Gas membrane installation in Worcestershire

Mill towns and works yards

Kidderminster grew on carpet weaving along the Stour, Redditch on needles, fish hooks and springs, and Worcester on porcelain, gloves and engineering beside the Severn and the canal. Redeveloping those mills and yards often turns up made ground, old tanks and fuel spills, and the ground investigation then moves on to a gas risk assessment under BS 8485.

Former landfill and infilled pits on the edges of towns bring methane and carbon dioxide. Hydrocarbons appear wherever there were depots, garages or fuel stores. The design sets which of these the building needs protecting from, and to what level.

The details carry the risk

A gas barrier fails at its weakest point, and on a slab that means the penetrations. Pile heads, soil stacks, ducts and columns are each collared and sealed separately, and the sheet laps are welded or sealed exactly as that product's maker requires. Before any of that, we walk the blinding and tell the site manager about anything sharp enough to cut through.

• Radon, methane, carbon dioxide and hydrocarbon barriers
• Lap joints made the way the product's maker sets out
• Pile caps, service entries and stacks detailed one by one
• Integrity testing and independent validation on request
• Installation records for the warranty provider

Who decides the protection

The ground investigation and risk assessment come from the client's consultant, and the membrane class and detailing follow from them. We don't assess the site or design the protection. We install what is specified, record it, and leave the work open for a third-party validator to check before it is covered.

Fitting the slab programme

Gas membranes go down at a busy point in a build, between the groundworks and the slab pour. We plan visits around that sequence so the membrane is laid, detailed and inspected without holding up the concrete, and we return for late service entries rather than leave them for someone else to tape over.

Riverside plots in Worcester and Stourport add one more consideration: winter groundwater can rise into the formation. We need a dry, firm surface to lay on, so we tell the site manager early if pumping or blinding is needed before the membrane can go down.