When you are constructing or renovating a building, there is no rule that says that you have to use a screed. However, despite the fact that they add somewhat to the overall cost of the project, most buildings use a screed these days. One of the reasons is that they can provide as near a level and flat surface as possible upon which to lay the final flooring. Another is that the precast concrete beam and block floors which are used so often today have a certain camber which can cause issues when you lay the floor finish.
There are two main types of screeds, one of which is a sand and cement screed in London, and the other which consists of gypsum, which is calcium sulphate mixed with water. When heated, gypsum converts to calcium sulphate anhydrite. This can lead to some confusion because they can be called gypsum screeds, calcium sulphate screeds, anhydrite screeds, anhydrous (dry) screeds, and also, simply, liquid screeds, because they are laid on the surface in liquid form, but they all refer to the same material.
Sand and cement screed has been used for many years and can consist of between a 1:3 up to 1:4.5 ratio of cement to sharp sand. Traditionally, this was mixed by hand in a cement mixer on site and then barrowed to wherever it is needed. This does have a certain disadvantage in that mixing is very labour-intensive, and also that each load is going to vary somewhat in consistency.
Ready-Mixed
For this reason, a lot of sites today use a ready-mixed cement screed in London. These cost a little more for the actual material but does save on labour costs. Most of these screeds have a retardant added to them so that they can last all day without drying out to the point where they cannot be laid flat with the trowel.
If underfloor heating is being installed, then sand and cement has the disadvantage of being laid by hand because it is almost impossible to cover the heating pipes without leaving pockets of air. That means that the transfer of heat into the room will not be even. It is also usually laid fairly thickly, typically from 65mm to 75mm deep. Obviously, this adds to the drying time. A sand and cement mix also has a tendency to curl as it dries, and in addition it can develop cracks, none of which is good if you want to lay the final flooring finish directly on to it.
All of these are reasons why liquid screeds, as we will call them, have gained rapidly in popularity over the last few years. These screeds are delivered to the site in liquid form pre-mixed and then connected to a hose and pump which delivers them to the area that they need to be laid.
It does take a certain amount of time to set everything up, but once in position the screed can be poured into position. Since it is liquid, it is self-levelling. All that needs to be done then is for it to be cleared of any air bubbles by using a dappling bar in two directions, and it can then be left to dry.
Screed Can Be Laid More Thinly
When it comes to drying time, liquid screeds have the advantage, despite being liquid when poured. Not only do they dry very quickly, but they can be laid far thinner than a sand and cement screed of 65mm to 75mm. If underfloor heating is being installed at the same time, then it needs to cover the heating pipes to a depth of 30mm. However, that still means that the overall depth is as little as 45mm. It follows that although the material itself is more expensive than sand and cement screeds, you don’t need so much of it.
Sand and cement screeds can take a considerable time to dry. The standard allowance is 1mm of thickness per day up to 50mm and 1/2mm per day thereafter. So, if the screed is 75mm thick it will take 100 days – over 3 months – to fully dry and cure.
Liquid screeds, on the other hand, will dry in 24 – 48 hours to a point where they can be walked on. This means that other contractors working on the project will not suffer any delays. The screed can then be force dried over the next seven days at which point it is possible to lay the final floor surface.
Laitance
One disadvantage of liquid screeds is that, as they dry, they produce a layer of laitance on the surface, and this must be removed before tiling. That means that it is necessary to sand the surface. However, when everything is added up, the overall benefits of a liquid screed are obvious.
However, the benefits do not stop there. As we said, the screed is in liquid form when poured, so it will totally cover heating pipes without leaving any air pockets, which is almost impossible to achieve with sand and cement laid by hand. So, the heat transfer into the room is perfectly even. In addition, the heat transfer properties of liquid screeds are almost twice that of sand and cement. Add to that the fact that the screed is laid more thinly, it uses far less energy to heat the room and takes less time, saving money forever.
Don’t forget sanding.
