Tiling on Anhydrite Screed: 0.5% Moisture, Sand, Then Prime

Tiling on anhydrite screed goes wrong in one predictable way: a cement-based adhesive is spread straight onto a calcium sulphate floor, the two react, and months later the tiles lift with a hollow sound underfoot. The screed itself is a very good base for tiles. It is flat, it suits underfloor heating, and it does not curl like a sand and cement screed can. What it needs is three things done in order: let it dry to the right figure, sand off the surface crust, and put a primer barrier between the gypsum and the cement. This page covers each step with the numbers the manufacturers and the technical press actually publish, so you can check your own floor or your tiler’s method before a single tile goes down.

We are Bromley Tilers, and liquid screeds now turn up in most of the new-build and extension floors we are asked to tile in Bromley and south east London, usually over wet underfloor heating.

What anhydrite screed is, and how to tell you have one

Anhydrite screed is a gypsum-based screed. You will see it sold and described as calcium sulphate screed, liquid screed, alpha-hemihydrate, or under trade names such as Gyvlon. It arrives in a lorry as a fluid, is pumped across the floor through a hose and self-levels, which is why it is so popular over underfloor heating pipes: it flows round them with no air pockets.

If you are not sure what is under your floor, the clues are:

It was poured, not trowelled. A sand and cement screed is laid semi-dry and compacted by hand; a liquid screed goes down in one pour and is dappled with a long bar to release air.
The surface is very flat with a skin on it. Fresh anhydrite dries with a thin, slightly shiny crust called laitance, which can be dusty and friable when scraped.
The paperwork says so. On a new build or extension, the specification, the screed contractor’s invoice or the delivery ticket will name the product. Ask before you tile, because the preparation is completely different from a cement screed.

How long anhydrite screed takes to dry before tiling

The rule of thumb quoted by BS 8204-7 and repeated in the manufacturers’ tiling guides is one day per millimetre for the first 40mm, then two days per millimetre for every millimetre over 40mm. That assumes good drying conditions: warmth, moving air and no damp coming up from below.

A 40mm screed: about 40 days.
A 50mm screed: 40 days plus 20 days, so about 60 days.

Writing in Tile and Stone Journal, Palace Chemicals’ technical director Jim Percival puts a screed of over 50mm, drying at 20°C and below 65% relative humidity, at roughly two months. Ventilation, a space heater or a dehumidifier will speed things up.

The calendar is only a guide. The floor is ready when it tests ready, and the target figures are:

75% relative humidity or lower, measured with a hygrometer, or
0.5% moisture content (w/w) or lower, measured with a carbide (CM) test.

That is a tighter limit than a cement screed, which is why anhydrite floors catch people out. A hand-held pin meter pushed into the surface is not a reliable test on gypsum. We cover the drying of both screed types in more depth in how long screed takes to dry before tiling.

Using underfloor heating to dry the screed

If the screed sits over wet underfloor heating, the heating is your best drying tool. Percival’s guidance is that the heating can be brought on gradually after just seven days to help drive moisture out of the screed. Follow the screed manufacturer’s commissioning schedule, which sets how fast the water temperature rises and how long it is held, and keep a record of it: a good tiler will ask to see it.

Two practical points. First, heat with ventilation, otherwise you are just moving moisture into the air of a sealed room. Second, once the floor tests dry, turn the heating down or off while tiles are being fixed and the adhesive cures, then bring it back up slowly. Our guide to tiling over underfloor heating covers adhesive choice and the restart.

Removing the laitance

Every guide we checked agrees on this step: the laitance has to come off mechanically before anything else touches the floor. It is a weak surface skin, and tiles bonded to it are only as strong as the skin.

Surface of an anhydrite screed over underfloor heating showing blistering — The surface of an anhydrite screed laid over underfloor heating, showing blistering in the top skin | Anhydrite screed for underfloor heating with undesirable blistering by GT1976 (CC BY-SA 3.0), via Wikimedia Commons, https://commons.wikimedia.org/wiki/File:2008-12-09_Anhydrite_screed_for_underfloor_heating_with_undesirable_blistering.jpg

When: light abrasion can start about seven days after the pour, which also opens the surface and helps it dry. Once the floor reaches its moisture target, abrade it again just before priming.
How: a floor sander or grinder with a coarse abrasive, then a thorough vacuum. Sweeping leaves fine gypsum dust behind, and primer will not bond to dust.
What you are aiming for: a dull, slightly open surface with no shiny crust and nothing that powders when you scratch it with a key.

If the laitance is left on, the usual failure is that the tiles come up with a thin layer of screed skin still stuck to the adhesive.

