Floor screeds and finishes

At a glance

The main purpose of a floor screed is to provide a finishing layer on to an existing structural base. Screeds are not intended to be wearing courses and usually require some form of protective finish. There are essentially 4 types of screed:

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

All types can curl as a result of shrinkage and all require careful mixing, placing and curing to reduce the risk of cracking and curling. Failure to control curing properly could, depending on atmospheric conditions, lead to premature drying. However, the surface of the screed is likely to become drier than the main body of the screed with the result that the top surface will shrink more than the lower parts, creating curling forces.

Rapid drying also results in cracking. Differential curling between adjacent sections of screed will affect rigid or flexible flooring systems. If the screed has curled away from the base sufficiently for it to deflect under load, there is a high likelihood that the screed will crack, with the risk of damage to the floor finish. See Screed collapse.

Resistance to applied loadings can be improved by increasing the thickness of the screed, using fine concrete or selecting a proprietary polymer modified screed. See Site investigations.

Applied floor finishes are sensitive to movements or defects within the screed on which they are laid. Typical defects include:

  • rippling of flexible floor finishes over crack lines;
  • damage to flexible floor finishes over movement joints;
  • blistering of flexible, impervious floor finishes;
  • osmosis in applied seamless floor systems; and
  • disruption and delamination of rigid flooring materials due to substrate shrinkage.

Magnesite floor finishes were once popular in housing and can often be found in industrial units dating back to the 1930s. The material resembles asphalt in appearance. It is made of caustic magnesia, additions (fillers), an aqueous salt solution of bivalent metals - generally magnesium chloride - and additives (like colouring materials). Magnesium chloride is an acid, hygroscopic and corrosive salt and can corrode ferrous metal that is in contact with it. The material is also very sensitive to moisture and deteriorates in damp conditions, for example where impervious floor finishes trap moisture.