Cantilevered framing

[Mike K]

This month's mag has an interesting article about framing a round bump out on a deck. My queston is this: even though the author says the framing was "engineered" and 2x10 at 12" oc w/ every 2 joists doubled were used, doesn't 5' seem a little much to cantilever any deck framing? Looking at span charts for PT SYP framing, typical cantilevers are in the 2' to 3' range.

What are your thoughts?

[Bobby Parks]

Mike, I was beginning to think everyone had gone into hibernation for the Winter. I saw the article you're speaking about. Good piece I thought written by a carpenter with a good brain for math. I'm not technically an enginneer but know enough to keep myself out of trouble. It looks like the actual 5' cantileever is only in the center and begins to drop back as the framing angles in. I'm reluctant to structure anything out that far but my guess is that the doubled 2X 10s , and only handling the rail weight are probably okay. Single 2x12s would probably have worked as well. I know people who cantileever 3' with 2x8s which I won't do only because I've seen that they can sag over time. My understanding is that when wood is treated it loses some of it's original strength. I'm sure someone will pop in here that has the technical data on this stuff . I did like the article and my hats off to the writer and builder of the project. Bobby

[Mike K]

Things have definitely slowed down around here for the winter...slow but not dead & forgotten.

I did look at some span charts and technically one can cantilever 3' with 2x12 (based on 16" OC framing), so the 12" OC could extend that a little. That being said, there was also mention that while the framing can handle a standard deck load at that distance, most only go out to 2'. Also, I can see that doubling every other joist (even with 2x10) would help with the load capacity of the structure but it still seems to be a bit of a stretch going 5'...even if the structural capacity is there, I would think the cantilevered portion would be very bouncy.

My interest in this topic direcely correlates to the extension I am going to be adding to my deck this spring. Due to a septic line interfering with my original design, I'm forced to figure in some cantilever in order to build close to the size I want. Right now I have figured on using 2x10 at 12" OC with blocking and 3' of cantilever. I can easily go to 2x12 and do some double joists but part of my problem is the ratio of the supported length of the joists to the cantilevered portion. I'm assuming this was not an issue with the deck in the article.

[Bobby Parks]

What you say makes sense and you're looking at specific charts. I have replaced balconies that were cantileevered 5'-6' with 2x 10s and they had developed a sag or droop. In a couple of cases I cut them back to 3' or 4' feet and redecked them etc. Hard to argue with charts or a man that has them so hopefully Andy, the author of the piece , or someone can shed more light on the subject. The writer sure seems to know his stuff and math so hopefully he'll have a comment here. I guess I sympathize with any one that writes an article because I always have in the back of my mind that I'll say something stupid when I write one, and someone will pick up on it. I had considered building a deck that would have protruded about 5' to 6' out over the edge of a lake. I was planning to use treated gluelams on each end of the deck and then frame across from one to the other. The trick on that job would have been dealing with the actual footings and water table.Never did the job. Are you coming to the Deck Expo? Bobby

[Bobby Parks]

Mike ,Part of what I said did not make sense. Actually, most of what I say does not make sense. What I mean't to say about using the gluelam was that I was going to use the beams perpendicular to the lake, cantileever the beams out 5' to 6' and then frame back across. It would have been a freestanding deck . It would have been 18' beams so that 12' would have been over columns that were locked into footings sizable enough to handle any uplift or teetering possibilities. I'd like to have a copy of the charts you're looking at if you did not mind sharing them .

[RobertCDF]

Engineering can allow you to do a lot of cool stuff. There is a deck local here that I need to get a pic of. It is at least 20' wide and 16-18' span and at the end is a very large glulam beam... with NO posts under it. Back a few feet are stone columns that support the roof. Somehow there must me something holding this deck up and the glulam is acting as a counter weight. I would be willing to bet that at the ledger the hangers are inverted or some sort of custom metal work that holds the bottom and the top of the joists. Using inverted hangers or 2 hangers should allow you to cant further.

