Mikeysp has just visited a friend’s lakeside cabin and come away with serious reservations about an attached deck built over a steep embankment. Scenic, possibly, but in Mikeysp’s view, a disaster waiting to happen.
The design includes a 4-foot cantilever of the 2×8 framing, structural posts that seem inadequately anchored at the ground, and a lack of lateral support.
“I warned him that it is unsafe,” Mikeysp says in a recent Q&A post. “If that were to fail, it could go down, into the water, and really harm someone, whether elderly, young, or rendered incapacitated from the impact with debris or entangled in the mess.”
The problem might be put on hold, except that the owner is planning to have 20 family members over for Thanksgiving. He wants to know whether anything can be done now to make the deck safe. Says Mikeysp,”If that thing went into the water with half his guests on there, it would surprise me if it did not end in at least one fatality.”
Among Mikeysp’s ideas are to add cross-bracing for the 4×6 support posts, find a way to anchor the posts to the ground, lift the deck up and put in new posts, or find a way to eliminate the cantilever with new bracing.
Or should the whole thing just be ripped down?
That’s the topic for today’s Q&A Spotlight.
It’s time to call in a pro
This is no time to rely on advice from an online forum, says Akos.
“There are a lot of structural issues here,” Akos writes. “You are right to be worried. Finding a fix through an internet forum is not the best way to go. I would get a structural engineer to spec the details.”
At a minimum, Akos recommends lateral support for all columns, meaning cross-bracing in both directions, and a more robust connection between the columns and the main beams than the bolts now appear to be holding the structure together.
“I second Akos’ opinion,” says GBA Editor Brian Pontolilo. “Get some local professional help with this.”
Bolt connections are a problem
Zephyr7 finds the bolted connections especially troubling.
“You’re not supposed to have bolts taking load like shown,” Zephyr 7 says. “The joist should rest on top of the column, or in a notch cut into the column so that the bolt is just holding the assembly together but not actually carrying any load.”
If bolts are used to carry that kind of load, the builder would have had to consider the shear stress in the wood and used better hardware than what’s typically on sale at the hardware store. “It gets a lot more complicated,” he says.
Among possible remedies are the addition of some steel in the cantilevered span so the outermost columns could be eliminated. That would get the posts a little farther from the water, a plus. He also wonders whether screw piles could be used to anchor the posts to the ground rather than concrete.
“Concrete can sometimes be difficult to work with near a body of water if the hole can’t be kept clear of water during the pour,” Zephyr7 says. “Screw piles don’t have this problem.”
Some of these details are covered in a publication of the American Wood Council that sums up basic code requirements. The publication is recommended by Malcolm Taylor.
The cantilever is too big
Graham Love suggests that the cantilever should be a maximum of 36 inches, not the 48 inches as built.
“The cantilever distance is determined by the length of the joist,” he writes. “As is stated in the [document] provided by Malcolm, the cantilever can be 1/4 the length of the span of the joist. With a 16-foot deck joist and a 4-foot cantilever, that means the span is 12 feet from ledger to beam.”
Tyler Keniston also points to an apparent anomaly in the code recommendations for overhangs that Taylor has cited. In some cases, the allowable cantilever is smaller when the joist spacing is tighter. For example, the overhang with 2×8 framing is 2-foot-4 when joists are on 24-inch centers but only 1-foot-10 when on 12-inch centers (as this particular deck is framed).
Zephyr suspects the reason is that the greater loads allowable with closer joists spacing also increases the dangers of shear stress. “Basically I think they’re assuming the closer spacing is to allow higher floor loads, and the ultimate allowable shear strength for the wood becomes more of an issue,” he says.
Maybe the cantilever exceeds code limits, but Keith Gustafson is more concerned with what isn’t visible: How is the deck attached to the building? What condition are the posts in?
“The deck is unlikely to flip into the water from the cantilever,” Gustafson writes. “Mode of failure is the posts fail sideways, the entire deck surface becomes a parallelogram and falls down as the bolts pull through the wood at the attachment. The right answer is to not let a bunch of people hang out on the deck. The legalistic answer is to post a sign saying the deck is a POS.”
If forced to come up with some repairs, he would diagonally brace each post in both directions, run new diagonal supports under the deck joists and screw the diagonals to the joists, and beef up the attachment points to the house.
Don’t get involved
The advice from James Someone is pretty simple: “Rebuild that mess.”
So would Peter Engle.
“As a forensic engineer, my alarm bells aren’t so much ‘Danger Will Robinson,’ but the team of attorneys that are waiting in the wings,” says Engle. “If you touch a POS like that, you own it and [it’s] liability. Even a well-crafted waiver isn’t going to help much if someone gets killed. If you do try to improve this deck as a ‘favor,’ drive a rented truck, get paid in cash, and wear a fake beard and hoodie.”
There are too many issues here to make fixing the deck worthwhile, he adds. “Doing things right sometimes costs actual money,” he adds. “But when the downside of doing things wrong is a serious risk of bodily harm, is the view from your cheap-ass homebuilt deck really worth it?”
Our expert weighs in
GBA Technical Director Peter Yost has a few closing thoughts:
It was easy to decide on adult supervision for this one; I enlisted the expertise of a leading local engineering firm, Stevens & Associates. I sat down with Ben Harwood, a project engineer, to discuss this Q&A Spotlight. Here are the key takeaways from my conversation with Ben:
Good prescriptive reference: There is a good reference for deck construction, including cantilevered decks: AWC Prescriptive Residential Wood Deck Construction Guide. But remember that if there is any deviation from the prescription of the detail, then all bets are off.
Good engineering discussion forum: There is a really active forum for engineers, including structural engineers, here. You can use the forum as a non-member but, like GBA, you get the best results if you sign up as a member, and like GBA there is no cost to being a member. The forum is highly technical, but typing in something as straightforward as “cantilevered deck” can lead to good results. It could only improve if you became a member and posted your structural question directly.
As with most internet sources, this forum is a good place to start your research and establish strategies for a solution, but you should not rely on it for final decisions or a design.
What an engineer needs to know to help with this deck: Ben Harwood asked the following questions about this deck:
- Is it tied to the building? No surprise this has a big impact on the deck’s stability. If it is attached to the building, how is it attached? Directly, or with a free-draining gap. What are the types and number of fasteners? Proper connection of any deck to a structure is complicated.
- What is known about the concrete piers? Are they reinforced? How deep are they placed?
- What is known about the soils and geology of the site? There are always rules you can follow, assuming worst-case conditions, but that’s not really engineering.
- A 4-foot cantilever can be done, but it needs to be engineered, particularly to include railings.
Are you surprised that an engineer essentially says that you need an engineer? I bet not. But in this case, as Ben remarked in closing, “This deck is not going to fail when not loaded and slip slowly into a problem. It is going to fail catastrophically when loaded.”
Final cut: Either find a prescriptive solution by reference and follow it to the T or hire a structural engineer to ensure it gets done right.
-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.