wall.

Working safely

Sanding, scraping and vacuuming produce fine dust. Wear an FFP2 dust mask, safety goggles and knee pads. In old floors in houses built before 1994 there may be asbestos-containing material under or in the floor covering. Stop and have it investigated.

Frequently asked questions

How long must a screed dry before tiling?

As a rule of thumb about one week per centimetre up to 4 centimetres thick, slower after that. Measuring remains necessary, because temperature, ventilation and thickness make a big difference.