Membrane Shrinkage

Membrane shrinkage is a permanent reduction in the dimensions of a roof membrane over time. It is different from the normal, reversible expansion and contraction caused by temperature. Shrinkage has been most widely documented in EPDM (ethylene propylene diene monomer) rubber membranes, though dimensional change can affect other sheet materials too.

How it shows up

As the membrane slowly gets smaller, it pulls against wherever it is held: perimeter edges, curbs, walls, drains and penetrations. Typical signs include:

• Bridging or tenting, where the membrane spans across an inside corner (for example where the roof meets a parapet) instead of lying tight into the angle.
• Base flashings pulled away from walls or curbs, sometimes with the edge securement detached.
• Stress at seams and splices, which can open.
• Membrane pulled tight across drains and penetrations.

The EPDM Roofing Association has described bridging at angle changes in some loose-laid applications, and the problem has been observed most commonly on loosely laid, ballasted systems and on some mechanically attached ones.

Why it matters

Shrinkage-related failures often occur at the perimeter and details, where leaks are hard to trace. Water can travel through the insulation before appearing inside, so the insulation may be saturated by the time a leak is noticed. This is a particular risk in recover projects where the new membrane is installed over existing roofing.

Remedies

Responses range from isolated repairs at pulled flashings to full replacement. Repairs typically involve releasing and re-securing the membrane edge, adding strips of new membrane or flashing to relieve tension, and adding perimeter restraint. Industry commentary has warned that repeated patch repairs treat symptoms rather than causes. Prevention focuses on adequate membrane restraint at edges and details at installation. Owners of older single-ply roofs should include perimeter and corner details in routine inspections.