There is no law that says that you have to screed a floor. It adds to the cost of a construction, and it can delay other workers on a site because it has to be allowed time to dry until it can be walked upon. However, drying until it can be walked upon is one thing: it is a completely different ball game waiting for a floor screed to dry to the point where it is safe to lay the final floor.

This is normally calculated as drying at a rate of 1mm a day up to 40mm thick. If the screed is thicker than that, then the remaining thickness will only dry at ½mm a day. So a screed of 50mm thick will take 60 days to dry before you can lay the final flooring.

However, that assumes ideal drying conditions, and in fact that same screed may not be ready for the final flooring for three months or so.

So why use screed at all? The answer is to ensure that the final floor is laid on a material that is as near level as it can possibly be, and this is why we use screeding in Dorset. If the floor is not levelled, there can be all sorts of problems down the line. Tiling, in particular, faces the problem of the tiles cracking. A screed is laid so that it produces as levelled a surface as possible to receive the final floor. The reason is that a concrete floor can have all sorts of rough spots and can also have a camber, so that it is higher at one point than at another.

Screeding in Dorset can take one of several different forms because there are different types of screed mixes available. The traditional type of screed mix is sand and cement, with one part of cement and between three and five parts of sand. These screeds are a fairly dry, powdery mix. Typically, they would be prepared on site in times gone by, but shovelling sand and cement into a mixer takes a lot of manpower and can obviously lead to inconsistencies in the mix. This type of screed can be used with underfloor heating but is prone to holding air pockets which can prevent satisfactory heat transfer.

This is why many larger sites now use ready-mixed screeds delivered by a lorry. These can now also come with retardants which will delay the set, so that a screed can be workable for several hours. This produces consistency in the screed, but unless the screeder is skilled it can still produce results which are short of what is desirable, and the ready mix also costs more.

However, today there is a new kid on the block in the form of liquid calcium sulphate screeds, or anhydrite screeds, which contain gypsum. They cost a little more than a ready mixed sand and cement screed but have several advantages. Since they are liquid, they are delivered ready mixed to site and poured on through a large hose. This makes them very quick to lay. Claims of up to 20 times faster than sand and cement screed have been made, and companies that provide these screeds say that they can lay as much as 2,000 square metres in a day. That saves a considerable amount on labour.

Because these screeds are liquid, they are also self-levelling. Furthermore, they can be laid much thinner than sand and cement screeds. Sand and cement over underfloor heating would typically be laid to a depth of 70mm – 75mm, whereas liquid anhydrite can be laid to just 30mm above underfloor heating pipes giving a total of 45mm which obviously saves a lot on material cost. In fact, these liquid screeds are ideally suited to underfloor heating because they totally envelop the pipes with no voids or gaps, and so transfer heat much more effectively. The actual material of the screed is also more thermally conductive than sand and cement, so the ongoing cost of energy to power the heating will save a considerable amount of money over the years.

There is one other consideration with liquid anhydrite screeds, and that is the film of laitance which forms on the surface as the screed dries out. This needs to be sanded off before the final flooring is laid, so there is an extra cost involved with that. However, all things considered, liquid anhydrite screeds are the future.