At UK Screeds there are some requirements that are necessary before we actually install underfloor heating and/or a screed on a project.
To begin with, the building should be weathertight, so the roof needs to be in place and the windows and doors fitted. If the windows and doors have not yet been fitted, at the very least they need covering with clear polythene. The subfloor surface needs to be clean and free of debris.
In most cases, the screed will be installed on top of some sort of insulation which will often be polystyrene, and between 50mm and 100mm thick. The insulation boards need to be laid flat and butt-jointed with staggered joints, and if there are any service pipes running across the subfloor the insulation boards need to have grooves cut into them to accommodate them. Once this has been done, any vertical surface such as a wall that will come into contact with the screed needs to have an edging strip attached to it to allow for any expansion once the screed is dry.
The next job is to install tanks on top of the insulation boards, and this will usually need to be of at least 500-gauge polythene. The purpose of the tanking membrane is to prevent the escape of the liquid screed when it is poured. The membrane needs to be taken to at least the height of the edge strip on walls and pipes and taped along the top edge. Where the floor meets the wall there needs to be a small amount of slack in order for the screed to form a perfect right angle when poured.
The next job is to install the underfloor heating pipes or cables if, indeed, underfloor heating is being installed. There must be no lengths of cable or pipe longer than 300mm between fixings. The fixings themselves will pierce the membrane but will also seal the holes so there will be no leaks. These directions need to be followed exactly because otherwise the heating cables or pipes will float up to the top of the screed when it is poured. Furthermore, if the heating system is water-based, the pipes need to be filled with water. It is also necessary to pressurise the system to check for any leaks. The screed needs to be poured to a thickness of 30mm minimum over the top of the heating elements.
Our liquid screed has minimal shrinkage, and this means that control joints are not required at lengths of less than 40 metres. They should be used at doors though.
Our liquid screed is delivered to the site premixed and we then pour it using a pump and long hose. First, we agree on a datum level with you and then set up tripods in order to ensure that the screed is level. It is finished off by dappling in two directions in order to remove any air bubbles.
The screed will be dry enough to receive light foot traffic after 24 – 48 hours. If heavier traffic is needed, then it is necessary to protect the screed with plywood sheets. The screed should also be protected from the sun for the first 24 – 48 hours, and any south-facing windows need to be covered temporarily.
As the screed dries, it will produce a layer of laitance which consists of particles that rise to the surface. This needs to be removed before the final flooring is laid and we do this by sanding between 7 and 10 days after pouring the screed. Any longer than this and the laitance becomes harder to remove and will also delay the final drying time.
If you prefer to do the sanding yourself, we can also hire you floor sanding machines in Cambridgeshire or wherever your project is.
It is a good idea to open windows after the initial drying period in order to improve ventilation. Unlike sand and cement screeds, our anhydrite screeds can also be force dried with dehumidifiers if it is necessary to speed up the process. Underfloor heating where fitted can also be commissioned after seven days, or after the sanding, and gradually turned up to a maximum of 55°C at which it can be left for a week before gradually being reduced again to 15°C – 20°C.