Traditionally, screed is laid around 75mm thick onto a concrete subfloor. Its a skilled job, usually undertaken by plasterers when the walls and ceiling are plastered. A screed mix is relatively strong, using one part cement to three parts sharp sand, and when done well it gives a smooth and level floor on which to lay your chosen floor finish. A screed is also the preferred medium when laying underfloor heating pipes.
You do not have to use a screed. Theres nothing structural about screeds and some designs do away with one altogether. However, the floor beneath the screed is usually not laid to nearly such a high standard. Also, suspended precast concrete flooring systems, which are now increasingly being used instead of solid concrete slabs, have a noticeable camber which causes problems when it comes to laying most floor finishes. So although a screed can be expensive costing between £15 and £20/m² most self-builders choose to go with one.
The traditional way to lay screed is to mix the sand and cement on site with a mixer. Screeders are always looking for the perfect mix. This is pretty dry, almost powdery, when compared to brick and render mortars. The trouble is that mixing this by shovelling sand and cement into a mixer, and delivering to the screeders, keeps a labourer fully occupied in itself expensive. So there has been a marked shift towards using ready-mixed screeds, delivered by lorry at the beginning of the day. In fact, according to Andy Nevitt of Cemex, only a third of sites still use sitemixed screeds. Ready-mixed screeds come with added retardants to delay the set, so that you can be working all day with one load.
To standardise the process, there is a new trend towards pumped screeds, supplied and fixed by specialist crews. These pumped screeds use gypsum as a binder; they are also known as calcium sulphate or anhydrite screeds. They cost about 50% more per cubic metre than a cement mix, but they are much faster to lay and can be successfully laid at 50mm, or even 35mm depths where there are no underfloor heating pipes to cover, whereas a conventional screed needs to be at least 65mm deep.
The process of laying is very different to what we have grown used to with cement screeds. Bevin Pilling, of Selfbuild & Contract Floors, comments: We do try to visit and survey each project before quoting where possible. That way the price we quote is fixed and there are no hidden costs or surprises the biggest one being volume of screed used due to increased depths, especially on irregular sub-bases. It is not uncommon to receive an enquiry for a 50mm depth of screed and to find that it runs from 40mm to 80mm, therefore resulting in increased costs. Typical costs for, say, 100m² at a 50mm depth would be between £1,800 and £3,000.
The actual pumping process is very quick. The screed has to be carefully separated (via a polythene sheet) from any insulation sheeting placed below, because chemical reactions can (rarely) take place.
Whichever system you use, ensure ample time for drying out. The rule of thumb is to allow a day for every millimetre depth, so that 70 days is the standard time recommended. If you have underfloor heating pipes buried in the screed, you can speed the process up but only very carefully: experts recommend doing nothing for a month, then putting the heating on at its lowest setting, turning it up by 3°C per day until the working temperature is reached.
Finally, there is the issue of underfloor heating. Best practice recommends that, to avoid movement issues, screeds should not cover an area of more than 40m2 or a distance in any one direction longer than 8m. Expansion joints should subdivide larger screed areas. Alternatively, you can place an anti-crack mesh in the screed or add fibres to the mix itself.