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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.
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.
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.
IR Plastering can recommend the best way to screed to your floor with the years of experience we have gained. For a FREE no obligation quote, contact us to arrange an appointment.