How to Allow for Thermal Movement in Metal Roofs: Clips, Fixed Points and Oil Canning

Step by Step

1. 1Step 1: Calculate how much the panel will move

   The linear expansion of steel is about 0.0000065 inch per inch per degree Fahrenheit. For a 30 foot panel (360 inches) and a 100 degree Fahrenheit swing, that is 360 times 100 times 0.0000065, or about 0.23 inch. For a 100 foot run it is about 0.78 inch. Aluminum expands about twice as much, and copper somewhat more than steel.

   Use a realistic temperature swing for the panel itself, not the air. A dark panel in full sun can be 30 to 40 degrees Celsius warmer than the air, and a clear winter night can cool it below air temperature. A range of 100 degrees Fahrenheit (about 55 degrees Celsius) is a sensible design number for the Fraser Valley, and colder sites such as Hope have a larger winter range.

   Tip: Darker colours run hotter, and so move more. Light colours reduce the swing.

2. 2Step 2: Use the panel length the manufacturer allows

   Manufacturers state a maximum continuous panel length, often 30 to 40 feet for fixed-flange panels, and longer for clipped systems where panels can float. Beyond the limit, the roof needs a mid-slope lap or a transverse joint designed to allow movement. See how to measure and order for planning lengths.

   Panels that are long enough to need a splice also need a stepped or expansion joint at the lap. Sealing a lap solid with rivets traps the movement. Follow the manual for the lap dimension and sealing.

3. 3Step 3: Anchor each panel at one fixed point only

   A panel should be held at one point and free everywhere else. On clipped standing seam, that is one or two fixed clips near the ridge, with sliding clips down the slope, so the panel grows and shrinks from the top toward the eave. On a fixed-flange snap-lock panel, the flange screws are placed in the centre of their slots, so the panel can move a little either way.

   A panel fixed at both eave and ridge is trapped. Its expansion has nowhere to go and it buckles, rips at the fasteners or pushes the trim off. Check that the eave clips are sliding, that no fixed clips were used by mistake and that the trims do not clamp the panel ends.

4. 4Step 4: Use sliding clips and slotted fasteners correctly

   Sliding clips have a slotted tab or a hook that holds the seam but lets the panel slide. They must not be overdriven, because the clip screw head must not clamp the tab. Keep the screw snug with a clutch-controlled driver. On flange panels, fasteners go centred in the slot and are snug. Off-centre or overdriven screws pin the panel. See metal roof screws.

   A slip sheet or smooth high-temperature underlayment lets the panel slide without gripping. See underlayment for metal roofs.

   Watch out: Clip screws driven tight are the most common cause of trapped panels. The clip should slide with a finger.

5. 5Step 5: Leave room at trims, closures and penetrations

   Panel ends at the eave and ridge need a gap, often about 1/2 to 1 inch at the ridge, so the panel can grow without pushing the cap. Closures should be compressible. Wall trims should overlap the panel without pinching it. A pipe or curb in the path of a long panel anchors it at that point and forces movement elsewhere. See ridge and hip caps and wall flashing.

   Sealant should be flexible and in a joint that can stretch. A rigid rivet across a lap does not allow sliding.

6. 6Step 6: Identify and reduce oil canning

   Oil canning is a cosmetic waviness in the flat of a panel, caused by stresses in the metal released when it is formed, installed or heated. It is not a structural or leak problem, and manufacturers normally exclude it from warranty. Causes include thin gauge, wide flat panels, light angle sun, an uneven deck or strapping, overdriven clips and panels pinned by fasteners.

   To reduce it, use 24 gauge instead of 26, choose panels with striations or ribs in the flat, build a flat deck, avoid overdriving fasteners and allow free movement. Darker colours and high gloss show it more than lighter and matte finishes. Careful handling matters too, since a panel that is kinked during carrying will be wavy forever.

Noise and Ticking

Fraser Valley Notes

Common Mistakes

• Fixing a panel at both ends, which buckles or tears it.
• Overdriving clip or flange screws, so the panel cannot slide.
• Using panels longer than the manufacturer allows for the profile.
• Rigid sealant or rivets across a lap that should slide.
• Tight trims that pinch the panel end.
• Ignoring the colour effect: dark panels move more.
• Treating oil canning as a leak problem rather than a cosmetic one.