Wood-framed deck: how tall is too tall?

[Ben29]

I have a wood-framed deck that is 4'-4" wide and is attached to the house via a ledger on one side. The top of the deck is 17'-9" above the ground elevation and the architect is suggesting 8x8 posts. See image below. The prescriptive design for post heights for decks only goes up to 14ft, and this just doesn't feel right the way they have it drawn. I could lower the braces, but then the braced would obstruct the window view. I'm not sure whey they couldn't just cantilever the floor joists to create this deck since it is only 4'-4" wide. Thoughts?

 

[jayrod12]

Cantilevering joists comes with it's own issues. Proper waterproofing at the junction between the deck and the wall is the most concerning one.

I feel that if you're outside the prescriptive part of the code then you need an engineered design. If you are an engineer, then you aren't obligated to follow prescriptive requirements. Make your analysis of the structure and determine if it is acceptable. If not, change it until it does.

 

[rowingengineer]

I would drop the braces and change the angle to miss the windows. I would treat it as a portal frame and analyse as such, and make sure deflections work.

 

[XR250]

I would look at cantilevered joists also.

 

[jayrod12]

You could probably omit the second brace on the right side of the right hand column if that's the one giving you view issues. Seems redundant.

 

[IRstuff]

I'm right in the midst of replacing such joists because of dry rot, which required ripping up the interior flooring so that the replacement cantilevered joists are properly replaced.

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[KootK]

I also don't love the cantilevered joists on account of the building envelope issues, at not least in snow country. At this very moment I have this same condition at my house with 18" of snow sitting up there waiting to be shoveled. All I can think about is it melting back into the interior and messing up the stuff that I can't afford to replace.

A deck that's 4' wide is going to derive it's lateral stability from the decking cantilevering laterally from the main building. The posts won't do squat unless you take the braces wayyyy down. I wouldn't sweat this.

 

[r13]

Will intermediate beams help?

 

[dauwerda]

I would investigate getting rid of the kickers/angles all together and put diagonal bracing in the plane of the deck to create a diaphragm that caries the lateral loads back into the main structure.

 

[STrctPono]

dauwerda beat me to it and I agree. I was also going to recommend that this perhaps might be a better choice. Run diagonal bracing straps across the joists in the plane of the deck and tie it back to the house.

 

[Ben29]

You have given me alot to think about! Thank you all!

 

[phamENG]

I agree with KootK - this is a really narrow deck. Use the deck boards as your diaphragm (which is allowed - there's numbers for it in NDS), use tension ties at each end to tie back into the floor diaphragm, and then use your columns as gravity only.

 

[dauwerda]

That depends what your deck boards are made of, I wouldn't count on it for most composite decking.

 

[XR250]

@KootK and Pham,

The problem with using deck boards as a diaphragm is that you have no control of the decking used. If they use a hidden fastener system, is there a way to quantify the shear capacity?
I typically show the under deck planar bracing in these situations. That being said, my parents have a 60 year old cantilevered balcony in MD that still functions fine.

 

[Canuck65]

Agree with dauwerda and STrctPono, for such a narrow deck, horizontal diagonal bracing attached to the underside of the deck framing is the way to go.

 

[Ron247]

I would avoid cantilever joists if possible. The Cons outweigh the Pros.

I agree with others that 4'-4" is so narrow I would prefer the diaphragm deck route. You can reinforce the corners of the outer frame (you would see the braces in plan view) in addition to any deck strength or diagonal. If it is a little shaky after initial framing, I would add the 2 diagonals but not the one in the window.

 

[phamENG]

Fair point on the deck material. If it's not standard lumber, then discreet bracing would be a more robust and predictable route.

 

[KootK]

My point was merely that I see horizontal plane system bracing being the way forward here. Beyond that, I'm happy to leave the design and specification of the details to Ben29.

 

[phamENG]

Speaking of narrow, what's up with that spiral staircase?

 

[STrctPono]

Good eye phamENG. The OP really should talk to the architect about that. I actually built a spiral staircase in my house a few years back at 60" dia. I get 25.25" walking room but mine is wood. Even at steel, a 40" dia. staircase will not be enjoyable and is questionable on whether it will be very functional. They should bump that up to at least 48" dia.