A liquid screed has several advantages over a traditional sand and cement screed, not the least of which is that it can produce a surface regularity which is greater than that of a screed that is laid by hand. It is also the best type of screed for use with underfloor heating because it totally envelops the heating pipes or electric cable, leaving no gaps. Liquid screed also has better heat transfer properties than a traditional sand and cement screed and so will use less energy for the same given amount of heat in the room.
Underfloor heating is being specified for more and more new build properties because it is a method of heating the room evenly using radiant energy. Traditional radiators heat the air, and once switched off the room will cool very quickly whereas when the whole floor is warmed it will cool at a much slower rate if switched off. Another advantage of underfloor heating is that it does away with the need for pipes and radiators in the room, which do take up some space and also affects the aesthetics. Certainly, over the last 50 or more years we have got used to seeing radiators in a room, but it is still nicer to do without them.
Not only do liquid screeds perform better in conjunction with underfloor heating, but they have many other benefits too. One of these is the speed of laying. A traditional sand and cement screed is mixed on site and laid by hand. This can lead to differences in each batch. Larger sites today now use ready-mixed sand and cement screed delivered to site which contain retardants so that they don’t set rapidly. However, they still have to be laid by hand using a trowel.
Liquid screeds, however, are also delivered to site ready-mixed and then simply pumped into place. This is a far faster method of laying a screed than by doing it by hand, and it is said that it can be up to 20 times as fast. On any building site, the faster you can get a job done, the better. Furthermore, because a liquid screed is just that – liquid – it is pretty much self-levelling. In fact, liquid screeds are often referred to as self -levelling screeds or self-levelling compound. When laid in conjunction with a laser leveller, a liquid screed will always attain Surface Regularity 1 – SR1 – and it is not possible to achieve better than that. Traditional sand and cement screed, on the other hand, will struggle to get past SR3.
It is true that liquid screed costs a bit more than a sand and cement screed, but this is offset by two factors. One is the speed of laying, which saves a considerable amount on labour. The other is the fact that liquid screeds do not need to be laid as thickly as sand and cement. A liquid screed used in conjunction with underfloor heating can be laid to a depth of 45mm whereas a sand and cement screed on underfloor heating needs to be laid to 75mm – 80mm, so there is a corresponding saving on materials also.
In addition, liquid screeds do not curl and bend, and they are far less likely to crack than sand and cement.
So with all these advantages, there must be something that offsets them, surely?
Well yes, there is. As it dries, a liquid screed produces a layer of laitance on the surface of the material. If you try to glue tiles or anything else to it, it won’t adhere properly, so this layer of laitance needs to be removed using floor sanding machines. Sanding the screed can be done somewhere around 7 – 10 days after the screed is laid.
First, the screed needs to be swept in order to remove any debris which could damage the sander. Then the screed needs to be sanded using a universal orbital sander. After that, the floor needs to be vacuumed using an industrial vacuum cleaner – a home vacuum won’t be strong enough and you could damage it. Finally, the screed needs to be primed using a high-quality acrylic primer. After that, the screed is ready to accept the flooring.
So, yes, this extra step is needed, but nonetheless the benefits of a liquid screed over a sand and cement screed far outweigh the fact that it needs to be sanded.