Cantilevered Wood Deck 1

[Don_100]

Hello,

I have recently purchased a home currently under construction that includes an attached cantilevered wood deck

The deck details are...22’-0” long x 11’-0” wide with 2x10 joists at 16”

The joists have a 5’-0” cantilever and a 6’-0” back-span attached to a ledger beam.

There is no vertical bracing anywhere in the design.

I have a solid background in structural engineering but I am not an engineer, however, I have never seen a deck with a near 1:1 ratio without vertical bracing and I am very concerned about uplift.

I can't find any documentation to support a design like this...Is there any way this deck could be engineered to these dimensions?

 

[XR250]

I have a similar design in my house and it works fine. The uplift is not huge and can usually be handled by the uplift capacity of a joist hanger. It does bounce a little which seems to be a combination of joist downward deflection and beam humping. Make sure the beam is tied down sufficiently.

 

[STpipe]

This would really depend on the stiffness of the joists and the ledger beam. I don't have much background in wood (or specifically residential construction), but my home has an issue with a cantilevered deck with no vertical bracing deflecting too much at the tip. When speaking with the inspector who looked at it, his comment was that a general rule of thumb would be a 1:2 ratio for the cantilever and backspan respectively. I was never able to inspect the framing since everything was covered by the time I purchased, so I have no idea what kind of back span exists.

 

[SlideRuleEra]

Is there any way this deck could be engineered to these dimensions?

Yes, but it may have just been "built", without any design.
Problem with outdoor decks is, given enough time, deterioration from weather. Wood rots (even treated lumber) and fasteners (including galvanized) rust. A cantilevered deck invites sudden collapse from a load (people) that it once carried easily.

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[XR250]

Problem with outdoor decks is, given enough time, deterioration from weather. Wood rots (even treated lumber) and fasteners (including galvanized) rust. A cantilevered deck invites sudden collapse from a load (people) that it once carried easily.

I would argue they are less likely to collapse as they do not rely on any exterior fasteners or hardware for support and have more redundancy. We tapered out joists to allow better drainage. Have not had any leakage issues in over 20 years.

 

[oldestguy]

I'd be concerned about failure under a full deck of people. Let's take the case where a big group is on the deck and most are on the overhang. The ledger connection at the house has to resist the uplift of that load. It is unlikely that the lifting resistance of the nailing to the house was done specifically knowing what that unusual loading might cause. In addition to beefing up that confection to the house, I'd consider adding inclined posts between part of the overhang and the foundation for the existing columns. Belt and suspenders maybe, but significant improving the support of the deck.

 

[SlideRuleEra]

XR250 - I was thinking of a deck where both the back-span and the cantilever are outside the building envelope and the back-span is "attached" to the house, but even with the back-span protected there can be problems. Either way, the OP implies he does not know if the deck was designed or not.

See the Engineering Tips thread on the balcony collapse in Berkley, CA.

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[hokie66]

Yeah, I agree with SRE. You read about decks collapsing all the time, reported on this site and elsewhere. And I think most of the problems are inadequate connections at the building and/or deterioration with time.

 

[Once20036]

The American Wood Council published a guide entitled, "Prescriptive Residential Wood Deck Construction Guide".
Its available all over the place, for free, and should be at your finger tips with a quick google.
Their allowable overhang for 2x10s at 16" centers varies with species, but never exceeds 3'-5" or 25% of the backspan.

Personally, I'd start by looking into uplift capacity of the joist hangers at the ledger.

A lot of residential construction is prescriptive. Do you have any indication that the house was engineered?
If the house wasn't engineered, I`m sure that they didn't hire someone just for the deck.

Personally, I`m not concerned by the lack of vertical bracing, as long as it feels stable under foot.
There's additional information on bracing in the AWC guide.

 

[Don_100]

Thank you everyone for the feedback

I do have access to the AWC Guide and have been referring to that as a reference.

After some further discussion I have been told that the deck was engineered but have no information on the type of hangers used.

I assume there must be a tremendous amount of redundancy built into the AWC guide if a near 1:1 ratio will work.

 

[chicopee]

And if you need additional feedback, your town engineering people will probably agree with the above respondents and apply pressure on the contractor for a proper design.

 

[jayrod12]

I'm not surprised a 1:1 ratio works. I've done it many times to fit the architectural constraints (in fact I've even exceeded it for items such as roof overhangs). As others have indicated, the key is providing adequate uplift connections at the rear connection. If you can get a good look at the hangers, you may be able to determine which manufacturer, and model, and then check accordingly.