For solid floor construction, a normal sand/cement floor screed can be used. No special additives in the screed are required. Where the pipe is laid on insulation, the minimum screed depth must be 65mm for domestic applications and 75mm for commercial applications as specified in British Standards. The optimum screed thickness is 75mm but no more than 90mm, for most applications. Where heavier floor loadings are required, the construction engineer should advise on the screed thickness.
Specialist Anhydrite (Calcium Sulphate) Screeds, when used with underfloor heating, must provide a minimum 30mm coverage over a pipe or conduit. Careful consideration must be given to the expansion of heated screed floors. As a guide when using semi-dry cement and sand screeds, BS EN1264 - Part 4 recommends a maximum screed area of 40m2 can be laid without expansion allowances. With Anhydrite screeds much greater areas can be laid without expansion joints; e.g. in Sports Halls up to 600m2.
When using an Anhydrite screed always check with the supplier for their requirements. The screed must be allowed to dry and cure normally, in accordance with the relevant BS Standards and manufacturer and supplier instructions, before initial heating and system start-up. The underfloor heating MUST NOT be used to speed up the curing process.
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