Liquid floor screeds such as the liquid anhydrite screed that we install at UK Screeds do need the preparation work carried out to the very highest of standards, but it is not difficult, and in any case, we carry out all the preparation work for you.
To begin with, the building itself should be weather tight with the roofing fitted and also the doors and windows. If the doors and windows have not been fitted, then all the openings need to be fitted with clear polythene. It is also best if the walls have been plastered and have dried out before commencing operations. Dust and debris need to be removed from the floor surface.
Before laying the liquid screed – also known as a self-levelling compound in Hampshire – it is necessary to lay insulation boards on the flooring. This helps to ensure that all the heat goes up into the room where it is needed. Sometimes there may be pipework running across the floor and if this is the case, the insulation boards need to have grooves cut into the underside in order to accommodate them. The insulation boards need to be laid flat and not be unstable when walked on, and they need to be butt jointed with staggered joints.
After this, the next job is to stick a fine foam insulation strip on the walls or any other vertical surface such as a door frame that will come into contact with the screed. What this will do is allow for any minor expansion that may occur as the screed dries.
The next thing to do is to lay a membrane over the insulation so that the liquid screed cannot escape and cause issues with the insulation boards, including in a worst-case scenario causing them to float upwards. The membrane is typically 500-gauge polythene and needs to be stretched out tightly and lapped up the walls at a tight right angle and glued into place with a spray adhesive. Any exposed edges such as door openings need to be provided with temporary formwork.
If underfloor heating is being installed, the next job is to lay the heating pipe and screw or staple it down. The membrane will not leak because the screws or staples will seal the holes themselves. What is important is to ensure that the screws or staples are no further apart than 300mm because otherwise, the heating pipes could start to float upwards when the liquid levelling compound is installed. For the same reason, the heating pipe should be filled with warm water. They should also be pressurised in order to check for any leaks.
The next job is to take measurements in order to ensure that we have sufficient screed ordered, and then we will set up levelling gauges, known as tripods, and set them to the correct depth using a laser. We work from a datum point which is usually the stairs or a doorway. We then set up the pump.
When the screed arrives, we simply pump it into position, and this is a very quick job. Laying a sand and cement screed by hand can take days in a large building but using our pump we can lay 100 square metres in as little as 45 minutes and can lay a total of as much as 2,000 square metres in a day. Next, we need to remove the tripods, and then we use a dappling bar in two directions in order to remove any air bubbles. After that, the screed is left to dry.
The screed will be dry enough to walk on in 48 hours, and very often even less. This means that other contractors don’t get held up. However, in order for the screed to be fully dried so that the final flooring can be laid it needs to dry out for longer. This is usually calculated as a rate of 1mm per day for a depth of 40mm and 1/2mm per day for anything over that. It can vary according to the relative humidity and the temperature but if time is of the essence the process can be speeded up by using dehumidifiers.