Car park lining in Sheffield

Why Sheffield car parks get lined

Letting rain soak through paving into the ground works only where the ground can take it safely. Much of Sheffield can't: mudstone and clay drain poorly, and contaminated fill on former industrial land shouldn't have water pushed through it. A lined stone layer turns the car park into storage instead, which helps a scheme meet its runoff limit under the national SuDS standards.

Retail parks and supermarket rebuilds along the Parkway and in the Lower Don Valley, hospital and campus car parks, and residents' parking on new apartment schemes are the typical sites.

The stone also does some cleaning on the way through, so what leaves the outlets is clearer than water off a gullied tarmac surface.

Slopes and stepped storage

A car park on a Sheffield hillside can't simply be one lined tray, or every drop of stored water would gather at the low end. Designers usually terrace the storage layer with internal check dams. Where they do, the groundworker forms each step to the drawn levels and we carry the membrane unbroken over every step, so the liner stays one sheet from top to bottom.

On the steepest plots the designer may split the car park into separate lined cells, each with its own outlet. We treat each cell as a welded tray of its own and log it separately.

What lies under the blocks

Our part starts on the trimmed formation and ends before the aggregate:

• Formation walked for level, fall towards the outlets and loose stone
• A needle-punched fleece over the floor and the side slopes
• LLDPE, HDPE or PP in panels, every joint fused by heat
• One weld peeled and logged at the start of each shift
• Each outlet, overflow and duct made watertight in the sheet
• Fleece over the top so the stone can't cut the liner

Working round a store that stays open

Retail car parks are often rebuilt in phases while customers keep parking on the rest. We line each area as the groundworker releases it and hand it straight back, so the stone can follow and the bay count doesn't stay down for long.

Where the design adds attenuation crates under the paving, we build those too, under one set of weld records. A liner uncovered for days on a working site is asking for damage, so we hand back promptly and expect the stone to follow close behind.