Creep in Lead and Zinc Roofing

Creep is a gradual deformation of a material held under constant load. On roofs it affects soft, heavy metals, above all lead, and to a lesser degree zinc. Gravity pulls the sheet down the slope, and daily heating and cooling repeatedly stresses it, so over years the metal stretches, thickens at the bottom, thins at the top and can eventually crack or tear.

Lead

US preservation guidance notes that lead roofs tear because of creep, the gravitational stress that moves the metal down the slope. Lead roofing practice in the UK manages creep by:

• limiting the size of each bay and piece for each lead code;
• breaking long runs with drips and rolls;
• fixing sheets at the top and along edges in patterns set out in the Lead Sheet Association manual;
• avoiding over-long pieces, which raise the risk of creep and fatigue cracking.

Zinc

Unalloyed zinc can also creep. Architectural titanium-zinc includes about 0.06 to 0.2% titanium specifically to improve creep resistance, alongside copper for strength. Without it, zinc panels can thicken towards the eaves and slide gradually down the roof.

Other metals

Copper, steel, stainless steel and aluminium are much less prone to creep at roof temperatures; their main movement concern is thermal expansion and contraction.

Signs of creep

• Sagging or bulging sheets, thickening near lower edges.
• Fixings pulled through or elongated nail holes at the top of bays.
• Splits at fold lines and at the head of sheets.

Repairs usually involve replacing affected bays with correctly sized pieces rather than patching.