Deck spans and limits for deck addition
Adding on to an existing deck. Existing second floor deck will be taken down to post/beam/joists. It’s 16’ wide, ~18’ deep, attached at house. The new section will be 21’ wide, 16’ deep attached at house with 2×10 ledger. Posts will be 4 individual 6x6s spaced 7ft apart. Beam is 3ply 2×10, sitting on top of posts. Deck floor will be 8.5’ above grade. My question is I have a couple different options for joists, post placement, and cantilever. Ideally, we’d put the posts at 16ft from house, and 2×10 joists 12”oc, which is the max span for those joists. Would this be pushing it too much, too much deck bounce? Other option is to put the posts 13’ or 14’ out from house, 2×10 joists either 12”oc or 16”oc, with a 2’ or 3’ cantilever beyond the beam, hitting 16’total. In that case I think a 3’ cantilever would be better with how we are placing grill area, allowing a bar/seating within that 3’ overhang from the posts. We want as much open space as possible under the deck, as we plan to waterproof the ceiling and have a covered patio beneath it. We just don’t want to push the span limits beyond what is safe by a decent margin. Any suggestions are appreciated. Not sure how close people tend to get to the span limits. Attached are basic drawing and house.
The span will be safe. A good row of bridging will help a lot at midspan. It may bounce a little, but it won’t be like a trampoline. I did have a problem once (will never do it again) using composite decking on long spans. Composite (especially if blind screwing with clips) does not really add much structural integrity to the joists, therefore the joists will start to warp a bit more with the deflection and your deck boards will look wavy and out of level of one another. Face screwed, solid lumber adds more structural integrity keeping all joists at same level and more evenly deflecting with one another. If using composite I would definitely bring the header in to shorten span. I personally would go with a 2’ cantilever instead of 3’. There’s quite a few other options as well, but I’m assuming you are trying to build as easily and cost effective as possible. Good luck!
I would go with 12" centers on joists with solid blocking over beam if canterlevering as well as a double rim
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Based on what I've read, I would strongly recommend that anybody building a deck to be reliant on a ledger-board reconsider, and try to design something free-standing. Ledger-board pullout is possibly the single most common major structural failure in a home, and damage to the wall & the wall's moisture management as a result of the ledger attachment is common... when the attachment is even made right at all.
Going to 12" centers (or 6" centers, or 3" centers) is the expensive way to minimize deflection. The cheaper way is to use deeper joists. In this case, would the height limitations permit you to go with 2x12's?
I favor overbuilding decks to 100psf floor loading, because you can't be certain that a party won't get overcrowded; If you aim for that kind of ultimate strength, you end up with a deck that doesn't flex much under the weight of one person.