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Flat roof questions

jedoyle | Posted in GBA Pro Help on

I have a mid-century modern house in New England with a low-slope (1/2 pitch) old school tar and felt built-up shed roof that needs to be replaced. I’m pretty sure there is no insulation to speak off and I would like to add a continuous layer on top. Also, I would potentially like to be able to add roof-top solar sometime down the line. 

Insulation- what kind? how thick?

Underlayment/ventilation details?

Best low-slope roofing material for solar?

Thanks in advance

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  1. jberks | | #1

    Hi John,

    I'll take a stab at your questions. I'd say my first concern is the weight/stability of your future solar panels. I'm not a solar guy so I don't have any practical experience, I hope someone else can chime in on this, but I'd say place 3/4 ply on top of polyiso insulation as decking. Then roof on top of that.

    The thickness of your insulation will depend on your climate zone and how much insulation you have between the rafters.

    You'll have to tell us if your roof is vented or not. But otherwise, you don't need venting of your overroofing, unless it's a steel roof or something.

    As for the roof material itself, I say go with polyurethane. It's easy and it works well. But I'm biased because I just love polyurethane in general.


  2. GBA Editor
    Brian Pontolilo | | #2

    Hey John.

    I suggest you start with this article, Insulating Low-Slope Residential Roofs, and consult a PV installer before proceeding to make sure your roof will be ready for the hardware and panels when you decide to add them.

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