Reading an Ewell screed specification: CA grades explained
Calcium sulphate screed is specified by a short strength code on the drawings. The C figure gives compressive strength, while the F figure gives bending (flexural) strength, each measured in N/mm2. For a lightly used domestic room, CA-C20-F4 is enough; CA-C25-F5 is the usual pick for houses, apartments and offices; and CA-C30-F6 is kept for floors that take harder wear. If your architect or engineer has named a grade, we pour to it; if not, we can advise.
Within calcium sulphate sits a second group, the alpha hemihydrates. People pick them when a floor has to dry sooner or be laid thinner, and since every one comes with its own data sheet, that sheet sets the rules for the pour instead of a rule of thumb.
What makes anhydrite attractive is how little it shrinks. It can be laid in large bays with few joints, which suits the open ground floors of a bigger rebuild such as those possible on Ewell Downs. Purpose-built flats make up 18% of Ewell's homes, and apartment floors in blocks like these are a typical place for a CA-C25-F5 specification over insulation. We plan the bay and joint layout before the pour, especially where screed crosses doorways or meets a different base.
Water is its weak point. Keep it out of wet rooms, shower areas, outbuildings and anywhere else a floor could get wet; for those spaces we lay a flowing screed with a cement binder, which goes in by pump just like anhydrite. For the chemistry in more depth, see our anhydrite screed explainer.
Laitance, moisture and primers on Ewell floors
As anhydrite cures, a fragile skin known as laitance forms on top. Leave it there and adhesive or levelling compound may let go, so it is normally sanded off once the screed is dry enough. We explain this in writing at handover.
Checking moisture is more critical here than on a cement floor. Plenty of flooring makers allow less moisture in calcium sulphate than in cement screed, which means a reading that would be fine for tiling sand and cement can still fail on anhydrite. We take the reading with a hygrometer; whether it passes depends on the limit your chosen flooring sets.
Heating speeds this up. Where there are heating pipes in the floor, they can normally force dry it from about day 7, in line with the commissioning schedule.
After sanding, and once the reading passes, it gets a primer made to work on calcium sulphate. Some primers and adhesives made for cement do not suit it, so check with your flooring fitter before they arrive. If the surface still needs smoothing, a thin compound goes on after priming; our floor levelling page covers that stage, and floor preparation covers mechanical work on larger areas.
Anhydrite & Flow Screed prices in Ewell
Liquid flowing screed (which includes anhydrite): £24 to £31 per m² supplied and laid (MyJobQuote, August 2026), so £960 to £1,240 for 40 m². MyJobQuote does not say whether these figures include VAT. See our floor screeding cost guide or screed calculator.
Who lays your anhydrite and flow screed in Ewell
K & B Screeding’s own team, led by director Donatas Bardisevicius. Founded 2019, company 12298780.
See what customers say in our Google reviews and on our testimonials page.
Anhydrite & Flow Screed from Slough to Ewell
As the crow flies, our yard in Slough is about 21.2 miles from Ewell, which lies to the south-east. Postcodes here include KT17 and KT19, and Epsom (1.4 miles), Kingston upon Thames (3.2 miles), Sutton (3.6 miles) and Merton (4.5 miles) are close by.
More on anhydrite screed, part of our screeding services.