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I am trying to design a roof top deck above a flat roof on a row house. The deck must bear on the parapet walls (4 layers thick structural brick walls on both sides of the roof), it can not bear on the roof itself. Because of the distance between my two parapet walls I must use 3 laminated 2x12s for beams on the north and south ends of the deck with 2x8 joists running between the beams. Because of a variety of zoning restrictions the surface of my deck must be lower than the height at the top of the 12 inch high beams (otherwise the railings would have to be too high for zoning).

Ideally I could hang my joists below my beams. That would definitely allow my railings to satisfy code and zoning and, if there is a code compliant way to do that for deck joists, solve my problem. Then my beams would essentially form the bottom part of the railing on those ends (capped by a deckboard or something to prevent rot, with the railings then going up from there). Alternatively, I might be able to attach my joists to my deck beams so that they are flush with the bottoms of the beams, as opposed to the top (this might or might not allow my railings to satisfy zoning) - but as I read it neither of those possibilities appears to be contemplated by the code?

I realize that another possibility would be to find a way to lower my beams. In the case of the south beam, this would interfere with the roof's attachment to the parapet walls and would create an unhelpful potential leak.

Thanks!

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  • Hello, and welcome to Home Improvement. If an answer is helpful, please click the large check mark next to it to accept.
    – Daniel Griscom
    Jun 29, 2020 at 10:45
  • You may want to hold on a bit on that check-mark. It's only been 6 hours since you've posted your question and most of America is just waking up (yes, I know there are other parts of the world, but a lot of the answers come from US participants). That said, have you considered adding another beam between the North & South beams to shorten the span allowing you to use 2x6 joists? That will give you 2 more inches.
    – FreeMan
    Jun 29, 2020 at 12:13
  • The 2x6s are a good idea - unfortunately though I can't add another beam since it would stick up beyond the level of the deck.
    – Bill
    Jun 29, 2020 at 18:27

2 Answers 2

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Joist hangers are used all the time to fix joists to the side face of the beam. I see no particular reason that you wouldn't be able to hang the joists even with the bottom of the beam using standard joist hangers like these:

enter image description here

Picture Source

In fact ceiling joists may very well be hung with similar brackets like these where the ceiling height is set to be at the bottom of the support beams.

Hanging your deck joists totally below the beams seems a bit problematic. My guess would be that even if you could come up with some type strapping to securely hold the joists up against the bottom of the beam that an inspector may very well reject the concept, especially if they figure out that you are playing games to work around your railing height restriction.

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  • Thank you! There wouldn't be anyway to drop the beam instead, right? I.e. have it bear on a surface at x height and then have it drop down and run horizontal at a height of x -10? I'm guessing that would take an engineered solution...?
    – Bill
    Jun 29, 2020 at 18:29
  • @Bill - Since you are doing something that seems to twist standard practice to try to squeeze under some other rule thresholds I strongly suggest that either do it yourself or hire someone to draw up plans of what you think you want to and take it to the local community and submit for a building permit. You will need this to proceed anyway and this is the only for sure way you can get some feedback from the folks that would review, inspect and approve your construction plans,
    – Michael Karas ♦
    Jun 30, 2020 at 3:15
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Attaching joists (2x8 for example) to bigger beams (4x12) so that the bottom of the joist is flush with the bottom of the beam is done all the time. Here's one I did for an indoor project-

enter image description here

Should not be any different for your roof deck. Just make sure you use the right fasteners for outdoor use.

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