New low slope metal roof that interfaces with higher slope existing shingled roof

Randall Moore
I have a new porch roof that I am laying out for my backyard. My wife would like the new porch roof to be of metal roof construction, and not very steep. However, at the ends where the new roof will mate up to the existing roof at the house, it will overlay an existing asphalt shingle roof. The new metal roof will be approximately 2 vertical to 12 horizontal slope. The existing asphalt shingle roof is approximately 9 vertical to 12 horizontal slope. Is it practical to have a new metal deck roof that at its high point interfaces to an existing roof, with that roof be asphalt shingles? Can that work? If so, what details and or precautions need to be taken? If you have reference to any details or illustrations or photos that would be greatly appreciated. What details or precautions also need to be taken with concerns towards replacing the shingles on the older roof in say 5 to 7 years? Thanks
Todd Miller
Isaiah Industries, Inc.
This is possible. You need a transition flashing that goes up under the shingle and out on top of the new roof. Exactly how that flashing interfaces with the metal roof depends upon the profile of metal roof you install. Make sure the transition flashing is mounted high enough no the steeper roof that it maintains a positive pitch.
Lynn Noesser
As mentioned, the transition, or step flashing, is the proper piece to use to change from different pitches, regardless of the roofing materials. One other note, you mentioned that the low slope metal roof section is a 2/12 pitch. Depending on the length of the porch and the profile of the panel used, in a strong rain you could get flooding of the overlaps. If you are using an exposed fastener panel with a rib height of less than 1" you may want to invest in overlap tape to ensure the laps are sealed for that potential.

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