Floor slabs and gas-resistant membranes

The floor slab is the first barrier against gas ingress. There are several types of floor slab that are commonly seen on developments in the UK. Each type will offer a degree of protection against gas ingress into the finished sfrucfure. 

Reinforced concrete cast in situ suspended floor slabs can give reasonable gas resistance, if they are detailed correctly with anti-crack reinforcement. They also need to be sealed at joints (possibly with water bars a design measure to ensure joints are watertight) and sealed at all service penetrations with water bars. Evidence from Building Research Establishmenf (BRE) research has shown that reinforced concrete rafts with a venting layer below give the best level of protection to gas ingress. 

Block and beam slabs used in many housing developments have many joints between the blocks and beams which means they provide minimal resistance to ground gas ingress. However, a well-constructed block and beam floor should have a sand cement grout brushed into the surface to fill all the joints which can improve gas resistance to a limited extent. This also gives a better surface into which to lay a gas-resistant membrane (see Eigure 7.4). 

The gas resistance of floor slabs is improved by providing gas-resistant membranes. The purpose of the gas-resistant membrane is to provide a low permeability barrier against the ingress of gas from the ground into the building fabric. 

It is recommended that gas membranes are specified on the basis of the need to survive construction, i.e. use one that is resistant topimcture and tear. The joints should be sealed in accordance with the manufacturer s instructions. Thickness should not be the main method of determining the suitability of a membrane for a particular site, although it can indirectly influence some of the key performance parameters. 

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