When laying flooring in a new building, or for that matter in a refurbishment project, it is necessary for the subfloor to be as level as possible, and this is achieved by the use of a screed. A screed is a material which can be laid over concrete in order to level out the dips and bumps as far as possible, and for many years has traditionally been made of sand and cement. However, today, calcium sulphate screeds are becoming more and more popular because of the advantages that they have over sand and cement, and they are the screeds that we supply and lay at UK Screeds.
These screeds are also known as gypsum screeds. When water is added to calcium sulphate it turns into calcium sulphate dihydrate which is more often known as gypsum. The screeds also go under names such as self-levelling screeds, floor compounds, anhydrite screeds, and more, but all refer to the same product.
Our screeds have many advantages over sand and cement, not the least of which is the speed of laying. Because they are delivered in liquid form, we lay them using a hose and a pump. This means that laying can be as much as 20 times faster than laying a sand and cement screed by hand. We can often lay as much as 2,000 square metres in a day. Furthermore, when laying our floor compound in Berkshire – or anywhere else – it will be dry enough to walk on in 48 hours or less. Those two things combined mean that there is no disruption to other contractors who may need to work on the site.
Not only that, but our self-levelling screeds are perfect for use with underfloor heating. When installing water-based underfloor heating, the screed is poured over the heating pipes. Because it is in liquid form, it totally envelops the pipes and won’t leave any gaps or air pockets, which is what usually happens with a sand and cement screed. When you have air pockets, it interferes with the thermal conductivity, so more energy is required to heat the room, and the heat will not always be perfectly even.
In addition to that, the gypsum material itself has almost twice the thermal conductivity properties of sand and cement, so less energy is required to bring the room to the level of heat desired.
It gets even better, because the screeds that we produce at UK Screeds can be laid far less thickly than sand and cement. Our liquid screed only needs to be 25 mm above the heating pipes, making a total of 40 mm – 45 mm as opposed to the 70 mm+ of sand and cement, so again less energy is used. All of this put together means that our screeds are going to use less energy all round, and in turn that makes them more environmentally friendly.
As with sand and cement screeds, our liquid screeds need to be fully dry before the final flooring is laid, and if underfloor heating is being installed this can be turned on after the screed has been laid for a week or so. The heat should be increased by 5°C each day until the maximum required working temperature is achieved, but should not exceed 55°C. the maximum temperature should be maintained for a week or more, and then gradually reduced to between 15°C and 20°C. In addition, dehumidifiers can also be used.
Before the final flooring is laid it is necessary to remove the layer of laitance that has formed on the surface of the screed. This is a layer of fine particles that forms on the surface as the screed dries and is removed by sanding. This should be carried out between 7 and 10 days after the screed is initially laid: if left longer, the laitance will become harder and more difficult to remove.
If laying tiling, it is recommended to apply a primer to the screed surface before using the adhesive. This creates an effective barrier and makes the surface more stable. It is also necessary to use the appropriate adhesive as some are not suitable for use with calcium sulphate screeds.
