If you are going to lay a liquid screed in a new building, there are certain conditions that have to be met before doing so. Before commencing, the windows and doors must have been fitted, as must the roof, because there must not be any chance of rain or water entering the building. If the external windows and doors have not yet been fitted, the gaps will need to be covered with heavy polythene. The floor surface also needs to be clean and dust-free.

Very often, there will be some form of insulation laid on the floor such as expanded polystyrene (EPS) and this will need to be covered by a tanking membrane. Before laying the insulation boards, the floor must be clean and free from any debris.

The insulation boards need to be flat and level, so if there are any service pipes running across the subfloor the insulation boards have to have grooves cut into them in order to accommodate them so that the boards are laying flat on the floor. The insulation boards also need to be tightly butt-jointed with staggered joints. Once the insulation boards are in place, it is necessary to cover any vertical surface that will come into contact with the screed with a foam insulation strip in order to allow for any slight expansion that may occur as the screed dries.

The tanking membrane itself needs to be of heavy gauge polythene of at least 500 gauge and this is laid on top of the insulation. The tanking membrane must have all joints overlapping by at least 100mm and the abutments must be taped and watertight. Where the polythene adjoins the walls, it must go up to at least the height of the edging strip and be taped. It is also necessary to allow some slack where the polythene meets the walls so that the screed can go right into the corner and form a sharp right angle. There must also be no air pockets or voids of any description. The whole purpose of the tanking membrane is to ensure that the liquid screed cannot escape when it is poured.

If underfloor heating is being installed, the cables or pipes are laid on top of the tanking membrane and the screws or staples used to hold them down will pierce the membrane and automatically seal it themselves so they will not cause any leaks. It is important that there should be no longer of more than 300mm between fixings of the cables or pipes because if they are not rigidly fixed down, they will float up to the surface of the screed when it is poured. If a water-based heating system is being used, the pipes must be filled with water in order to help hold them down. Of course, if you need to hire a liquid screed company in Reading, at UK Screeds we will undertake all of this for you so that everything is perfect before we pour the screed.

The screed needs to be laid so that it covers the heating pipes or cables by 30mm, and we will work from a place which is called the datum point which is usually the stairs or a doorway. We set out levelling gauges and set them to the right depth by using a laser and this will ensure that the screed surface is level through the whole of the floor. Then the screed is poured.

Once the screed is poured, we run over it with a dappling bar in two directions in order to ensure that there are no air bubbles left in the screed.

After this, the screed can be left to dry for up to 48 hours. When it has solidified, the windows can be opened in order to help with evaporation, and if required the screed can be force-dried after a period of about a week.

As it dries, the screed will form a layer of fine particles on the surface called laitance, and this must be removed by sanding before laying any final floor surface. We will do these 7 – 10 days after pouring the screed.