Anhydrite screed
Anhydrite screed is a liquid screed made with a calcium sulphate binder. The minimums: 25 mm bonded, 30 mm unbonded, 35 mm floating in a house and 40 mm in commercial buildings, with at least 25 mm over heating pipes.
It dries at about 1 mm a day to 40 mm, can be dried with the heating from day 7, and usually needs its surface sanded and primed before flooring.
Free, no obligation. One local screeding contractor prices it, not five.
Anhydrite and hemihydrate
Both are calcium sulphate. The difference shows on the surface. Anhydrite screeds can form a laitance, a weak skin of fines, that is sanded off before flooring. A traditional mix can form one; a low laitance mix has an additive that stops it. Hemihydrate liquid screed forms no laitance and needs no buffing or grinding.
The figures
| Calcium sulphate | A thin calcium sulphate screed+ | |
|---|---|---|
| Bonded, minimum | 25 mm | 25 mm |
| Unbonded, minimum | 30 mm | 30 mm |
| Floating, house | 35 mm | 35 mm |
| Floating, commercial | 40 mm | 40 mm |
| Over heating pipes | 25 mm minimum, 30 mm nominal | More than 20 mm |
| Thermal conductivity | 2.2 W/mK | 2.3 W/mK certified minimum, 2.5 nominal (BBA) |
| Strength | Not given | Over C30, over F8 |
| Biggest bay, heated | 300 m², 20 m long | Up to 300 m² |
| Biggest bay, unheated | 1,000 m², 40 m long | 1,000 m² |
| Movement while drying | 0.02% at most | 0.012 mm/mK thermal expansion |
Why it's used with underfloor heating
Two reasons: it wraps the pipes fully, with no trapped air, and it conducts heat about twice as well as sand and cement. Thinner screed over the pipes plus better conductivity means the floor reacts faster and can run at a lower flow temperature. Underfloor heating and liquid screed.
What to watch
- Water after it's laid. Protect it from moisture getting back in, which re-wets it and slows the drying.
- Cement on top. Calcium sulphate and cement react. A primer goes down as a barrier under any cement-based adhesive or smoothing compound. Calcium sulphate based products are better where you can get them.
- The wrong primer. Neoprene and SBR primers aren't suitable; use acrylic dispersion or water-based epoxy, usually two coats.
- Gas heaters. No gas or paraffin heaters: they raise the humidity.
- Foil tape. Never use the foil-backed tape that comes with insulation boards.
Liquid screed poured by the contractor who takes your call
Calls on this site ring JAM Floor Screeding: James Mulhall, based in Portlaoise, 25 years laying floors. Lines open Mon to Sat, 7am to 6pm. These are his jobs.
“JAM were punctual, professional and efficient and did an outstanding job on pouring our new floor as part of a renovation. Very highly recommend!”
“Excellent work, laser level finish, neat and tidy worker, highly recommend.”
“Top class work by james and his crew, highly recommend”
Quoted word for word from JAM Floor Screeding's Google reviews. Read them on Google
Get your floor priced in 4 quick taps
Tell us about the floor. It goes to one screeding contractor who covers your county, and they ring you to price it.
Liquid screed guides & price breakdowns
Straight answers to what people ask most: itemised costs, specifications, planning or grants, and what to check before you book.
More guides & resources: Tiles, wood and vinyl on liquid screed · Joints and bay sizes · Is liquid screed right for my floor ? · All guides & calculator hub · Technical & trade glossary · Frequently asked questions · All areas covered · About this site
Anhydrite screed questions
Is anhydrite screed the same as calcium sulphate screed?
Yes. Anhydrite is a form of calcium sulphate. Hemihydrate is another; both are calcium sulphate screeds.
How thin can anhydrite screed be?
25 mm bonded, 30 mm unbonded and 35 mm floating in a house, with at least 25 mm over heating pipes.
Does anhydrite screed need sanding?
A traditional mix can form laitance that is sanded off before flooring. Low laitance mixes need less, and hemihydrate screed needs no sanding.
Can I tile onto anhydrite screed?
Yes, once it's dry and primed. Cement-based adhesive needs 75% RH; a calcium sulphate based adhesive can sometimes go on at 87% RH.
Liquid screed across all 26 counties
Requests from all 26 counties go to one vetted local specialist who covers your county — from Dublin, Cork, Galway and Limerick to Kildare, Meath, Wicklow, Waterford and Donegal. Pick your county below for local prices, specifications and advice. Compare all areas covered →
Get a free quote for your floor
Four taps and your number. One screeding contractor who covers your county rings you with a price. Nothing is booked until you say yes.