There was some talk at MetalCon last month about the thermal coefficient of metal roofing materials. It seemed like a topic worth addressing, even briefly, to give you an idea of what it means and why it’s important when you are looking for the metal roof that’s right for you.
Thermal Movement: temperature fluctuations throughout the day and year cause your metal roof to expand and contract.
This actually affects everything that is susceptible to expansion and contraction from temperature change (from your hardwood floors to your water bottle). But metal roofing is of particular concern for many because it is so structurally important for your home or building.
In a nutshell:
Hotter Temperatures —-> The roof expands
Colder Temperatures —-> The roof contracts
Those are the main points of thermal movement. But I’ll give some more details for those of you who want to impress your friends (or contractor or client) with some metal roofing know-how.
Length and Linear Accumulation
Metal roofs are manufactured and installed in long sheets. The short, wide side of the sheet is not susceptible to thermal movement simply because of the shortness of it. The width is a broken seam (although the panels are obviously joined quite securely) there is still some flexibility for contraction and expansion without causing any issues.
The unbroken length of a metal roofing component, which is often 2 or more long sheets from end to end are what you need to pay most attention to. It’s this long side that accumulates the tension of metal surface temperature changes (not ambient air temperature change).
The color of the metal panel as well as any absorption and reflection characteristics will change the maximum and minimum temperature capacities. As it heats up and expands, this will be compounded across the full unbroken length of metal roof, causing buckling and fractures.
Attachment Clips Save the Day!
As a result of these concerns, metal roofers have designed attachment clips that allow for this movement to take place without creating undue strain on the metal roof or the structure itself.
After all, a single metal roofing panel can exert several tons of pressure. You can imagine how quickly that would cause the roof to fatigue and eventually fail – and probably quite sooner than the metal itself starts to break down.
So they created attachment clips that connect the panels to each other but leave room for shifting and flexing.
1) A one-piece clip allows the panels to slide, so to speak, against each other.
2) Two-piece clips attach one part to the roof structure, the other part to the roof panel and the clip itself has a little wiggle room. This second option is nice in that the seal of the clip to the building and the clip to the roof doesn’t come undone, but the two part clip is what moves.
Thermal Movement FAILS
There are several things that people try to get away with when accounting for thermal movement that simply doesn’t work. Please remember these and don’t fall for them.
Not doing any calculations involving thermal movement can be a problem for designers and contractors. But because so many metal roof manufacturers already account for typical movement, this doesn’t always result in a fractured roof. However, if you have an extreme climate, you definitely want to make sure it’s been factored in.
Measuring thermal movement using ambient air temperatures will never result in real world accuracy. The temperature of a metal roof, in some climates, can fluctuate as much as 250 degrees.
Some manufacturers claim that a roof sheet will buckle (during expansion) before a joint, seam, or clip breaks. This is all fine and good but most roofs actually fracture during contraction, or cold temperatures. So don’t let this falsely lead you to believe that one way thermal movement protection is enough to save your metal roof.
Ok, that’s probably enough on that for now! Thanks for reading. I hope you’ve learned some useful things about Thermal Movement.
~J.C.
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