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What’s the best way to ventilate a flat roof with an EPDM membrane?

sdaboulder | Posted in GBA Pro Help on

I often design houses with low slope roofs, ranging from 1/4″ per foot to 1″ per foot. EPDM has been a reliable roofing material, but because of its lack of any permeability, the potential for condensation in the roof assembly is increased. One option to address this in the Colorado Front Range, seems to be to apply 2″ of closed cell polyurethane to the underside of the roof sheathing and then fill the rest of the roof cavity with blown-in cellulose. This results in an unventilated roof cavity.

A less expensive option is to use just the blown in cellulose, but then the roof cavity requires ventilation. On flat roofs with an overhang, it is possible to install continuous soffit vents tied into a continuous ventilation baffle between the rafters. But unlike a more steeply pitched roof, there is no low soffit vent and higher ridge vent to set up a convective ventilation current. I am skeptical that soffit vents on each end of a rafter spacing that are both at the same height will provide effective ventilation. Does anyone have some thoughts on this? Thanks.

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  1. Riversong | | #1

    Cellulose has been used successfully on unvented roofs. The essential element on any unvented roof/ceiling assembly is thorough air sealing and a permeable inside finish to allow drying to the interior.

    If you do decide to ventilate, studies by Rose have shown that a ventilation cavity between insulation and roof sheathing improves moisture response even without air flow.

  2. homedesign | | #2

    Steven, There is a very simple and durable solution ... always provide an overhang and do not design flat/low-slope roofs.


    Any breeze will create a barometric pressure differential between the area under the soffit on the windward side of the house and the same area on the leeward side. You are looking for drying potential so I'd think that would be enough. You do need to air seal that assembly to keep bulk air from bringing humidity into it. so a well detailed ceiling plane is critical.

    We designed and helped to build an "ocean wave" roof a few years back that was 22" flat floor trusses on a shaped (vertically curved) wall with wedges under the trusses for bearing. we had 18" overhangs that were 2x6's sistered to the tops of the trusses. The insulation was vented with regular styrofoam baffles from soffit to soffit and the whole thing was blown full of fiberglass using a mesh blown-in-batt system. In this case the ceiling was wood 1x6 T&G. At the time I think we were still using a poly vapor barrier under wood ceilings, I'd probably use Tyvek if I were doing it today. More likely I'd use open cell spray foam if doing it today. I wouldn't use cellulose because of its ability to hold moisture but I realize that I'm in the minority on that concern at GBA and it's probably a regional thing in that I'm in North Carolina where humidity is more of an issue than in other parts of the country.

  4. John | | #4
  5. AN3RPvYXjG | | #5

    Don't try to vent the attic space, you likely will not have much success. You can install a vapor barrier to prevent the possibility of condensation. Ex example design may consist of the following (from the deck up):
    Rosin Paper (to prevent asphalt from dripping into the residence)
    5/8" DensDeck Prime (Fire Barrier)
    Vapor Barrier (2 plies of base sheet mopped in hot asphalt, shingle fashion)
    Insulation system
    1/4" DensDeck Prime
    Roof membrane.

    Michael H. Lichy
    [email protected]
    RC Lichy & Associates

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