Liquid screeds have grown in popularity considerably over the last 20 years because of the distinct advantages that they have over and above the traditional sand and cement screeds. They are very easy to lay, simply being pumped into position with a long hose, and they can be laid far faster than sand and cement which is trowelled into position by hand.
Liquid screeds, or liquid floor compounds, also dry faster than sand and cement, and can indeed take foot traffic within 24 – 48 hours of pouring. They also have minimal shrinkage and do not crack as sand and cement can. They have benefits when used in conjunction with underfloor heating because the thermal conductivity is almost twice that of sand and cement. That means that less energy is required to achieve the same temperatures. Not only that, but they can be laid thinner than sand and cement, which typically needs to cover the heating pipes by as much as 75mm, whereas the liquid anhydrite screeds only need to have a depth of 30mm above them.
Liquid screeds use anhydrous (dry) calcium sulphate and aggregates as a binder and when water is added to it the calcium sulphate becomes calcium sulphate dihydrate which is commonly known as gypsum. The reaction stops when the calcium sulphate binder is mostly used up which is generally after 3 – 7 days and leaves the rest of the water to evaporate through the surface of the screed.
Before you tile onto a calcium sulphate screed it is necessary to ensure that it is dry enough and this is calculated as being a drying rate of 1mm per day up to a depth of 40mm and 1/2mm per day for anything over that. So, for example, if the depth was 50mm the total drying time would be 60 days.
However, this is based on an average temperature of 20°C and humidity levels of not more than 65% throughout the whole of the drying time, and it is unlikely that this can be achieved in UK weather conditions. So, in fact, the overall drying time is likely to be long before the final tile flooring can be fitted.
Nonetheless, this drying time can be speeded up by doing one or two things. If underfloor heating has been installed, then after 7 days it can be commissioned and increased by 5°C per day until the maximum working temperature is achieved. It can then be left at this level for at least a week, and then gradually reduced to 15°C to 20°C. The temperature should not exceed 55°C.
In addition, you can also use force drying in order to speed up the drying time. The trick here is to use the correct specialist drying equipment and control the environment including the humidity, together with appropriate moisture testing. Doing this correctly will ensure that the essential heating and cooling programme can be carried out accurately in terms of drying results. This can reduce the overall drying time before tiling can be laid down to just 28 days.
A recent study showed that installing underfloor heating compared with a radiator system actually had comparable costs. However, by using force drying and taking into account the cost benefits of time-saving, the cost of commissioning underfloor heating was considerably reduced.
Before laying tiles onto a liquid floor compound in Herefordshire, it is essential to remove the layer of laitance that will form on the surface as it dries. This is a layer of fine particles that is too weak to tile on to and it can also inhibit the drying of the screed. It needs to be removed between 7 – 10 days after the screed is poured by light sanding. Left any longer and the laitance will cause issues with the drying time and may also become harder to remove.
Before tiling, it is best to use a primer before applying the tile adhesive, which will make the surface more stable. When choosing the adhesive, it is necessary to consider the type of tile and the method of floor cleaning that will be used which may cause the floor to become wet and permit limited water penetration.
