Reading an anhydrite specification for Southwark blocks
The code on a calcium sulphate specification comes from the European standard that covers screed materials. C gives the compressive strength in N/mm2, and F gives the flexural strength in the same units. In practice you meet three of them. CA-C20-F4 is adequate for a lightly used bedroom or hallway; homes, apartments and offices normally get CA-C25-F5; anything heavier needs the CA-C30-F6 grade.
On regeneration schemes such as Elephant Park, the phased rebuilding of the Aylesbury in Walworth or the Canada Water masterplan, the grade is set by the project's engineer and we pour exactly what is specified. Now and then an engineer specifies an alpha hemihydrate product rather than one of those grades. These get picked when the programme needs quicker drying or the floor needs a shallower layer, and their depths and drying times come from the maker's own product sheet, which is what we follow.
Calcium sulphate shrinks very little, and that is its main strength. So an entire flat on one of these schemes goes in as a small number of large bays, and joints are kept to a minimum. The few that are needed, at doorways or where one heating zone meets another, are set out on plan before pouring starts. That is different from the everyday question of whether to pump a screed at all, which the Southwark liquid screed page deals with. For more on calcium sulphate as a material, read our anhydrite guide.
If a floor is wrong for calcium sulphate, we can pour a flowing screed with a cement base instead.
Sanding, testing and wet areas in Southwark flats
With calcium sulphate, walkable and ready for flooring are two separate milestones. During drying, a fine, weak film known as laitance rises to the surface. It must be removed, because adhesive and levelling compound will not grip it. Once drying is far enough along, that film gets sanded back, and the bare screed is sealed using a primer that works with calcium sulphate.
Moisture is the next check. Many flooring makers set calcium sulphate a tighter moisture figure than they set for cement, which means a hygrometer result that tiles on a cement screed would accept may still fail on this one. Readiness is judged with a hygrometer, not by the date. In flats with underfloor heating, switching it on from about 7 days usually lets us force dry the screed.
Bathrooms and wet rooms in a flat are another matter, as is anywhere the floor might stay damp. Calcium sulphate gets weaker the longer it sits wet, so those rooms change to a flowing screed with cement as its binder, or to a hand-finished sand and cement base. Bases that need attention before the pour are covered under floor preparation.
Anhydrite & Flow Screed prices in Southwark
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 Southwark
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 Southwark
Southwark is about 24.7 miles east of our Slough base as the crow flies, with postcodes including SE1, SE5, SE21 and SE22. The nearest towns to it are Lambeth (2.1 miles), Lewisham (3.2 miles), Tower Hamlets (3.5 miles) and Westminster (4.2 miles).
More on anhydrite screed, part of our screeding services.