Floor screeds and finishes

The primary purpose of a screed is to provide a finishing layer onto an existing structural base or substrate. Screeds are not intended to be wearing courses, they must be overlaid and protected with another material such as carpet, vinyl or other traditional finishes.

Given the range of problems experienced with screed as well as the time needed to lay, cure and dry out, it is unsurprising that there has been a tendency to move away from screeds altogether in favour of power-floated finishes, where the surface of the concrete or the topping is finished to a high degree. However, power-floated floors can be difficult to achieve in all cases, and the areas around columns or other obstructions can be difficult to finish adequately. The marks left by the power-floating paddles can often be seen showing through vinyl floor finishes unless a self-levelling compound is added. Further, it may be difficult to correct a lack of tolerance in the floor structure or to accommodate services within the floor thickness.

Materials constructed using ordinary Portland cement have long been known to shrink as they dry out. Sand and cement screeds are no exception to shrinkage problems, but while shrinkage can often cause cracking in larger concrete elements, floor screeds, because of their relative thinness, can be prone to a problem of curling up at the edges. To the designer, the spectre of a curling floor screed can come as a nasty shock: it often becomes apparent at a critical time, such as the fitting out of a new building.

In the UK a screed is defined as a layer of well-compacted material applied in-situ to a structural base or other substrate as opposed to a topping that provides a wearing layer (Screeds, flooring and finishes: selection, construction and maintenance, Part 5 – Screeds, Report R 184, CIRIA, London, 1998). There are 4 types of screed that need to be considered:

• bonded;
• partially bonded;
• unbonded; and
• floating screeds.

Bonded screeds are usually up to around 40mm thick, while unbonded screeds are usually in excess of 50mm. Floating screeds are applied to a non-structural base such as a layer of insulating material or a damp-proof membrane. All types depend on careful laying and curing in order to avoid shrinkage and curling problems. It is not a case of simply specifying one in preference to the other to avoid curling. Generally, a bonded screed has more resistance to curling than an unbonded screed, although when laid to a greater thickness, the screed has a greater resistance to the curling force than a thinner unbonded screed.

Over the years a variety of different screed mixes have been designed and manufactured - each with its own properties, benefits and limitations, for example:

• cement/sand, fine concrete;
• synthetic anhydrite;
• lightweight, flow-applied cementitious or anhydrite;
• no-fines.

Comparison of screed types