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I am trying to design a replacement deck for my home. Local regulations require IRC 2015 and the area is designated SDC D2. The surface height of my deck will be ~8' 8" above grade and there is a cement patio under the deck.

In DCA6 page 11, it shows that "Joist on Beam" and "Joist hung from Beam" are allowed. I'm trying to preserve as much headroom on the patio as possible and have chosen to hang the joists. Do the top of the joists have to lay in the same horizontal plane as the top of the beams as shown in DCA6 or can I set the joists lower so the decking will line up with the top of the beams?

Joist layout illustrations

Using the middle figure concerns me that water and moisture retention may cause rotting where the decking lies on top of the beam. Are there other concerns in leaving the beam exposed as decking surface?

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  • If your goal is to maximize headroom have you considered using steel? It won't be much more than wood, is faster and easier to install, and should last longer. Steel beams on helical piles.
    – Matthew
    Jun 30, 2018 at 20:09
  • @Matthew I would love to use steel. Unfortunately, the only code documents I can find are all related to wood design. Any references?
    – psaxton
    Jul 1, 2018 at 14:59

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Yes, by code you can raise the top of the beam to be at the top of the decking. However, 1) it will give you a “pattern” on your deck that you probably won’t like, 2) you’ll need to strap the two sides together, because you’re relying on withdrawal to keep the deck together. Usually we run the decking across the beams and then the decking acts as a “strap” too.

Also, having the top of your “built-up” beam exposed will allow moisture to lay between the members and cause dryrot. Having the deck on top of you “built-up” beam will “protect” the members and keep moisture out.

Where you live with seismic D2 requirements, you’ll need diagonal braces (especially that high in the air) and the code now requires a minimum of 2 hangers like this to attach to the house:

http://embed.widencdn.net/pdf/plus/ssttoolbox/sonpfljoja/F-DECKCODE17-p019.pdf

Also, consider using stainless steel hangers with either pressure treated or cedar or redwood lumber for longevity.

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  • So, it seems important to plan the decking layout to not have any seams over the beam to block moisture. Is this an accurate statement?
    – psaxton
    Jun 30, 2018 at 19:06
  • Thanks for the look-ahead at diagonal bracing. I've already considered that here, if you have anything to add to that question as well :)
    – psaxton
    Jun 30, 2018 at 19:20
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Another issue that you are going to have with either of these methods:

enter image description here .. or .. enter image description here

..is that you cannot simply support a stub end of a joist using the brackets. You have shown a cantilevered overhang in your other posting and for such application you have to extend the joists out over the top of the beam. If you wanted to use the hangers at the outside ends of the joists then your beam would have to be at the extreme perimeter of the deck.

enter image description here

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  • The other illustration is from DCA6. I'm posting a low res version of my overall remodel plans on my profile for anyone interested.
    – psaxton
    Jul 1, 2018 at 4:45

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