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Eric and Todd, you are awesome for answering all these questions so thoroughly. Thank you.
My house is in the southern AZ desert but in the mountains so it doesn't see the 120 heat of Phoenix but we see high 90s in the summer and 20s in the winter. The 1930s 800 sq ft house is adobe brick covered in stucco. It originally had a parapet 1:12 flat roof with what looks like poured hot mop tar covered with gravel. In 1955 they converted the porch to a room and not long after that they simply built another roof on top of the original roof and connected it to the porch roof so I basically have two roofs and a kind of attic between the two roofs. I vented the attic with 2 gable vents and vented both soffits with many intake vents since they didn't vent anything...or insulate anything...or ground any electric. So, the roof is my DIY latest project and budget is a factor.
The rolled roofing is definitely multiple layers but it's built on a very stout frame that's built on the original roof and big beams inside the building and adobe walls. The decking is 1''x12''x10' planks that are in good condition and it's relatively flat. Attached are photos although I recently relocated all the vents since the plumbing vent is no longer attached to any plumbing (all studor vents) and I have a heat pump instead of a wall furnace now and the hot water heater now exits through the gable wall. So there are no duct/vent penetrations in the roof since I wanted to avoid cutting holes in the metal.
Disposing of the old layers of asphalt roofing may be an issue at the local landfill so I want to simply attach some perforated radiant barrier to the old rolled roofing, screw some horizontal 1''x4''x8' battens over the radiant barrier, and install a concealed fastener roofing panel such as Skyline panel from ASC. Then I was going to lay some more perforated radiant barrier inside the "attic" on top of the original hot mop roof. I'm trying to optimize heat reflection with the radiant barrier.
I'm leaning toward cutting a ridge vent but to be honest the two gable vents and a few dozen soffit vents keep the attic well ventilated. I've been up there on hot sunny days and it's basically cooler than my garage. The free air space math added up to exactly the correct intake for the square footage but a few inches short on the exhaust, which I could change with a larger or more gable vent.
I'm capable of cutting the ridge vent but if I have already ventilated the attic adequately then why should I do something that drastic? the ridge was built with a top ridge plate between the rafters.
I'm curious if you have an opinion on ridge or gable vents.
Another puzzle I'm trying to figure out is how to best ventilate the void created by the battens. With vertical battens?
Thanks. It seems like you have a good plan. Of course, I really can't comment as far as whether the structure can handle the additional weight load. Slightly pressurizing the "attic" with more intake than exhaust vent usually works just fine. You could "cross batten" by putting down vertical battens followed by horizontal if you want to vent directly beneath the new roof. Make sure that your new metal roof is approved for installation over battens. Good luck!
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