Liquid screeds are becoming used more and more in the UK, and according to one estimate now account for around 20% of the market for screed of all types. The main reason for using floor screeds in homes is that they are very useful for levelling uneven floor surfaces which is especially important with today’s precast concrete beam and block floors which can have a definite camber, causing problems when it comes to laying the final floor surface.
Liquid screed is also extremely useful for use with underfloor heating because it totally surrounds the heating pipes leaving no voids, and the material has no air pockets. It is also excellent at transferring heat, being highly thermally conductive.
Traditional sand and cement screed has a fairly dry, almost powdery, consistency, and is a relatively strong mix of one part cement to three parts of sharp sand. In days gone by it would be mixed on site, but this is a very labour-intensive way of doing things, and on most sites today, certainly the larger ones, it is delivered to site ready mixed. This also has the benefit that the mix will be consistent, whereas when done by hand shovelling materials into a mixer there can easily be errors creeping in. Ready mixed sand and cement can also have retardants added in order to slow the set and allow the mix to be workable all day.
However, despite improvements in sand and cement screeds, liquid screeds are making definite inroads into the market. These are delivered to site ready mixed and simply poured into place using a screed pump and large hose.
The terminology about liquid screeds can be confusing with words such as "anhydrous", "gypsum", and "calcium sulphate" being bandied about, all to describe the same thing. The important ingredient in liquid screed is calcium sulphate, and "anhydrous" simply means dry. When it is mixed with water, the calcium sulphate turns into gypsum, which replaces cement as a binder, and is the same material found in plasterboard.
However, in the last couple of years, cement has been making something of a comeback, and there are now some liquid screeds consisting of cement instead of calcium sulphate. They are applied in the same way as anhydrite screed and have similar properties as regards being quick to lay and being self-levelling. However, manufacturers claim that they are even faster drying than anhydrite and need no surface preparation because they do not produce laitance in the same way that anhydrite does.
Traditional sand and cement screed is fairly cheap to produce as it can be prepared by unskilled labour. Using ready-mixed, the consistency of the material can be improved, but that makes it more expensive, and the finish will only be as good as the individual laying it. It often has a patchy quality and an uneven finish. It also has a tendency to develop cracks, so sometimes fibres or anti-crack mesh are added. Sand and cement screed can be used with underfloor heating, but also has a tendency to have voids and pockets of air that affect the heat transfer. In addition, it has to be laid relatively thickly, up to 75mm deep, which also adds to the time it takes the room to come to temperature.
Liquid floor screed costs about half as much again as sand and cement screed but there are considerable savings on labour costs because it is so much faster to lay. It is claimed that it can be as much as 20 times faster: certainly, companies that provide liquid screeds claim to be able to lay up to 2,000 square metres in a day. Furthermore, there are materials savings as well, because liquid screed can be laid very thinly: even when laid over underfloor heating it can be as thin as 45mm overall.
It is also self-levelling and perfect for use with underfloor heating because it completely covers the pipes and won’t leave any gaps. The material itself is also better at thermal conductivity. In addition, liquid anhydrite screed does not curl and is highly resistant to shrinkage.
If there is a downside to liquid anhydrite screed it is that as it dries it produces laitance, which is a fine layer of particles on the surface. This means that it must be sanded before laying the final floor covering.