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Community and Q&A

Insulation and ventilation in a flat roof

Matt Jennings | Posted in General Questions on

Hello all,

I’m re-roofing the flat roof that’s above part of my house (bedrooms, bathrooms and hallway). The current roof has pretty bad ponding and drainage problems so the plan is to build a slight slope by laying diagonally ripped 2x4s (or 2x6s) on top of the joists, then laying the plywood roof decking and installing flat roofing material (torchdown product). Target slope is about 1 in. per foot. The plywood roof decking is delaminated in some places.

While the roof is completely opened up from above, I’m going to take the opportunity to do air sealing and improve the insulation in the roof cavity. The current insulation is R-13 fiberglass bats laid in the 2×8 (16″ o.c.) joist bays. The current ventilation for this roof uses soffit vents, two inch diameter circles, centered between the joists. The space between any two joists is isolated from the other joist spaces. As far as I can tell, the “ventilation” system for this roof depends on pressure differentials to force air up to 20 feet through the joist bay from one two inch soffit vent to the other.

I believe that a better ventilation system for the roof would include a ridge vent (I have been reading a lot of Q&A posts here at GBA), but can a ridge vent be done on such a low slope roof? Is there another venting solution available?

My insulation preference is cellulose, is there any other insulation product that I should consider in this case? What if additional venting is not possible?

I live in Santa Cruz , California (climate zone 3). House is 37 years old and 1628 sq ft. Recent blower door test yielded 2981 CFM50.

Thanks in advance for any help you can provide,


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  1. RaterPaul | | #1

    Presence of ductwork in joist bays may dictate exterior XPS below sheathing ... could curb "ridge" & install ridge vent

  2. GBA Editor
    Martin Holladay | | #2

    Commercial flat roofs are almost never vented. Historic experimentation with roof vents for flat roofs proved problematic, with more water entering the vents than leaving the vents.

    The standard solution to your three problems -- old roofing, ponding, and insufficient insulation -- is to install tapered rigid foam on top of the existing roof deck, followed by new roofing. Tapered rigid foam will provide a slope to your roof. Contact a commercial roofing supply house to learn more about tapered rigid foam and which roofing products are compatible with available foam systems.

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