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Seeking advice on proposed EPDM low pitched roof new construction

| Posted in General Questions on

I am planning a post and beam house with a 1/10 shed roof for climate zone 5 in eastern PA.  The beams are on 5′-4″ centers across the 42.7′ width shown below, and the house is about 25′ in the other dimension.  The stock plan has 12″ rafters on 24″ centers, likely for standard insulation.  A span calculator shows that 2×8 rafters on 24″ centers will handle a snow load up to 90psf – where 50psf is the requirement in my area.

*After* researching low pitched roofs here on GBA and coming up with this EPDM solution, I discovered that metal roofs can handle a 1/10 pitch.

There is a R-49 requirement for roofs here, and I discovered that polyiso’s R-value decreases to about 2.8 per inch in cold weather.  However, polyiso can withstand EPDM adhesives, whereas EPS and XPS can not.  A mimimum ratio of exterior R to total R of 41% is required.  I am proposing therefore 2″ polyiso on top of 4″ EPS outside, and either 4″ of spray foam (R-28) or possibly R-23 batts somehow attached to the underside of the sheathing (it is recommended here that the interior insulation is in contact with the sheathing).  The batts would technically satisfy the overall R-49 requirement, but at R-2.8 per inch estimated for the polyiso worst case would result in a total just below the requirement.

Am I anywhere near close to something viable?  I tried to distill what I saw in different threads here for my circumstances.

One question I have no clue the answer to is what happens along the four sides of this giant sandwich – how are they structurally finished?  I see that this kind of roof relies heavily on proper sealing along the edges, between the membrane and the drip edge, and drip edge to deck.  But what will the fascia attach to?

Thank you in advance for any assistance.  Obviously I will have to consult a professional, but I am trying to gain an understanding before taking that step.

Don

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