We’re about to build a small summer cabin on the lake. The design is a one storey Swiss chalet style cabin with a 2 in 12 low pitch roof supported on exposed roof purlins. We want to use metal roofing because there is so little maintenance for a seasonal building like this. However, we’ve heard different opinions about using metal roofing on a low slope roof. What do you suggest?

It’s true that if you look around the Kootenays you’ll see a lot of metal roofing on low slope roofs because it is easy to install and virtually maintenance free. However, the rule of thumb and the BC Building Code requirements specify that metal roofing should not be installed on roof slopes less than 4 in 12 pitch. 

There’s a good reason for this. When rain or snowmelt water rolls off a low pitch roof it travels relatively slowly along the roofing material. The metal sheets overlap each other by only one corrugation at the seam between each sheet. The slow moving water can wick up under these seams and leak through into your attic space. So a 4 in 12-roof pitch is considered the minimum slope that will keep the snowmelt and rainwater moving fast enough to avoid possible leaks.

As well, metal roofing over living areas should have 15 pound breather paper installed between the metal roofing and the strapping or roof sheathing. This breather paper ensures that if condensation builds up on the underside of the metal roofing (from moist heated house air), it will not drip on to your attic insulation. Good attic ventilation will also decrease the possibility of condensation on the underside of the metal roofing. If you really don’t want to change the slope of your roof, consider installing torch-on roofing, tar & gravel or “low slope application” asphalt shingles. These materials are all reliable for low slope roofs.

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This article was written by Steven Cannon, a Registered Building Official and partner in Lynch Building Inspection Services Ltd. of Nelson, British Columbia. It originally appeared in The Nelson Express.

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