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Flat roof detail

mountaincabin | Posted in General Questions on

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,

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  1. Expert Member
    Dana Dorsett | | #1

    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?

  2. Expert Member
    Michael Maines | | #2

    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:

  3. GBA Editor
    Martin Holladay | | #3

    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?

  4. Rontheroofer | | #4

    As an old roofer, and new commentor here, I would like to say something about insulation above deck and below roofing material.
    Adding polyiso above the roof deck is standard for most commercial projects.
    Attic spaces are normally insulated below the roof deck, above the sheet rock usually.
    Unless you are spray foam insulating the bottom of the roof deck and including the attic area in your heating/cooling envelope, the polyiso is just for protecting the roof deck itself from temperature extremes, which is extremely smart. Expansion and contraction of the roof deck over the life span of the roofing material(usually 20-50 years) loosens the nails in the sheathing from the rafters or trusses. Removal of the loose nails, and re-nailing, or replacing them with screws is something a good roof does without thinking about it. It is part of every re-roof we do. As to the vapor barrier, it is the base sheet for the torch down, you don`t need it under the iso board. For single ply roofs, it would go above the iso, right below the membrane. I have done custom residential projects with four layers of 3 inch iso board, which I thought was extreme, since the ceiling was insulated, sheet rock hung on hat channel, and the volume in the jack trusses was relatively small. Thermal transfer, through the wood of the trusses would have been minimal. I thought 2 or 4 inches would have been sufficient .

  5. Rontheroofer | | #5

    Also, I only see the bad effects of the spray insulation to the bottom of the roof deck. The condensation being trapped in the plywood has resulted in rotted roof deck and trusses. B especially aware of this effect where there are over builds on the primary roof deck. Please ventilate these space to allow the moisture and heat to vent, one intake high and one exhaust low, convection will do the rest of the work for you.

  6. Rontheroofer | | #6

    I prefer a two ply torch system. A full base, a full layer of smooth, and a full layer of granulated.
    If the base sheet is self adhesive, then Dens deck or polyiso is advisable. It is very difficult to remove SA bas sheet from plywood, usually damaging the plywood.

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