# Please help with foam over roof plan



## idahoan333 (Sep 18, 2021)

Hey Guys,
I am trying to redo a roof an a very old cabin I moved and i've been having a hell of a time. Most roofers don't want to touch it and i'm in an area where building has absolutely blown up and getting anyone over to even look at it has been a nightmare. I thought I had a decent plan for this project but the roofer I have found (who was recommended by the building inspector and has a good reputation) is now convinced that my plan will end in failure and says he won't warranty his work if I proceed with the plan I have. I had another company give me a bid earlier this summer and said it was fine but their bid was so high I couldn't afford it. Anyways, the roof:

It's a log cabin with post and beam inside and 2x6 t&g deck right over the beams. The roof is gambrel/barn style roof with a 22 degree pitch on the top part, and 55 degree pitch on the bototm. Zone 6b with 90psf snow ground load. The ceilings are too low on the second level to add insulation to the interior side. From the inside out it's currently:

2x6 t&g
tar paper
2x2 running horizontally with 1.5" rigid foam cobbled between.
2x actual 3" running vertically with two layers of 1.5" rigid foam cobbled between
1x4 strapping horizontal
metal roof

I bought two pallets of reclaimed foam for a very good price and was originally thinking of adding the foam to the top side, but after a lot of deliberation (and the roofer originally convincing me this was a better plan than what i'm about to propose) we ditched this plan as it required screws that were too long and hitting too small a target. The changed plan:


take the roof down to the t&g deck
ice and water shield directly on the deck creating an air barrier.
2x3 or 2x4 purlins vertically on flat with 1.5" rigid foam cobbled between
a continuous layer of 4.5" rigid foam with the seams staggered from the bottom layer
2x3 or 2x4 purlins vertically on flat again with screws going through the 4.5" foam layer and into the purlins below.
Then EITHER leave and air gap and vent to the ridge, or cobble in a 1.5" foam layer again, then 7/16 deck and underlayment
new metal.

So this roofer is freaking out about three things:
1. He is convinced that the moisture in the house will hit the ice and shield, form condensation, and drip back through the t&g deck into the house. I have read every article and comment thread I can find on this, talked to a local energy consultant, and posted on green building advisor, and everyone it telling me that this is wrong. Except this roofer who has been roofing here for 30 years. Everyone says I MUST have an air barrier at the deck with this roof foam over plan, or moisture will get into the roof. I'm not a pro, but my understanding is condensation happens when warm moist air meets a cold surface. With ALL of my insulation on the exterior side of the roof, the deck is open and drying to the interior and the condensation point should be in the foam somewhere. Am I wrong?
2. He thinks that the wood being separated by 4.5" of foam will put all the force on the screw head and this is not structurally sound. Now, I completely get what he's saying here but I talked to an engineer and again researched the hell out of it, and while this isn't a common way of doing roofs, it is not an unheard of method of over roofing, especially in retrofitting. The engineer says no problem whatsoever, but he's really freaking me out saying if I get iced up the extra weight may pull the roof down. I'm skeptical of how much ice/snow build up i'm even going to get on a 55 degree pitch but this guy has been roofing in this area for 30 years and says he's seen it.
3. He is convinced I need an air gap betwen the top of iso and the second roof deck/metal to prevent ice damming. Energy consultant said don't need it, fill it with foam. I have read articles completely contradicting each other on this topic. This is the one thing item i'm least concerned about, because I only lose 1.5" insulation if we do it, and I do get the premise behind the air gap, it definitely couldn't hurt lower the risk of damming, just wondering what people's thoughts are on if its necessary in this roof assembly.

Sorry for the length and thank you in advance for any help - my head is spinning at this point and I don't know who to believe.


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## Deerfly (Mar 14, 2021)

Get some GAF thermacal 1 nail base. That should work for the application you describe. I believe it comes in different thickness and also similar products from other manufacturers as well


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