When you are constructing a building, one of the most important jobs is to ensure that the flooring is as flat and level as possible and the way to do that is to lay a screed on top of the concrete base before fitting the final floor surface.
There are different types of floor screed, and the traditional mix is one of 1:3 or 1:4.5 cement to sharp sand. The screed can be directly bonded to the base or it can be laid unbonded on to a damp-proof membrane. If insulation is being applied, the screed can be laid as a floating screed over the insulation and this can be combined with underfloor heating pipes. The screed can also be reinforced if required with a fine metal or polypropylene mesh.
Sand and cement screed in Bristol, or anywhere else, can be mixed on-site by unskilled labour and is tipped on to the floor and then levelled out by a worker on hands and knees using a trowel. The sand and cement screed can also now be pre-mixed and delivered to site, and if this is the case it will contain retardants in order to slow the process of drying and keep the mix workable all day. This is necessary because laying the screed by hand is a lengthy process. Pre-mixed sand and cement screed will provide a more consistent mix than hand-mixed on site.
When the screed is bonded to the substrate below the main way that it may fail is if the bond between the substrate and screed fails, and this may happen if the screed is too thick. If an unbonded screed is used, the main way that the screed will fail is if the screed is too thin and lifts or curls. An unbonded screed should be at least 70mm thick and if curling must be avoided has to be at least 100mm. A bonded screed should be thinner – less than 50mm.
Today, we also have liquid anhydrite screeds which are made from anhydrous (dry) calcium sulphate. This becomes gypsum when mixed with water and these screeds are also known as gypsum screeds and calcium sulphate screeds. In addition, they are known as flowing screeds and self-levelling screeds.
The liquid screeds that we install at UK screeds have several advantages over and above sand and cement screeds, Not the least of these is just how fast we can lay them. The screeds are pre-mixed and delivered to site and we then pour them into position using a pump and a long hose. We then use a dappling bar to smooth them out in two directions and remove any air bubbles. This is up to 20 times faster than a sand and cement screed can be laid by hand. After this, they must be left to dry for 24 – 48 hours, at which point it is possible to walk on them, although heavy traffic must be avoided.
Our liquid screeds can be laid unbonded or bonded and can also be used as a floating screed. They are perfect for use with underfloor heating because, as they are liquid, they will completely envelop the heating pipes leaving no voids or gaps as often happens with sand and cement screeds. This means that the heat transfer into the room above will be 100% even. Furthermore, gypsum screeds have nearly twice the thermal conductivity of sand and cement which means that less energy is required to heat the room to the same temperature. Not only that, but our liquid screeds can be laid thinner than sand and cement screeds, which also contributes to heat transference, and in a multi-storey building can result in a considerable reduction in weight.
Our liquid screeds can be laid in large areas as they do not crack or curl, and shrinkage is to the absolute minimum. In addition, this means that they can be force dried after seven days and this can reduce the drying time down to as little as four weeks, at which point the final flooring can be laid.
