Thermal expansion
Thermal expansion is the size change of a material with temperature. Why a steel roof moves about 6 mm over 10 m, and where expansion joints go.
Ask Chalkline about this →Thermal expansion is the change in size of a material as its temperature rises and falls: it grows when heated and shrinks when cooled. How much depends on the material’s coefficient, the length of the run, and the size of the temperature swing. It is the movement long runs of metal roofing, cladding, and masonry have to be detailed to absorb.
How much a metal roof moves
Steel roof sheet moves about 0.012 mm per metre for each 1 degree C. Roof temperatures swing hard: in summer a sheet sits near 50 degrees C in a light colour and over 80 degrees C in a dark one (verified 2026-05-26, Lysaght). So:
- A 10 m sheet moves about 6 mm over a 50 degree C change, and around 9 mm over 75 degrees C.
- Aluminium moves roughly twice as much as steel for the same change.
The longer the run and the darker the colour, the more it moves.
Why it matters on site
- Run length. Long unbroken runs need somewhere for the movement to go, or sheets buckle, fixing holes elongate, and the roof ticks and oil-cans.
- Joints and fixings. Expansion joints and movement joints take up the movement; concealed-fix clips and slotted holes let sheets slide. See metal roofing and AS 1562.
- Masonry too. Brick and block move with temperature as well; AS 3700 control and articulation joints absorb it.
It is not the same as thermal bridging, which is about heat flowing through a material, not the material changing size.
Also known as: thermal movement, expansion and contraction.
Category: Building physics / movement
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Last updated: 2026-05-26. Verified: 2026-05-26. Quarterly review for currency.