Anhydrite grades for Harpenden homes and flats
The designation on a calcium sulphate screed tells you its strength. Under the European screed standard the code works like this: CA stands for calcium sulphate, then the C value gives compressive and the F value flexural strength, each in N/mm2. Domestic floors that get little wear can take the lighter CA-C20-F4. CA-C25-F5 covers the bulk of houses, flats and office floors. CA-C30-F6 is the heavier duty option.
Purpose-built blocks hold 16% of Harpenden's homes. Apartment floors are a natural fit for calcium sulphate: large continuous areas poured in one visit, low shrinkage so bays can be big, and even cover over heating. In a block like that, CA-C25-F5 would be the normal choice.
Alpha hemihydrate products appear in some specifications, chosen where faster drying or a thinner section matters. That can suit a Harpenden retrofit where every millimetre counts. Their own data sheet then governs depth and drying time, and that sheet overrides any general rule we would otherwise apply.
Low shrinkage is what lets anhydrite run across a whole open plan ground floor with few joints. We still plan joints before the pour: at door thresholds, where heating zones divide, and wherever a specification asks for them. Agreeing that layout early, with the heating engineer as well as the designer, avoids cracks later.
If the grade on your drawings is unclear, anhydrite screed explained sets out the background.
Sanding and moisture testing Harpenden anhydrite floors
Calcium sulphate dries with a weak surface skin called laitance. Anything bonded on top, whether adhesive, primer or a levelling compound, would grip that skin and not the screed, so it has to go first. With the moisture down far enough, we sand that skin away and put on a primer made for calcium sulphate.
Moisture is judged by testing, not by the calendar. Flooring makers often hold calcium sulphate to a lower moisture limit than cement, so a Harpenden floor that reads dry enough for tiles on a sand and cement base can still be too wet here. We test with a hygrometer and share the reading. If pipes are already in the screed, force drying through them can generally start around day 7.
Under stone, porcelain or engineered wood, a failed bond means lifting an expensive finish, so the test is worth the wait.
Wet areas are the exception. Calcium sulphate goes soft if it stays wet, so wet rooms and any utility floor that could flood get a cement based flowing screed or sand and cement laid by hand in Harpenden. If the floor then needs a thin smoothing layer, see levelling floors in Harpenden.
Anhydrite & Flow Screed prices in Harpenden
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 Harpenden
K & B Screeding’s own team, led by director Donatas Bardisevicius. Founded 2019, company 12298780.
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Anhydrite & Flow Screed from Slough to Harpenden
Harpenden is about 23.2 miles north-north-east of our Slough base as the crow flies, with postcodes including AL5 and AL3. The nearest towns to it are St Albans (4.5 miles), Luton (5.1 miles), Welwyn Garden City (6.5 miles) and Hemel Hempstead (6.6 miles).
More on anhydrite screed, part of our screeding services.