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Community and Q&A

How do you properly build a deck on a flat(-ish) roof?

Armando Domingos | Posted in Green Building Techniques on

I live in a home in NJ (4A/5A climate zone border) with a 3rd floor wooden deck over a a slightly pitched flat roof with conditioned living space below it. This deck was built about 7 or 8 years ago, and is supported by 4×4 posts which are going through the roof and on top of the rafters on the second floor. The ceiling on the second floor is leaking now, and I’m pretty sure the deck isn’t too safe.

But I do enjoy the deck, and I’m hoping to do some renovations soon and wanted to ask everyone on here what is the best way to build a rooftop deck that allows the building envelope to remain intact, but is still supported properly. I know how to make the roof well insulated, I know how to properly support the deck, but how does I combine the two?

Any help would be appreciated, and thank you in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    You are requesting more information than can easily be conveyed in a typed response to a question posted on the Web.

    The short answer: this is not a question of proper support; it's a question of proper flashing. You need a skilled roofer, not a carpenter.

  2. Expert Member
    Armando Cobo | | #2

    The way I design decks over flat roofs in the SW is to flash and waterproof your roofing surface accordingly without any penetrations, then install your deck floor rim and joists to counter the roof pitch so you can install a flat flooring surface (Trex usually). You can install your corner posts bolted to the rim and joists or to a parapet wall if you have it. Make sure your roof decking is strong enough to support the additional weight and traffic of your deck.

  3. Armando Domingos | | #3


    Thank You (its not often I can tell another Armando thank you). I was hoping to build the deck over rigid foam as I am trying to avoid thermal bridging.

    I don't think the rigid foam will support the weight of a deck + foot traffic without it compressing. I found is this: which is made to support foot traffic. I'll have to give it a shot and see what happens.

  4. Expert Member
    Armando Cobo | | #4

    Dow makes great rigid insulation boards and 40psi board is plenty. You need a 2” minimum, and then I would apply a Modified Bitumen roofing membrane, and then your deck floor joists on top. Make sure you have good flashing details as well.

  5. Armando Domingos | | #5


    Thank you for the response. I've been doing a bit of research, and on, I found this:

    which is great, except that I noticed he only recommends felt over the foam board. I don't know if this is specific to a deck that is going to be tiled over, but I was pretty surprised, considering I know how much these rigid boards break down with UV light.

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