Last year I built a 600 square foot a garage / workshop with a 2:12 pitch, low slope gable metal roof. The rafters are cross strapped with 1x4 so you can see the strapping and the metal roofing when you stand in the shop and look up at the roof. Last year in November, I was getting condensation drips all over my car and tools. Occasionally, during a hard rain, the roofing leaks water through to the interior. This is brand new, well-installed roofing. What’s happening?

This is a very common problem with metal roofing. Unfortunately, it’s not the fault of the product: it’s the fault of the installer. First, metal roofing is not designed or recommended for roof pitches less than 4in 12. The reason is simply that the water from rain and snow-melt moves off low pitch roofs very slowly. While the water is meandering down the roof it will occasionally work its way back up under the lap joint between two adjacent metal sheets. This is particularly probable when the roof is holding thawing snow. So your roofing will likely leak occasionally, not due to product failure but to inappropriate application of metal roofing on a low pitch roof.

From your description it seems that the roofing didn’t include a tarred “breather paper” on top of the 1x4 strapping, under the metal roofing sheets. This paper has one-way breathing characteristics, allowing moisture from the warm garage / shop air to flow through the paper, condense on the underside of the metal and drip onto the topside of the paper. This “ breather paper” ensures that this condensation will not return into the building envelope. If it is not installed under the metal, you can expect that this condensation will continue to occur whenever certain interior / exterior temperatures and building humidity levels converge to create a dew point on the underside of the metal. Unfortunately, the only solution will be to remove the metal and install this breather paper under the roofing. Then you are just left with the possibility of occasional leaks from water seepage that may also be mitigated by this breather paper.

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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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