Asphalt shingles on low slope/flat roof in a very specific climate?
I will be roofing the small house I’m building with asphalt shingles and I’m just barely okay with that since I had my heart originally set on metal and then composite but had to settle for asphalt for cost reasons. The majority of the roof is 4:12, even over the extensive West patio. The roof over the North patio sharply flattens at one point, though, to be closer to 1.5:12… or essentially “flat”. I realize that shingles are no longer an official option for that part of the roof — but all of the options I do see are simply a bridge too far! I compromised enough to settle on shingles for the majority of the roof and having to compromise again with an even worse aesthetic option thoroughly ruffles my feathers. Thus, I want to continue the shingle install on the low slope / flat section of the roof, as well.
Wait wait wait! I know that doing so is definitely not recommended. I know that it will instantly void any and all warranties on the shingles. I know that it would be patently foolish in any climate with snow or ice. But hear me out.
I live in Phoenix AZ; it never snows here; there is never ice buildup; it barely rains at all; the wind only rarely blows from the North (up to 10% of the time in the Dec-Feb months); and the winds never get to any substantial speeds compared to most parts of the country.
So I’m thinking that I will apply a self-adhered underlayment over the low slope section of the roof — something like Grace Select or even Grace Ice and Water Shield (since it is thicker). That will serve as my “true” waterproof layer. I would then attach the shingles on top almost normally but squished just a bit to have a little less of a reveal. My thinking is that the shingle layer will mostly just provide UV protection for the waterproof underlayment. There is zero chance of ice or snow building up underneath the shingles. There is extremely low chance of wind driving up the shingles (rarely from the North and relatively slow). Any standing water will tend to evaporate pretty quickly and even if not, the Grace Select is waterproof. If there was a freak (and I do mean freak) wind storm like a micro-burst blowing South and a few shingles did blow off or shove water underneath the shingles, then no big deal because of the waterproof underlayment… and besides, MOST of the low slope part is just over a patio, not over the main part of the house.
I’m not hoping for official validation here because that’s unlikely to come. Instead, I want to know, is my logic fundamentally flawed in some way? That is, is my approach here a terrible idea even in my specific climate? If so, what factor am I completely missing?
Heck, let me know even if it’s just a bad idea and not a terrible one. Just let me know why you think it’s bad.