When many people think of the process of laying a liquid floor screed they envision the mixer truck turning up at the site, two or three men connecting a hose and pump to it, they run the hose to where it needs to be, turn on the pump until the screed covers the floor, pack everything up, and go home for lunch.
As it happens, as liquid screed contractors in Northampton, at UK Screeds we can tell you that, along with many other processes in construction, there is a bit more to it than that. To begin with, the building is only partially constructed and there may be some processes completed and others still to come. What is necessary is to ensure that the building is weathertight with roof covered and external doors and windows fitted, but if not, the gaps need to be covered in polythene. The substrate needs to be clean and free of dust and debris.
If insulation is being fitted, which is usually the case, the boards must be laid flat and butt-jointed so that they are stable. If there are any service pipes running across the substrate, the insulation boards need to have grooves cut in the underside in order to accommodate them. Alternatively, if two layers of insulation boards are to be used, the first layer can be placed between any service pipes and any gaps filled with sand. The second layer is laid on top of the first, making sure that it is flat. Then an edge strip must be fitted to walls and any other vertical surface such as the bottom of stairs that will come into contact with the screed.
Next, a tanking membrane needs to cover the top of the insulation boards in order to prevent the screed from escaping when it is poured. This needs to be of heavy polythene, and joints must overlap by a minimum of 100mm and tape, and at the edges, the polythene must run up the walls, pipes, etc., to at least the height of the edge strip and be taped at the top. There must be no voids under the polythene membrane and no creases, and there must be sufficient slack where it meets the walls to enable the screed to flow and form a full right angle.
The next job is to fit the underfloor heating pipes, electric cables, or heating mat, depending on which system is being installed. In the case of electric cables or pipes, they must be fixed down at intervals no greater than 300mm. Whether screws or staples are used to hold them down, they will pierce the polythene and seal the holes themselves. Fixing them down is critical because if the pipes or cables are not fixed, they will float upwards when the screed itself is poured. In addition, if a water-based heating pipe system is used, the pipes must be filled with water to reduce the chance of them floating upwards. A pressure test must also be performed in order to ensure that the pipes hold water and do not leak.
Next, we will set the levels using a datum point which is usually the stairs but could be a doorway. We then set out the tripods, which are levelling gauges, and set them to the correct depth using a laser.
Around this time, the mixer truck should arrive, and only then do we connect the hose and pump and pour the screed into position. We then use a dappling bar to level off the screed in two directions and ensure that there are no bubbles in it. Now the screed is laid to the correct depth and can be left to dry.
The screed will be dry enough to walk on in 24 – 48 hours, depending upon temperature and humidity. This allows other trades to carry on with their work without interruption.
The final flooring cannot be laid until the screed is fully dried, and the time taken for this will depend on screed depth, temperature, and humidity. Drying can be hastened by turning on the heating and gradually increasing it, but the temperature must not exceed 55°C. It can be left at this level for a week and then gradually reduced to 15°C to 20°C. Even faster drying can be carried out by the use of dehumidifiers in addition.