Flat roof detail
mountaincabin| Posted inGeneral Questionson
Hi all,
I have contractors in the middle of a torch down roof install and am concerned about the process they are going through. I’m affraid it may be too late to redo without redoing the whole project but for piece of mind I have to ask this community to know it there is a huge miss or not.
This is their detail:
1st layer – Replaced flat roof decking with OSB CCX 1/2″
2nd – 3″ polyiso insulation on top of OSB
3rd – base layer for torch down
4th – white modified bitumen torch down roofing
Something I caught too late was no use of a vapor barrier on top of the OSB. Is this a huge miss? Any other thoughts on the layers they put down? Thanks,
Zach
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Replies
Both the polyiso & bitument torch down are extremely vapor retardent. What would be the purpose of vapor barrier between the OSB & polyiso?
Most torch down bitumen stackups don't include a vapor retarder at that layer, but some will have a slip surface, depending on the roof.
Is there going to be fiber insulation under the roof deck, or is the polyiso the only insulation?
Zach, the polyiso is a vapor retarder, and the torch-down is a full vapor barrier, so another vapor-retarding layer above the OSB would not do anything, as far as I can tell without seeing your project. The bigger concern would be the rest of the roof assembly, as low-slope roofs tend to be problematic. More information here, and in the related articles listed://m.etiketa4.com/article/insulating-low-slope-residential-roofs.
Zach,
The main issue is the relatively low R-value of the assembly (R-18 or at most R-20). That's below minimum code requirements in every location in the U.S.
Where is this building located?