How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
Why is my metal roof leaking?
Thursday, September 2, 2021
Most of the time a leak in a metal roof can be traced to an installation problem. Here's our list of problems that lead to leakage:
Panels improperly installed - Homeowner installed roofs often put the male (flange) edge exposed on top, which is opposite of the manufacturer's recommendation. See photo of wrong installation and diagram of correct way below. This defect leaves the panels vulnerable to wind blown water intrusion and back up from rain not exiting the roof quickly such as at valleys and gutters. The lap acts as an anti-suction device when installed correctly.
Flashings missing or not down-lapped - Flashings are intended to waterproof transitions from one plane to another. They occur at walls, chimneys, plumbing vent pipes, skylights, electrical service masts, vent hoods and from one roof slope to another.
Pitch too low - Traditional exposed-fastener roof panels such as 5V-crimp and corrugated are rated for use down to a 3/12 pitch — three inches of vertical “rise” for every foot of “run”. Standing seam and PBR (shown below) can be installed as low as 1/4” or 1/2” in 12” pitch because of their taller profile and either concealed fasteners or screws at the top of the panel ridge. Roofs installed below the manufacturer’s listed minimum pitch are prone to leaking during a heavy rainstorm.
Poor quality or incorrectly installed fasteners - The hundreds of screws at the roofing components are all candidates for leaks if not properly set. Screws can be misaligned, overdriven, under driven or missing. We have seen fasteners that have “backed out” of their original position over time and roofs where all of the screw heads have been buried in mastic as a temporary fix. It’s alway a good idea for pay a little extra for a premium quality fastener.
Dissimilar metals - can cause corrosion and leaks over time but we see this mostly in costal areas where the salt air compounds the reaction. You will likely find corrosion at roofing repairs or vent hoods installed with a dissimilar metal than the panel, as in the picture below.
Age - Most metal roof are warranted by the manufacturer for about 25 years, but can possibly last as long as 40 to 50. However, eventually the protective layer over the steel core deteriorates, exposing it directly to the weather, and rust-through spots start to appear.
Also, metal roofs are just as vulnerable as asphalt shingles for sheathing damage under the roofing due to long term gutter back ups, and panel and flashing damage from acidic leaf build up.
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Here’s links to a some of our other articles about METAL ROOFS:
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