Would I cantilever 5'?... maybe... As long as I had the 3-1 ratio covered and I could block that crap out of it, and oversize the joists. I guess I would.

[Andy Engel]

I too wondered about that cantilever, and I checked with the author. I don't think I'd have published the article without some serious disclaimers if that cantilever hadn't been engineered. Another factor to consider is that it was a semi-circular cantilever, so it was 5 ft. out only at its furthest point. Most of the cantilever was less than 5 ft.

Of course, the original Tacoma Narrows Bridge was engineered, too....

A great book to pick up is Why Buildings Fall Down.
http://www.amazon.com/Why-Buildings-Fall-Down-Structures/dp/039331152X

[Mike K]

I found charts online a while back and have used them as reference for some of my projects. They don't necessarily deal with cantilevers save for the standard 2' overhang with normal framing.

Again, I wasn't disputing the engineering behind the deck in the article and I do understand that oversizing and doubling joists with blocking will increase their capacity...it's just that I'd never heard of framing a 5' cantilever w/ wood joists.

I do have 1 question on the 3:1 rule...say I have a 4 1/2' to 5' span from a ledger board attached to the posts of the existing part of the deck to a beam and I want to cantilever around 3' by doubling every other 2x12 (just a portion of the angled expansion would have the issue with minimal supported length to the cantilever). To prevent the "uplift" factor from having less than 3:1 I can add a bracing board above the joist ends to secure them from lifting vertically. There is a step or 2 down to the new part so I can hide this board under the step. Do you think this would pass the mustard?

[Bobby Parks]

Mike, Happy New Year!! Almost. I've built a couple of jobs in the past where I dealt with a similar situation that you speak of. I used joist hangers (full size/length) for 2x10 joist, with the hangers installed upside down. I figured if they can handle the expected normal download, they should be able to handle the potential uplift. I then used a 2x ledger under the joist as you normally would.( or used to be able to/ depends on how your inspector looks at it in terms of hangers vs ledger and revised codes etc. I have no doubt that the 5' with a 3' cantileever could be handled, the 4'6" with the 3' cant. would probably be okay but might be a question for someone sharper than me to cover yourself. Some USP J-9 framing angles or Simpson equivalents, along with the top ledger if I understand you, should work together to be more than adequate.

[gjustynski]

Here in the Chicago area we are allowed 20% overhang of the joist lenght. (20 ft joist 4 ft overhang) This works out in most local towns alright but there are some that state you are only allowed 2 ft max. But if you are unsure or just don't feel comfortable with the over hang the customer would like, hire a structural engineer and charge the customer for his services. You will be surprised how quickly the customer will use your suggestions for a cantilevered deck.

[ctdecks]

5' is a lot of cantilever! you may engineer the deck with that much cantilver with big enough joists/beams and it may be solid enough now (less then 3/8' deflection) but it will sag over time. gravity eventually wins. 2' to 3' cantilevers with 2x8 or 2x10's will hold up well, and if the joists are crowned corectly(maybe not the way you think) the cantilever section may not sag over time and will proalby be the strongest(less defelction) section of the deck

[Glenn Mathewson]

06 IRC Table R502.3.3 (1)
2x12, 8" O.C. 30 psf ground snow load. End of cant. carries a bearing wall carrying 16 horizontal feet of roof. (that means snow load and live load are accounted for). 3:1 backspan required. = 48" allowable cant.


06 IRC Table R502.3.3 (2)
2x12, 16" O.C. 30 psf ground snow load. Not supporting wall/roof. 2:1 backspan required, but must account for uplift resistance at the other end. = 72" allowable cant.


You can cantilever floors further than jurisdictions would like you to think you can...and that's using the prescriptive code to do it...no engineer required. This is of course, the minimum standard. That's right...6 foot cant...probably would make a good diving board, but not prohibited.

[ctdecks]

i have done the math in the past and 5' cantilevers are possible or rather the deflection is within the comfortable level 3/8-5/8" ish
haven said that i have been on balacony deck with similar cantilevers and i did not like the way that much overhang feels, and they all sag over time.