Car park lining in Bradford

Fast runoff in steep valleys

Bradford Beck runs in culvert under much of the city centre, with the becks of the surrounding valleys feeding the Aire. Water off a new car park arrives there quickly unless something slows it. A lined permeable pavement does that: rain passes through the blocks into the aggregate, the membrane stops it entering the ground, and the outlets meter it out at the flow the drainage design allows. It also means fewer gullies on the surface, and the stone strains out some of the grit before the water leaves.

Old mill yards and retail parks

Many car parks here sit on former mill and works land, where fill and possible contamination mean the water shouldn't soak in. Others are on glacial clay that won't take it anyway. Supermarkets and retail parks along Canal Road and around Thornbury, college and hospital car parks, and parking on mill regeneration schemes are typical.

Ground under a mill yard can hide old cellars, flagstones and setts. The groundworker deals with those; we look for anything left standing proud of the formation that could press through the fleece over time.

What we lay, in order

The dig and the trimming to falls are your groundworker's. We inspect the result first and flag it on the spot if it needs more work. Then:

• Fleece underneath, carried up the side slopes
• A heat-fused LLDPE, HDPE or PP sheet, laid in panels
• A sample weld peeled and logged that morning
• Outlets, overflows and any services sealed into the liner
• Edges dressed up to the design level
• A second fleece on top before any stone arrives

Fitting in with the paving crew

A liner left uncovered is a liner at risk, so we line in step with the stone-laying programme and release each area as soon as its weld log is signed. Bradford weather means planning around the forecast; we weld only when a seam can be kept dry, and we never tape.

On a live retail site we also agree how deliveries and customer traffic will be kept off the lined area. Tyres on bare membrane, even briefly, can do damage nobody sees until the outlets run dirty.

If the design needs more storage than the stone layer alone, crates can go beneath the paving. We install them as well, and both end up in the same handover file.