When you are in the construction industry, one of the things that are necessary for any building is to ensure that the floor is flat and level. That may sound obvious, because we don’t want to walk on bumpy floors and when placing, say, a table on the floor we want all four legs to be in contact with it so that the table stays level.
What all this means is that before fitting the final flooring, whatever that may be, the subfloor needs to be flat and level as well, especially when fitting something such as tiling, and for this purpose a screed is added to the concrete. This can be directly bonded to the base or laid unbonded on to a suitable damp-proof membrane which is placed over the slab. Alternatively, it may be applied as a floating finish over a layer of rigid insulation material.
Traditionally, a screed as been made out of sand and cement and is laid by hand with a labourer levelling it out as flat as possible using a hand trowel. This is a lengthy process and there is a propensity for it to develop cracks. The finish can often be uneven and of patchy quality. However, sand and cement screed can be prepared on site by unskilled labour, and it is ideal for use in wet rooms with sloping floors where a liquid screed is unsuitable.
Liquid anhydrite screeds have several advantages over and above sand and cement, not the least of which is that they can be laid far faster. The screed is delivered to site pre-mixed and our team at UK Screeds will pump it into position using a pump and a long hose. On any construction site, time is money, and we can lay as much as 2,000 square metres of liquid screed in a single day, which is up to 20 times faster than a sand and cement screed can be laid by hand. We then use a dappling bar to remove any air bubbles and achieve the perfect final surface. With a sand and cement screed it is difficult to achieve anything better than the levelling standard of SR3, which allows for a discrepancy of up to 10mm over a 2 metre distance. Our liquid screed will always achieve SR2 with a variation of +/- 5mm but will usually be SR1 which only allows for 3mm.
Anhydrite screeds have the advantage of being very resistant to cracking and curling and are ideal when underfloor heating is being installed. This is because, being in liquid form, they flow and fully envelop the heating pipes which makes for 100% efficient transfer of heat into the room. When you use a sand and cement screed, it is very difficult to wholly cover the heating pipes and there are usually gaps and voids which have an effect on heat transfer.
Another advantage of liquid screeds is that they have nearly double the thermal conductivity of sand and cement. They can also be laid thinner without fear of cracking and shrinking, which again helps with heat transfer into the room and means that less energy is used to attain the required level of heat. Yet another benefit of anhydrite screeds is that they will be dry enough to walk on within 24 – 48 hours, depending upon temperature and humidity, so this means that there is no delay to other contractors who need to work on the site.
If there is a downside to liquid anhydrite screeds it is that as they dry they produce a layer of laitance on the surface which is fine dust-like particles, and this needs to be removed before the final floor is laid. We do this by sanding the floor between 7 and 10 days after the screed is laid. Left any longer than this, the laitance becomes more difficult to remove.
However, if you prefer to undertake the floor sanding yourself, we are happy to rent you our floor sanding machines in Berkshire, or for that matter anywhere else.
The other thing to note if you are going to lay tiling or stone slab flooring is that some adhesives can react with the screed material, so it is necessary to use an adhesive that is compatible with anhydrite screeds.