A compression strip is a profile, usually aluminium or impact-resistant PVC, that mechanically clamps a roof membrane or a lead flashing against the wall. It fixes the turned-up edge of bitumen or EPDM at the top of the upstand and, together with a sealant, forms a junction that water cannot get behind.
Two versions are common:
- Membrane compression strip — an aluminium wall or edge profile holding the upstand of bitumen or EPDM; the top edge is sealed or tucked under a cover profile
- Lead compression strip — clamps sheet lead tight against the inner leaf of a cavity wall. PVC versions with a rubber seal, a waterproof tape and drainage holes need no mastic at all
Where it is used
- Where a flat roof meets a rising wall or parapet
- Dormers, extensions and canopies where the roof plane abuts existing brickwork
- Cavity wall junctions with lead dressed into or against the inner leaf
- Refurbishment, as an alternative to chasing a groove into the facade
Practical tips
- Sealant is not a fixing — the strip must be mechanically fixed with plugs and screws at regular centres; the sealant only closes the top edge
- Always seal the top edge where the profile does not finish under a cover flashing: an open top edge is a water entry point
- Work above the ponding level — keep enough height between roof surface and top of upstand so the detail stays dry in heavy rain or with a blocked outlet
- Watch material combinations — aluminium in direct contact with fresh concrete, mortar or copper corrodes; separate the materials
- Allow for movement — long straight profile runs expand; use butt joints and laps rather than one continuous clamped length
Related terms
- Upstand
- EPDM
- Bitumen
- Lead sheet
- Roof edge trim
- Cavity inner leaf
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