Wrong screed for the room
Gypsum Screed in a Wet Room: The Problem
Anhydrite screed is a great product in the wrong room. Wet rooms need a screed chemistry that tolerates standing water, and gypsum isn't it.
Get your wet room screed assessed
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Anhydrite (gypsum/calcium-sulphate) screed is excellent for the vast majority of a house. It self-levels beautifully, wraps UFH pipework, and costs less than cement-based alternatives. But it has one hard limit that keeps coming up on site: it does not cope with sustained or repeated water exposure.
Wet rooms, by definition, put water directly onto or very close to the floor build-up on a daily basis. If an anhydrite screed ends up under a wet room without the right waterproofing detail, or without having been substituted for a cement-based screed at spec stage, it will eventually soften, lose strength or debond.
We see this most often in renovation projects where a single screed type was specified for the whole first floor and nobody flagged the en-suite as a special case. Here's what happens, how to check for it, and what the fix looks like.
Why gypsum and water don't mix
Calcium sulphate is soluble in water over time. A tanked wet room with a proper waterproof membrane above the screed protects it from direct wetting, but any breach in that membrane — a poorly sealed drain upstand, a cracked tanking corner — lets water sit against or soak into the anhydrite.
Repeated wetting and drying cycles progressively weaken the screed's surface strength, and in serious cases can cause it to turn soft or crumbly, undermining the tile bond above it.
Anhydrite vs cement-based screed for wet areas
Anhydrite (Gyvlon/Gypsol)
- Excellent thermal performance
- Lower cost per m²
- Vulnerable to repeated wetting
- Needs full tanking above it in wet rooms
Cemfloor (cement-based)
- Stable under repeated water exposure
- Compatible with cement tile adhesives
- Slightly higher cost
- Our default recommendation for wet rooms
Get your wet room screed assessed
If a wet room floor feels soft, sounds hollow or has visible damp, get it checked before more tiles fail. We'll survey, diagnose and quote the fix.
How we check and remediate
- 1
Establish the waterproofing detail
We check whether a tanking membrane exists above the screed and whether it's intact — this often points to the real fault before we even touch the screed.
- 2
Assess the screed condition
Sounding, moisture testing and a scrape test tell us whether the anhydrite has actually softened or is still structurally sound under an intact membrane.
- 3
Remove and re-pour if compromised
Where the screed has degraded, the affected area is cut out and re-poured in a cement-based screed appropriate to a wet area.
- 4
Correct the waterproofing
Any tanking or drainage falls are corrected before re-tiling, so the same failure can't happen twice.
Gypsum Screed in a Wet Room — questions we get asked
Keep reading across the site
Get your wet room screed assessed
If a wet room floor feels soft, sounds hollow or has visible damp, get it checked before more tiles fail. We'll survey, diagnose and quote the fix.