Don’t want to lay your fancy finish on that bumpy, lumpy sub-floor? You need a decent screed – but there’s more to applying one than mixing cement and sand, says Peter Caplehorn of Scott Brownrigg
One of the keys to good-quality solid flooring is the use of a screed. In many cases, it is not desirable or practical to apply the floor finish directly to the structure. So a screed, usually a cementitious material, is applied to create a smooth, level surface. Screeds can also be used to enclose heating elements or as a path to route services.
Three basic criteria are at the heart of good screed design: strength, bonding and moisture control.
Structural floor base
Concrete is the most common base. The designer must establish the surface characteristics of the slab and the nominal thickness being allowed for the screed early in the design process. The thickness will have a large influence on the make-up and method of laying, as well as the nature of the bond. Screeds can be laid on timber but it must be stable and a good-quality damp-proof course must be used.
Quality control standards
Most screeds can achieve a high-quality finish with less than 3mm deviation over any 3m. BS 8204-1:2002 is the code of practice for cement-based screeds and it defines surface regularity standards SR1, 2 and 3, which allows the designer to be very specific about the quality. The finish material defines the quality needed. For ceramics or stone using thin-bed adhesive, a good general quality is sufficient. For thinner materials such as a vinyl less than 2mm thick, an even and blemish-free finish is needed.
Floor loads and floor traffic need to be established early on so that the strength of the screed needed can be built in to the specification.
Screed is usually a cementitious material made from a 1:3 to 1:4.5 ratio of cement to sand. A simple cement and sand mix will be fine for a simple application but more sophisticated mixes are suitable for a wider range of applications.
Some of these new compounds can be laid on substrates other than concrete. Others – largely compounds of latex or epoxy – can be applied to create strong screeds that are less than 20mm thick.
Manufacturers also offer screeds that can be pumped in to place and used to achieve very level finishes. Most pumped screeds are anhydrite compounds based on a calcium sulphite binder. These must be applied by a specialist and allowed to dry completely. While pumped products allow flexibility in laying, they also make the finished screed susceptible to water damage. Protection proportional to the final use should be specified.
Screed thickness and moisture control
Modern construction programmes do not usually allow enough time for the moisture in a new screed to evaporate. Screeds more than 100mm thick contain large quantities of water that needs to dry out and can cause significant disruption late in the build process. Moisture content is important, as most floor finishes and adhesives are sensitive to it. They are also sensitive to the build-up of water vapour if it is sealed in to the screed. This has resulted in the development of screed compounds that chemically control the water used. In some cases, a dpc on top of the screed can offset its high moisture content, but this should be seen as a compromise.
Optimum screed thickness is from 30 to 70mm as there are a large number of sophisticated and versatile compounds available for this range.
Screeds can be bonded to the sub-floor or unbonded. Bonded screeds are laid directly to the new concrete sub-floor or a chemical agent is applied before laying to ensure a good bond. This ensures the screed acts as one with the slab below. However, the laying process must be undertaken carefully to ensure de-bonding does not occur – this will make the screed unstable and it will fail.
Usually, thinner screeds are bonded. An unbonded screed of less than 50mm must be designed using a specific mix to ensure it is strong enough.
Unbonded screeds ranging from 60 to 75mm thick can be laid at any time after the slab has been constructed. They will usually have a damp-proof course separating them from the sub-floor. They must be allowed to dry slowly, however, as fast drying will make them cure like a curly sandwich.
If the floor loading requires it, the screed may need reinforcement. This can be achieved either by the addition of plastic particles to the mix or the use of one layer of fine reinforcement.
Thicker screeds can accommodate services, but it is rarely a good idea to place these directly in the screed. It is better practice to feed pipes and cables through a conduit, particularly heating pipes, as these expand and contract.
Typically, the heating elements in underfloor heating systems are surrounded by a specially designed screed. This includes plastic reinforced pipes that can flex with temperature and a screed compound that will not crack even if relatively thin.
Movement joints must be considered as these restrict the stress that can build up within the material and control cracking. Ensure that any building movement joints are mirrored in the screed, too. No large areas or significant changes in finish should be designed without a movement joint so you will need to seek expert advice.
Shape should also be considered. Keep each area of screed as rectilinear as possible and incorporate movement joints at all changes of level, structural elements and fixed points.