When constructing any new building, one of the most important considerations – among many others – is to ensure that the final flooring is laid on to a surface that is as smooth and flat as possible. This is always important, but especially so when the final flooring is going to be tiling or stone because if the subfloor is not level, then the tiles or stone will not last for very long before they begin to crack.
This is quite logical if you think about it because if a tile is laid on a surface that has a small dip underneath it, when someone walks on that particular part of the tile there is going to be a downward pressure that makes it very easy for the tile to snap, for the simple reason that it is not fully supported in that area. To all intents and purposes, there is nothing underneath it.
Furthermore, the dip does not have to be very deep. A few millimetres is sufficient to cause serious problems.
Unfortunately, a concrete base is extremely unlikely to be level enough to lay tiling on. Certainly, concrete is a strong material and is used to provide a base that will take the weight that is going to be placed on it, whether that is foot traffic or something heavier such as a forklift truck. But getting it level is almost impossible, and this is why another layer of material is laid on top of it in order to produce as level a surface as possible for the final flooring. This is what we call a screed.
A screed can be made of a variety of different materials, and for many years has been good old sand and cement. It is mixed in a ratio of 1.3 or 1.45 cement to sharp sand and is laid and levelled out by a labourer working on hands and knees using a trowel. This is a very hard job, and it takes a lot of time to get the surface of the screed as flat as can be.
Even then, the surface will not be as level as it can be. Surface regularity (SR) is measured by laying a straightedge on the floor over a distance of 2 metres and measuring any variations using a slip gauge. The highest standard is SR1 and allows for a variation of up to 3mm. SR2 is a normal standard and allows for up to 5mm, while SR3 is a utility standard and allows for up to 10mm. Using a sand and cement floor compound in Buckinghamshire, or anywhere else, SR3 is as good as you are likely to get. 10mm is nearly 0.4 of an inch and is not going to be level enough for tiling.
However, over the last few years a new type of screed has been developed using calcium sulphate instead of cement and this is known as anhydrite screed. This is what we use at UK Screeds, and it has several advantages over sand and cement, not the least of which is that it will be far more level. This is because we lay it in a liquid format, and as such it will pretty much level itself. It is similar to pouring wine into a glass: if you leave it alone for a minute the surface will be flat. Not only that, but we use a laser to ensure the level as well.
Another very big advantage is that it is far faster to lay. We pre-mix it and deliver it to site when we then use a long hose and pump to pour it into place. This can be up to 20 times as fast as trying to level a sand and cement screed by hand, and that is a big benefit on any project because it means that there is no delay caused to other contractors. Our liquid screed is ready to walk on within 24 – 48 hours as well.
Using liquid screed also has other advantages, particularly when underfloor heating is being installed because it totally covers the pipes leaving no voids, so the heat transfer into the room is completely even. It can also be laid thinner than sand and cement, and in addition, has nearly twice the thermal conductivity – saving on energy bills.