The basic method of laying a screed floor as the underfloor heating floor layer is firstly to lay the damp proof membrane (DPM), followed by the structural slab, insulation, heating pipes, clips, rails, or steel mesh, and finally the layer of screed.
Screed Floor Composition
A screed floor is usually composed of a ratio of 1:3 or 1:4:5 cement to sharp sand, and may be laid upon either a solid in-situ concrete ground constructed at the building site, or onto a precast concrete floor slab. Ready-mixed sand and cement screed mixtures offer quality assurance and a more consistent material. Manufacturers do provide screeds which can be pumped into position, and which can achieve an otherwise unattainable levelness. These screeds are mostly anhydrite compounds, based on a calcium sulphite binder. They can be applied much more quickly than traditional sand and cement screed, and to a thickness of 25mm minimum if bonded, 30mm unbonded, or 35mm if a floating finish is desirable. They can also be used in conjunction with underfloor heating systems, for which a minimum 30mm of mixture is necessary to cover the pipes. Up to 2,000M²/day may be laid using manufactured screeds of this kind.
Before applying the screed floor, the underfloor heating pipes must be fitted on top of the insulation layer using rails, staples, or a steel mesh to hold them in the desired shape. The rail is fitted to the insulation, and features a series of clips into which the pipe must be held to keep it in place. If using staples, the pipes are first laid out onto the insulation, and the staples are applied over them, keeping the pipes in place by sinking into the insulation on either side of them. If using the steel mesh, the pipes are first attached to the mesh, which is then embedded within the screed. This process can afford flexibility at the expense of greater time consumption.
The screed floor may be applied directly to the base with a minimum thickness of 65mm if applied over heating pipes; of 50mm if applied over a suitable damp proof membrane (DPM) laid over the slab; or of just 40mm if bonded directly to the base. Otherwise, a thickness of 75mm minimum allows the screed to be applied as a floating finish directly over rigid insulation and heating pipes. A thickness of just 65mm is sufficient applied over an area devoid of heating pipes.
Depending upon the nature of the building which the screed floor is laid within, reinforcement of the floor and pipework may be necessary. The screed can be reinforced with a fine metal mesh or polypropylene fibres. The underfloor heating pipes used for a solid screed floor typically range from 15mm to 20mm.
When working with a screed floor composition, the screed must be allowed to dry thoroughly. A typical sand and cement screed could require up to 28 days drying time. Moisture levels must be controlled throughout the drying period, lest disruptive remedial works should become necessary. Shrinkage can be controlled with metal mesh. Once laid, the screed may be left as finished or floated to produce the smooth surface necessary to lay the final flooring finish.