Screed mix and thickness

The effectiveness of underfloor heating in a screeded floor is, in part, reliant on the quality of the screed. A poorly mixed screed, unevenly laid with many air pockets will conduct less heat than a smooth, well compacted, level mix. Plasticisers should generally be avoided as their characteristic entrainment of air into the screed may slightly diminish the heat transfer. The consistency of the screed mix must be maintained across the entire floor.



Standard screed is 3:1 or 4:1, sharp sand to cement, semi-dry biscuit mix. A polypropylene fibre additive may be used to improve surface appearance and strength.



A screed thickness of 65mm is recommended, to match the height of the Fastfit™ distributor floor box whose lid and top surface will be flush with the top of the screed (10mm Fastflo® systems only). As a thicker screed will lead to increased warm-up times, screeds should preferably be kept below 85mm. Screeds more than 100mm thick will be difficult to control and should not be used.



Alternatively, liquid screeds are available and may be laid to a minimum of 50mm. Liquid screeds have good heat transfer characteristics and are strongly recommended for systems powered by a heat pump or for other systems using reduced water temperature. Get advice from the screed supplier as to what screed thickness is suitable.