Thermal Movement in Metal Roofs

All metals expand when heated and contract when cooled. A metal roof can swing from below freezing at night to well above air temperature in sun, so a long panel changes length noticeably every day. Good metal roof design does not try to stop this movement; it controls where it happens.

How much metals move

Approximate coefficients of linear expansion:

Metal	per degree C (x 10^-6)	per degree F (x 10^-6)
Carbon steel	10.8-12.5	6.0-6.9
Copper	16-16.7	8.9-9.3
Stainless steel 304	17.3	9.6
Aluminium	23	12.8
Lead	29	16.1
Zinc	30-35	16.7-19.4

As a worked example, a 10 m (33 ft) aluminium panel warming by 50 degrees C (90 degrees F) grows by about 11.5 mm (0.45 in). One zinc producer's rule of thumb is about 1 in per 30 ft of panel. Aluminium moves roughly twice as much as steel.

Design responses

• Clips: standing seam panels use a fixed zone (often at the ridge or mid-slope) and sliding or expansion clips elsewhere, so the panel can move away from the fixed point. Sliding clips matter most on long panels and on aluminium; nail-strip (fastening flange) panels restrict movement and are limited to shorter lengths.
• Slip sheets: on copper and other seamed roofs, rosin paper stops heat-softened asphalt underlayment from bonding the metal to the deck.
• Bay and pan limits: lead, zinc and soldered flat seam copper are laid in limited bay sizes or with expansion battens and drips.
• Fasteners: through-fastened sheets must not be overtightened; repeated cycling can loosen or snap screws.
• Flashings: trims at eaves, ridges and walls must not lock panels at both ends.

Symptoms of restrained movement

Waviness that comes and goes with temperature, popping or ticking noises, elongated fastener holes, broken clips, cracked solder joints and torn seams all suggest that movement has been restrained.