Most concrete floors today are covered with a screed of some sort or another. The exception would be a warehouse or a factory where the concrete may be left exposed since there is no need for aesthetics or something such as underfloor heating.

Traditionally, a sand and cement screed has been laid with the object of providing as near a level surface as possible for the final flooring to be laid on top of it. Plain concrete does not provide a sufficiently level surface on its' own, and other floorings such as precast concrete beams and blocks can have a distinct camber which causes trouble when laying the final flooring. Today, another type of screed is rapidly coming to the forefront and that is liquid screed.

Liquid screed can actually be a liquid cement screed but is more commonly anhydrite screed which contains gypsum. There are several advantages to a liquid screed, not the least of which is the speed of laying. Whereas a sand and cement screed has to be trowelled as flat as possible, which means a labourer working on his hands and knees, a liquid screed is delivered to site ready mixed and is then distributed over the cement substrate by a pump. This is known as a self-levelling screed, although it will require some levelling with a laser screed leveller. This type of screed is usually level to SR2 (Surface Regularity 2) although in practice will often be level to SR1 which is as good as it gets.

Following the British Standards Code of Practice, the surface regularity is calculated by laying a 2-metre straight edge on the surface and measuring deviations from the points which are in contact with the floor surface using a slip gauge or similar device. For most final floor surfaces, SR1 must have deviations no greater than 3mm, SR2 5mm, and SR3 10mm. Screeds which are going to receive timber flooring must have deviations no greater than +/- 3mm from the mean when measured over a 2 metre distance using a straight edge.

Types Of Screed

Screed can be laid in different ways. One is bonded screed where the screed is fully bonded to the substrate using a bonding agent or primer. It is frequently used for thinner types of screed where there is going to be a heavy loading on the final floor surface and/or where there may not be enough room to lay an unbonded screed. If no underfloor heating is being installed, the thickness of a bonded screed can be as little as about 15mm for anhydrite screed, or between 25mm and 40mm for a sand and cement screed.

You can also have an unbonded screed where a damp-proof membrane is laid over the substrate and the screed laid on top of that. Here, the flooring is not in direct contact with the substrate so the problems that can be associated with shrinkage or settlement can be avoided. The damp proof membrane also prevents damp rising from the concrete. A cement and sand screed would be laid to about 50mm or more, while anhydrite could be a minimum of 30mm.

What is known as a floating screed is one that is laid on top of a layer of insulation. This creates a floor which is thermally efficient and also has acoustic properties. It is often used where underfloor heating is being installed, and in the case of a sand and cement screed will need to be 70mm thick, and a little more if heavy loads are expected. With an anhydrite screed, the thickness could be installed at as little as 35mm thick for domestic use, and 40mm for commercial.

A floating anhydrite screed in conjunction with underfloor heating will ensure that the heat is conducted evenly across the floor surface without any hot or cold spots and it will also help to retain the heat for longer. Some anhydrite screeds can be installed with as little as 20mm covering the heating pipes, meaning a total thickness of 40mm or less. Another advantage of anhydrite screeds is that they can be force dried only seven days after laying by incrementally turning on the underfloor heating.