One of the most important qualities of a floor screed is surface regularity. This is often referred to as the measure of “waviness” of the surface. This is measured by placing a straightedge on the surface of the screed and using a slip gauge underneath to measure any gaps. The results are known as SR1, SR2, and SR3.
SR1 means that the maximum gap measured was 3mm, SR2 shows up to 5mm, while SR3 is up to 10mm. SR1 is referred to as High Standard, SR2 is Normal Standard, and SR3 is Utility Standard.
For many years a screed has been made of sand and cement which is mixed on-site in a cement mixer in a ratio of 1:3 cement to sharp sand. However, shovelling sand and cement into a mixer is very labour intensive and there will be resultant inconsistencies, and for this reason, it is becoming more common for larger sites to use ready-mixed screeds delivered by lorry. These ready-mixed screeds contain retardants in order to delay the set so that the screed can be worked all day.
A sand and cement screed is then laid by hand by a worker on hands and knees using a trowel in order to level it out. This is extremely time-consuming and also results in a screed surface that is never likely to be any better than SR3.
However, today there is also the use of a liquid screed which is becoming far more common and is taking over from sand and cement. As liquid screed contractors in Bristol, at UK Screeds we provide and lay liquid screeds which are composed of sand and gypsum, and they have several advantages over sand and cement. Our screeds are known by various names such as gypsum screed, anhydrite screed, calcium sulphate screed, flowing screed, self-levelling screed, and more.
The screed is composed of anhydrous (dry) calcium sulphate which becomes gypsum when mixed with water and is the binder. It is delivered to the site ready-mixed and then we attach a pump and a long hose, and the screed is simply pumped into place. As you can imagine, this is a lot faster than someone working on hands and knees, and in fact, it can be anything up to 20 times as fast. Our teams can lay as much as 2,000 square metres in a single day.
Because the screed is liquid, it is also more or less self-levelling. All we need to do after the screed has been poured is to dapple it two ways in order to remove any air bubbles and then we can just walk away and leave it to set. Better still, our liquid screed will always achieve at least SR2 and usually SR1. So, you get a better finish much faster than with a traditional sand and cement screed.
Not only that, but the screed will be dry enough to walk on in 24 – 48 hours, depending upon the level of humidity, which in turn means that there is a minimal delay to other contractors who need to work on the site at the same time. Time is money on any building project so the less delay for other contractors, the better.
Furthermore, our liquid screeds can be laid thinner than sand and cement. Certainly, the cost of the product is rather more than sand and cement, but because is it laid thinner, we don’t need so much of it. There is also a huge saving on labour compared with a sand and cement screed.
However, there is one thing that our screed needs which sand and cement don’t, and that is that it will need to be sanded after it has dried. This is because as it dries it forms a layer of tiny particles on the surface which are known as laitance, and this needs to be removed before attaching the final floor surface. We will sand the surface about a week after pouring the screed.
Because the screed is thinner, it also requires less overall drying time than sand and cement before the flooring can be attached. So, when all is added up, the costs probably work out at about the same, yet with all the additional benefits.