Priming: the barrier that stops the reaction

This is the step that matters most. When a cement-based adhesive sits directly on calcium sulphate in the presence of moisture, the two react and form ettringite, an expansive salt. It grows at the bond line and can push tiles off the floor. Percival is blunt about it: anhydrite screeds should never be overlaid directly with tiles fixed in cementitious adhesive without a barrier.

The barrier is a tiler’s primer, and the products differ in how they are diluted, so read the label on the tin you are actually using. Three published examples show the range:

Palace Chemicals’ advice in Tile and Stone Journal: two coats of an SBR or acrylic tiler’s primer, the first diluted 1:1 with water and the second applied neat.
Mandarin Stone’s anhydrite method, written around Ardex products: flood-prime with ARDEX P 51 mixed 1:3 with clean cold water, brush it in, and let it dry to a thin blue film.
Walls and Floors suggests a diluted SBR primer such as BAL Bond SBR at 1 part primer to 3 parts water.

Whichever primer you use, let each coat dry fully before the next one or before the adhesive goes on. A primer that is still wet is not a barrier. For the wider question of which surfaces need primer and which do not, see our page on tile primer.

Choosing the adhesive and laying the tiles

Once primed, a good flexible cement-based adhesive is normal practice. The Mandarin Stone method names ARDEX X 7 R S1 rapid-set or X 7 standard-set flexible adhesive, and gives their pot lives at 20°C: about 30 minutes for the rapid-set version and about 5 hours for the standard-set one. Mix only what you can use in that time.

Solid bed. Use a notched floor trowel, push each tile down with a twist to collapse the ribs, and lift one now and then to check full contact. Large porcelain and keyed-back stone may need back-buttering as well.
Keep off it. Allow roughly 3 hours for a rapid-set adhesive and 24 hours for a standard-set one before walking on the floor.
Perimeter joint. Leave a gap of at least 6mm, the full depth of the tile, where the floor meets the walls, and fill it with a flexible sealant rather than grout. Anhydrite floors over heating move with temperature, and the edge is where that movement has to go.

Can you tile a damp anhydrite screed with a membrane?

Sometimes, with the right system. An uncoupling membrane changes the rules because the tiles are no longer bonded straight to the screed. Schlüter’s guidance for its DITRA matting notes that calcium sulphate screeds normally must not exceed 0.5 CM-% residual moisture at the time of tiling, but that with DITRA the tiles can be laid at less than 2 CM-%. The screed surface still has to be prepared as the regulations and the manufacturer require, and the membrane’s own fixing instructions for gypsum substrates must be followed. You can read the detail on the Schlüter DITRA product page.

A membrane is not a way to skip the laitance or ignore an obviously wet floor. It is a way to tile a few weeks sooner on a floor that is close to, but not yet at, the 0.5% figure. Our decoupling matting guide explains how the matting works and when it earns its cost.

A quick checklist before the first tile

Confirm it is anhydrite, from the paperwork if you can.
Work out the drying time from the thickness, then test: 75% RH or 0.5% CM or lower.
If there is underfloor heating, follow the commissioning schedule and keep the record.
Sand off the laitance and vacuum thoroughly.
Prime with a tiler’s primer at the dilution on its label, and let it dry.
Fix with a flexible adhesive in a solid bed, and leave a 6mm sealed perimeter joint.

For a fuller technical read from a manufacturer’s point of view, the Tile and Stone Journal article on tiling over anhydrite screeds is worth ten minutes.

Frequently asked questions

Can you tile directly onto anhydrite screed?

Not with a cement-based adhesive. The screed needs its laitance sanded off and a primer barrier applied first, because cement and calcium sulphate react to form ettringite, an expansive salt that can lift tiles. With the surface prepared and primed, tiling is routine.

How long should anhydrite screed dry before tiling?

Allow about one day per millimetre for the first 40mm and two days per millimetre beyond that, so roughly 60 days for a 50mm screed in good conditions. Then test it: it should read 75% relative humidity or less, or 0.5% or less by carbide test.

Do you need to sand anhydrite screed before tiling?

Yes. The surface dries with a weak skin called laitance, and it has to be removed mechanically with a sander or grinder and the dust vacuumed up. Tiles bonded to laitance fail with the skin still stuck to the adhesive.

What primer do you use on anhydrite screed?

A tiler’s primer, usually acrylic or SBR based, applied at the dilution given by its manufacturer. Published methods range from two coats (1:1 then neat) to a single flood coat at 1:3. Each coat must dry fully before the adhesive goes on.

Can I turn the underfloor heating on to dry the screed?

Yes, and it is the quickest way to dry it. Guidance from Palace Chemicals says the heating can be brought on gradually after seven days. Follow the screed manufacturer’s commissioning schedule and ventilate the room while it runs.

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contact@bromleytilers.co.uk