Upper floor and roof decks

The advent of steel framing introduced a need to reduce the dead weight of floors while at the same time maintaining their fire resistant capabilities.

Early systems utilised a framework of steel joists surrounded with coke breeze concrete. There were various forms of this, but primarily they were similar in concept. The loadbearing function was provided by the steel joists, typically at 600-900 centres and spans of around 2.7m. See Introduction.

Coke breeze is corrosive in damp conditions and severe corrosion can damage the web of the embedded steel such that its strength is compromised. Tell-tale parallel cracks on a plaster soffit usually indicate filler joist construction and a need for further investigation.

Later flooring systems involved hollow clay pots as void formers and woodwool slabs as permanent shuttering. Both systems could give rise to honeycombing of the concrete ribs and subsequent loss of fire protection or durability. The main difficulty is one of grout loss and the risk that the aggregate used in the concrete mix is too large to pass around the rib reinforcement. Since the ribs are concealed by the woodwool or clay spacer tiles, diagnosis and repair during construction is unlikely. See more.

Lightweight reinforced autoclaved aerated concrete (RAAC) planks were a popular choice for roof decks in some buildings, particularly schools and shopping centres. In section, the planks resembled ordinary hollow cored planks save that they were made from aerated concrete. Some records refer to the planks as 'Sipporex' after the manufacturer. The planks are no longer specified in UK construction, but many examples still remain. Some early examples suffered from deflection and associated cracking due to inadequate bond between the reinforcement and the aerated concrete.