Roof options for mid-century modern
I need to re-roof my mid-century modern home. It has a low-slope roof (1.5:12 pitch). The roof deck is 2×6 T&G, supported by 4×6 rafters, 4′ on center. The rafters are exposed and the underside of the deck is my ceiling. The rafters span 12′ and I don’t think they can support much more weight. This is in Michigan, climate zone 5.
The current roof assembly is decking, felt, 2″ of poly-iso board insulation, EPDM membrane roofing. I want more insulation, and I’d like a metal roof but could settle for another membrane if cost for metal is prohibitive or if there are good reasons for avoiding it.
What are my options? Can I re-use the poly-iso? Do I need to put ice and water shield directly on the decking before laying the insulation? Should I sheath the assembly before putting on the metal roof?
(see also: https://www.greenbuildingadvisor.com/question/is-sheathing-a-metal-roof-necessary and https://www.greenbuildingadvisor.com/question/roof-design-and-exterior-insulation-for-cabin-structure-with-wood-beam-ceiling)
Any other advice or considerations for my re-roof?
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Replies
You don't want regular I&W over the existing t&g. The issue is most I&W has a modified bitumen adhesive (ie fancy tar) which you don't want exposed to your interior as it can smell when it gets hot.
A peel and stick is a good idea just make sure it has an acrylic or butyl adhesive. Give it a sniff test first as some can be strong. This will seal up the air leaks through the boards but you still need to seal up along the gaps between the boards over the walls. What I've done in the past is when the roof was off, drill a 1/2" hole between the T&G boards over the exterior walls and inject spray foam or flexible caulk (see attached picture). On that specific roof, there was enough airflow along gap between the boards that each had a spider colony.
If your EPDM has failed, In most cases good chuck of existing polyiso will be saturated, plus peeling the roof off will take the facer with it. Chances or re-use are pretty slim.
You can do metal over low slope but that requires mechanically seamed and gasketed roof. I would only go this route if you have a simple flat roof with minimal roof penetrations (ie no skylights). A metal roof at that slope relies on the sealants for water tightness, so getting a good installer is critical. In most cases a new EPDM or PVC roof is simpler.
Thanks, Akos. This is helpful. I wouldn't want the smell of tar in the house.
The EPDM is patched, but overall in pretty good shape.
I do have a sun tube, fan vents, and and water heater vent and intake going through the roof, as well as a chimney. So I might be stuck with the membrane roofing, which will eventually end up in a landfill :( It also acts as a drumhead when it rains!
I am staring at the same issue for a Deck House built in the mid-70s that is in desperate need of a "deep energy" retrofit.
Most of the roof is asphalt shingles, but I do have one area with a rubber roof ...
An Acton Mass Deck house with the extended living room? I don't think they ever figured out how to seal those up....
I&W doesn't smell since it doesnt get hot since it is under the insulation.
15 years not overinsulated, nary a whiff
Good to know. Thanks for sharing your experience.
Planning to re-roof your Michigan home? Consider checking with a structural engineer to ensure your roof can handle the extra weight of more insulation. If the existing poly-iso is in good shape, adding more could boost your home's warmth. Think about using an ice and water shield for added protection against winter weather. When deciding between metal and membrane roofing, balance cost, benefits, and any structural requirements like sheathing. Also, make sure your project aligns with local building codes, focusing on insulation and moisture control. Need more advice? I'm here:https://stripesroofing.com/ !
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