There is no law that says that you have to use a floor screed in the construction of a building, but if you want a floor that is perfectly flat and level, then you need one.
Traditionally, for many years a screed was made of sand and cement, usually with a mix of 1 part cement to 3 parts up to 4.5 parts sharp sand. There are many proprietary screeds on the market.
However, over the last few years a new type of screed has become more and more popular and is a liquid screed which uses gypsum instead of cement. These screeds go under a number of different names, such as anhydrite screed, gypsum screed, liquid screed, calcium sulphate screed, levelling screed, and probably a few others besides, but they are all the same thing. At UK Screeds we usually refer to them as liquid screeds because they are applied in liquid form. The screeds are created by mixing anhydrous (dry) calcium sulphate which – when mixed with water – becomes gypsum, which is the same material found in plaster and plasterboard.
Pros And Cons
Old-fashioned sand and cement screed is cheap to produce and can be mixed on site by unskilled labour and trowelled into place. It is also suitable for use in wet rooms on sloping floors which liquid screeds are not. In addition, it requires very little preparation apart from taping over joints in insulation boards if they are used.
However, sand and cement screeds are very often prone to develop cracks and shrink. They are also frequently of uneven finish because they are laid on by hand. In order to avoid cracking, these screeds are usually laid in 5m long bays and may have mesh or fibres added to them.
In addition, although they can be used with underfloor heating, they very often have air pockets in them which can affect the heat transfer. Furthermore, this type of screed only has around half the heat transfer properties of a gypsum screed. Compared with a liquid screed in Northampton, or for that matter anywhere else, it is laid relatively more thickly, which also affects the amount of heat transfer into the room.
Using ready mix will give a better consistency but even so the finish is often not all that good. Cost-wise, sand and cement is cheap, working out at about £15 per square metre or £20 or so if using ready mix.
On the other hand, liquid screeds have several advantages. At about £25 per square metre they are only a little more than ready mix sand and cement but are also ready-mixed and delivered to site where they are pumped into place through a hose. This is far, far quicker than laying sand and cement by hand, and indeed it is calculated that it is up to 20 times as fast. At UK Screeds we can lay as much as 2,000 square metres in a day. When you take the labour into account, there is not much difference between the two.
Furthermore, liquid screeds can be laid thinner than sand and cement which not only saves on material but also means that heat transfer is better when using underfloor heating. They only need to be laid to a depth of 30mm above the heating pipes. In addition, liquid screeds won’t leave any air pockets as they will totally envelop the heating pipes. The material itself has almost twice the heat transfer properties of sand and cement as well.
Liquid screeds that we install are also self-levelling, because they are liquid, which is why they are often called levelling screeds. They can achieve a levelling standard of SR2 and usually SR1, compared with sand and cement where the aim is to achieve SR3 which allows for a difference of 10mm, plus or minus, over a distance of 2 metres.
In addition, liquid screeds do not curl and are far less prone to shrinkage than sand and cement.
If there is one downside, it is that as they dry, they produce a layer of what is known as laitance on the surface. This must be removed before laying the final floor surface and is done using floor sanding machines.
