Newbie / female / homeowner here researching for GC husband who is very knowledgeable about general construction, but not roofing in particular, and definitely not online discussion forums. Our questions have to do with finding the best construction methods and materials to give us the look and performance we want. We did have a plan, but are now second-guessing it and thought we’d better ask for help now…
Goal: We’re building a pair of near-flat roof canopies on the front of our house -
a) over the front door (82" wide x 96" long), and
b) over a 60" wide x 33’ long walkway between the house and garage
These canopies are a highly visible design element, so they have to look great from ground level, and if possible, also from above as they’ll be very visible from our 2nd floor bedroom. There are lots of trees nearby and we’re in a hot & wet climate, so we’d like to be able to walk on the canopies a few times a year as needed to clean and maintain.
Current Status: The visible vertical surfaces of the canopies are heavy-gauge steel C-channels hung on 8x8 cedar posts. Between the C-channels we have 2x8 wooden rafters on 16" centers. The tops of the rafters were trimmed so the ends are slightly shorter than the centers, creating a very small ridge slope (1" on 30" & 1" on 41" runs). We then installed 3/4" treated plywood decking over the rafters leaving a very slight gap for possible venting at the ridge. On the edges, the top of the plywood decking sits level with the top of the steel C-channels. All of the preceding work has been done. On the undersides of the canopies, we plan to install wooden beadboard or slats.
Our Original Plan was to install an impermeable membrane over the plywood decking topped with a steel roof treated with a rust-proofing clear-coat in a heavy sheet or light plate. The metal pieces would extend the full width of the canopy with a slight crease at the ridge and bends at the edges to clip onto the tops of the C-channels (possibly supplemented with tack-welds). We were planning to use a series of 48" long sheets with a 3" overlap and a sealing tape (?) between the plates at the seams.
But then we got to thinking about venting and got all confused! The treated plywood seems damp and we don’t want moisture/mold problems. But would we even need to vent a flat roof where the top and bottom are approximately the same temperature? But with a metal roof, wouldn’t that be hotter/colder? And if necessary, how could we safely vent a roof that’s so nearly flat…
Anyway – now we’re tied up in mental knots and trying to figure out what to do.
Our reasons for preferring metal roofing was mainly for appearance, but we don’t want a traditional standing seam because the seams would be visible from ground-level. Modified bitumen or some other membrane would be less attractive from above - but maybe much more sensible?