Gas membrane installation in Dorset

Where Dorset sites need protection

Much of Dorset is clean farmland, but the developed coast is a different picture. Land reclaimed along Poole Harbour, old wharves and boatyards, former depots and filled ground around the conurbation can all give off methane or carbon dioxide or carry hydrocarbons. Former landfill sites on the edges of towns are another common trigger. Around Weymouth and Portland, old port and military land brings its own history.

The decision is not ours to make. A consultant investigates the ground, assesses the gas risk, usually by reference to BS 8485, and writes the required protection into the design along with the class of membrane. We build to that.

• Made ground and reclaimed land around the harbours
• Former landfill and infilled pits
• Old garage forecourts, fuel depots and boatyards with hydrocarbon contamination
• Sites where the risk assessment flags radon

Laying the barrier

Before we lay anything, we inspect the blinding or slab, because a membrane laid over a sharp or uneven surface can be punctured before the concrete arrives. If it isn't ready, we say so. Each lap is then welded or sealed to the specification on the drawings, and only in weather and conditions that let the joint be made properly.

Penetrations decide whether gas protection works. Piles, service ducts, drainage pipes and columns each get a collar of their own, sealed to the sheet one by one. On a busy Poole site with several trades waiting to pour, that is the part we refuse to rush.

Working alongside the other trades

Gas membranes go down late in the substructure programme, after the piles and drainage and just before the reinforcement and concrete. That puts them in the middle of a crowded site. We agree access with your site manager, ask for services to be in and capped before we start, and inspect any damage left by following trades before the pour, so a tear from a steel fixer's boot is found and repaired rather than buried.

Coastal weather plays a part too. Wind off Poole Bay can lift loose sheet, and wet laps will not seal or weld reliably, so on an exposed site we ballast the membrane as it goes down and stop jointing when the conditions are wrong.

Proving it for the warranty

If the warranty provider or local authority wants proof, we can arrange integrity testing and certification by an independent inspector, which is now common on higher-risk sites. Installation records for the warranty file come with every job.