CA grades and alpha products on Letchworth projects
An anhydrite specification reads like a code, and each part matters. CA stands for calcium sulphate. Next comes C, the compressive strength, and F, the flexural strength, both measured in N/mm2 to the European standard for screed materials, and the higher the figures, the stronger the floor.
Three grades cover most work. In a house with only light foot traffic, CA-C20-F4 is enough. For the general run of houses, flats and offices, the drawings will usually say CA-C25-F5. CA-C30-F6 is for heavier use, where the floor has to carry more than normal domestic traffic. If your architect has written only "flow screed", we will check the intended grade before ordering, because a lower grade cannot be upgraded once it is in.
Alpha hemihydrate screeds sit apart from these grades. Designers pick them when the floor has to dry sooner or be thinner, such as a flat with little headroom to spare, and every one has its own data sheet setting thickness and drying.
The practical strength of calcium sulphate is low shrinkage. That lets it go down in big bays with hardly any joints, handy for an open-plan flat or the full ground floor of a new house. Bay layout and joint positions are settled before pouring, with particular care at door openings and on the line between heating zones. Our team also lays everything underneath, insulation and edge strip included. Our anhydrite screed explainer goes further into what the material is and how it behaves.
Before flooring goes onto Letchworth anhydrite floors
Three steps come between a dry anhydrite floor and the finish.
First, the laitance. As the screed cures, a thin and weak film builds up on top, and that film has to be sanded away once the floor has dried. Adhesive bonded to laitance can fail.
Second, the moisture test. On calcium sulphate, many flooring makers want the floor drier than they would accept on cement, so a hygrometer figure that passes on a cement floor can fail on this one. On a heated floor, running the system to force dry it can usually start around day 7, which brings the date forward.
Third, the primer. With the surface sanded and the dust cleared, it gets a primer made for calcium sulphate, and only then can adhesive or a levelling compound over anhydrite follow.
We hand this over in writing after every pour. Keep calcium sulphate out of any room that could get wet, and for those spaces we switch to a flowing screed made with cement. For how the screed is pumped into a flat, see our Letchworth liquid screed page.
Anhydrite & Flow Screed prices in Letchworth
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 Letchworth
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 Letchworth
On the map, Letchworth sits north-north-east of Slough, about 35.5 miles from our base in a straight line (postcodes including SG6), next to Baldock (2 miles), Stotfold (2.7 miles), Hitchin (2.9 miles) and Stevenage (5.3 miles).
More on anhydrite screed, part of our screeding services.
