Calcium Sulphate screeds, also known as liquid screeds, gypsum screeds, self-levelling screeds, anhydrous (dry) screeds, and a number of other terms, have many advantages over and above the traditional sand and cement screeds that have been used for years.
One of the big benefits is that these screeds do not need to be laid as thickly as sand and cement. This is because they do not curl, and they have minimal shrinkage. Sand and cement screeds suffer from both of these possibilities, and they need to be laid thicker in order to avoid curling, and also cracking. Although liquid screeds cost more than sand and cement, the fact that you use less of them balances out the extra cost.
With either type of screed, you have to wait until it is fully dried before the final flooring can be laid upon it. When using a liquid floor screed in Slough, it needs to achieve the correct level of moisture content for the required floor finish. Typical values for moisture sensitive floor finishes are less than 0.5% water by weight or less than 75% relative humidity (RH) for impermeable floors and less than 1% w/w for more permeable floors.
Drying times for screeds are calculated as 1mm per day up to a depth of 40mm and 0.5mm per day for anything thicker. A typical thickness for a liquid screed covering underfloor heating pipes would be 45mm overall, with the pipes covered by 30mm of screed. So, the drying time would be 50 days.
However, the drying times are based on an average air temperature of 20°C and relative humidity of no more than 65%. In practice, in the UK these conditions are unlikely to be met for 45 or 50 days, so in fact the drying time will be longer. However, it will still compare favourably with a sand and cement screed which would need to be a minimum of 70mm thick and very often quite a lot more.
Another Big Benefit
But our liquid screeds have another big benefit in that you can use the underfloor heating, if installed, to assist with the drying. Once they have been laid for a week or so, the underfloor heating can be commissioned and then increased by 5°C per day until the maximum working temperature is reached. It can be kept at this level for a minimum of one week and then gradually reduced to around 15 – 20°C.
However, when you use liquid screeds you can also force dry them as well. Using the appropriate force drying techniques can bring the overall drying time of the screed down to as little as 28 days, at which point you can lay the final flooring.
There are several other benefits to using liquid screeds, especially when used in combination with underfloor heating. They use a large proportion of recycled material so are more environmentally friendly. Because they are laid thinner, they use less material. This also has benefits for multi-floor buildings because that can add up to a significant height saving, and of course, thinner flooring means savings on overall floor weights.