Sometimes in the business of housebuilding, a way of doing things that has been the same for years may suddenly change. It doesn’t happen all that often, because builders are notoriously reluctant to change when things have worked well for years and only tend to do so if there are possible cost or time-savings, or if they are compelled to do so by changes in the law or by architects who have latched on to a new process.
However, when it comes to flooring, in the last few years there has been a major change in the type of floor screeds that are used. Traditionally a floor screed has been made of sand and cement, and that’s it. However, in the last 15 years or so there has been a considerable rise in the use of liquid screeds as it has become apparent that they have many advantages over staid sand and cement.
Not the least of these is the speed of laying. In any building contract, time is of the essence and may lead to severe penalties if contracts are over-run, so anything which can save time without cutting down on quality has to be a big benefit.
Liquid screeds, by definition, are liquid, compared with sand and cement which is mixed in a ratio of 1:3 cement to sharp sand to as much as 1: 4.5. This produces a mix that is almost powdery, and used to be mixed on site by hand using a cement mixer. Today, things have been speeded up, and most large contracts will use ready-mixed sand and cement screed with additives added that will keep it useable for most of the day.
However, it still has to be laid by hand and this is labour intensive and takes time. By contrast, a liquid screed is also ready-mixed but is pumped on to the substrate with a hose and pump which is up to 20 times faster. At UK Screeds, our teams can lay as much as 100 square metres in 45 minutes or so: to lay that by hand takes all day. This is just one reason for the almost inevitable rise of liquid anhydrite screeds.
Furthermore, our liquid screed dries fast. It will be at a point where it can take light foot traffic in as little as 24 – 48 hours. This means that other contractors who need to work on the site don’t get held up.
The there are other advantages, too. Liquid screeds can be laid thinner than sand and cement, so less material is needed. True, anhydrite screed costs more than sand and cement, but when you can use less material and save a huge amount on the labour cost there is not an awful lot of difference in the cost overall.
There are more advantages as well, particularly when underfloor heating is being installed, which it is today more and more. Liquid screed will totally envelop the heating pipes which means that heat transfer is perfectly even into the room. Sand and cement screeds laid by hand have a nasty tendency to leave air pockets which disturbs heating and may leave cold spots. In addition, the thermal conductivity of liquid anhydrite is nearly twice that of sand and cement, leading to faster warm-up times and less use of energy. On top of that, our liquid screeds don’t curl and are very shrink-resistant as well.
With liquid screeds of all types, it is necessary to carry out proper floor preparation in London, or wherever you are. The floor needs to be “tanked” in a similar way to a swimming pool by laying polythene sheeting over any insulation boards. This is necessary in order to prevent the screed escaping, and in a worst-case scenario lifting up the insulation boards. The polythene sheeting needs to be lifted up the walls at right angles and glued into place with a spray adhesive. Any debris needs to be removed from the sheeting so that it doesn’t float to the surface when the screed is poured.
After carrying out floor preparation in London, and the screed has been poured and dried, it will leave a layer of laitance – fine particles – on the surface, and this needs to be removed between 7 and 10 days after pouring by using an industrial sanding machine, before the final flooring surface can be laid.