Floor screeds

A floor screed is usually a cementitious material made from a 1:3 or 1:4.5 ratio of cement to sharp sand. It may be applied onto either a solid in-situ concrete ground floor slab or onto a precast concrete floor unit.

Application

The screed may be directly bonded to the base, with a minimum thickness of 40mm, laid unbonded onto a suitable damp proof membrane which is placed over the slab, with a minimum thickness of 50mm, or, if containing underfloor heating pipes a minimum of 65mm thick.

Alternatively it may be applied as a floating finish over a layer of rigid insulation material where the screed should be a minimum 65mm thick or 75mm if it contains underfloor heating pipes.

If reinforcement is required, this can either be in the form of a fine metal mesh or fibres which are normally polypropylene.

The screed may be left as finished, or floated to produce a smooth surface on which to lay the specified flooring or finish. Screeds can also be used to enclose underfloor heating pipes or as a path to route services.

Ready-mixed sand and cement screeds that are factory-mixed and then delivered to site offer additional quality assurance over site mixed screeds and offer a more consistent material.

Some manufacturers provide pumpable flowing screeds which can achieve very level finishes. Most of these screeds are anhydrite compounds and are based on a calcium sulphite binder. They are quicker to apply than a traditional sand and cement screed and may be applied to a minimum thickness of 25mm if bonded, 30mm if unbonded, or 35mm if a floating finish is required. They can also be used in conjunction with underfloor heating systems where a minimum 30mm cover to the pipes is needed. Up to 2,000M²/day may be laid using these screeds.

It is important to note however, that whichever type and depth of screed is specified, the decision should be made early in the design process.

Guidance should be sought from BS 8204 which is in several sections and covers a number of screeds, bases and in-situ floorings.

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Concrete Structures 08: Screeds and Toppings, page20