Hi All,
I’m sure most of you have heard the typical rule of thumb of being able to cantilever 1/3 of the total length of the structural member (2:1 ratio). At least that is my understanding over the years (someone correct me if I’m wrong). However, I just read something online that also stated that you can do this but the size of the lumber has to be at least 2×10. I’m designing (and developing a framing plan) a little ‘pop-out addition off of my kitchen and I want to cantilever the floor joists but I have 2×8 floor joists in my house.
Does anyone know for sure whether this issue of having to be at least 2×10 is true? I suppose I could use doubled 2x8s if this is indeed an issue. In my case I’ll probably be well ‘inside’ the 2:1 ratio….I’m planning on using joist hangers applied ‘up-side down’ to the end of the cantilevered joists at the other end from the cantilever (to resist ‘up-lift’)
Any thoughts or info would be very much appreciated! I do have an engineer involved on other parts of this project that I can get to OK my work and correct it. However, I’m trying to come up with a plausible plan on my own that will need minimal correction and input from the engineer…I just want his stamp on the plan. <g>
(I should also add that this addition won’t be completely straightforward; it will involve a curved wall (fragment of a circle). The curved wall will extend up past the existing roof and form a turret and have a conical roof. The little space will serve as a breafast nook off of the kitchen.)
regards,
jonR
Replies
Others will no doubt chime in with expert opinions, but I would caution you to be certain that the wall or beam from which your joists cantilever can bear more load. In the case of a 2:1 cantilever that would be 50% more load and if it's on a beam, there's a point load beneath each end of that beam.
The rule of thumb is that you can't cantilever more than twice the depth of the framing member.
And the back span has to be at least twice the cantilever distance.
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I think you're getting the 2nd rule confused with the first one. Longer cantilevers are possible, but it depends on the load on the end of the cantilevered member, among other things.
"The rule of thumb is that you can't cantilever more than twice the depth of the framing member. "So 2x12 joists can be cantilevered only 23"?I would have expected cantilever distance of a joist to be about half the allowable span distance, since the load forces would be the same.BruceT
"I would have expected cantilever distance of a joist to be about half the allowable span distance, since the load forces would be the same."
The loads aren't the same - Cantilevers usually have a lot of roof load on them.
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"The loads aren't the same - Cantilevers usually have a lot of roof load on them."Of course, I should have thought of that. My assumption would only apply for a balcony or bay window.Your rule of thumb limiting cantilever to depth of joist is based on a certain joist spacing, I assume, and would increase as spacing decreases? What is puzzling to me now is that I have read that load capacity of a joist member increases as the square of depth, but the cantilever = depth rule does not. Double joist depth = 4x load capacity but double cantilever = 2x effective load.What am I missing here?
BruceT
Thr rule of thumb is just a starting point. If you're halfway close to it you still have a shot of making things work. But if you're way beyond it you're most likely out of luck.
The world was my oyster but I used the wrong fork [Oscar Wilde]
Does the circular bumb out wall carry the roof load and the floor load or just the floor load. Is there a beam/header above that is carrying the roof load. Could make a big difference in your calculations.
What is the distance you will be cantilevering? What is the size of the existing floor joist?
It simply depends on the loads involved. And as mentioned before, there is really no rule of thumb. at least that I am aware of. If you already have an engineer on the project, why not just let him take this worry off you plate?
There are two critical pieces of info you leave out here.
how far do you plan to cantilever?
And
Does the wall on end of the cantilver support roof loads?
The 2/1 ratio you are familiar with is only half the basic rule of thumb and only applies for a porch deck or other where the load is the floor only. Adding a roof load makes for a different set of calculations and you have to limit extend to the depth of the joist unless you get the engineer in on it.
There are a lot of ways to work around that, but no sense detailing without seeing your plans. I'd be typing all morning about hypotheticals. given the turret cone and curved wall, I would expect that you will at a minimum be doubling up the 2x8s
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Edited 5/7/2009 6:40 am ET by Piffin
The rule of thumb is really dumbed down to cover virtually any circumstance and as such limits you to the depth of the framing member, which in your case would be 7 1/4" ('06 IRC code rule of thumb anyway)and require the backspan to be at least 3:1. You will hear anything from this to 24", and so on from others.
Most cantilevers really are not that complicated, but require an understanding of the dynamics of each particular situation (i.e. roof loads, floor loads, spans, tie-downs).
Since you've mentioned that you have access to an engineer on this project why not run it by him/her?
Thanks everyone for your feedback. I guess one thing I've learned is that 'rules of thumb' are really 'rules of dumb' (as in how they 'dumb-down' everything). <g>
I need to work on the framing plan a little more and yes, I intend to involve this engineer. I just wanted to work through the process on my own first for two reasons: 1) As a learning vehicle & 2) Since I'm building it, I'm thinking of that process while I'm generating the framing plan. I sort of felt that if I come up with something that is plausible, the engineer calcs it and it passes or at the worst he just has to make minor changes, then I get to build something that I've already thought through. If I leave it only up to him, then I end up building something where I had no input towards the framing design.
Yes, in my case the cantilever will have roof load. Based on how the circle interacts with the corner of the house (looking at it in plan view), the cantilever will be picking up around 60% of the roof load.
The cantilever distances vary (because of the circle), but the farthest one is around 3'-6". Of course I am working on two different schemes for laying out the joists; one that calls for them to be rotated 45 degrees from the common layout and the other that calls for some on common layout and some 90 degrees rotated from that. So, yes, it's largely a work in progress at this point.
So, I do acknowledge that the roof load interjects a variable that precludes using the dumbed down rules of thumb effectively. As someone pointed out, these rules of thumb only assume floor load and nothing else...which probably is the case. With that said, let's assume that I don't have any roof load (as in a deck). I've heard different, and in most cases 'conflicting' rules of thumb being stated with respect to backspan, depth of framing member, allowable cantilevering distance based on depth of framing member.
Would it be safe to say that if someone did want to come up with an accurate rule of thumb by stating up-front that it doesn't include roof load, is there a correlation between depth of framing member and backspan requirement?
thanks,
jonR
Just to put a different spin on cantilevers.
Here is photo of a roof cantilever on a project I did. My apologies about the quality, it is a scanned photo, not digital. 12' cant , 8' inside the building.
Engineered by a good friend of mine using Engineered Mono Trusses tied into stick framing.
It survived 30" + of wet snow this last winter and winds up to 75MPH.
They can't get your Goat if you don't tell them where it is hidden.
You mention that it was engineered underlines exactly what is needed here. Rules of thumb only work where we who are all thumbs are building basic shacks, but when doing things out of the ordinary, it takes real calculations and a design to make it work.
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With what you are proposing, I would laminate a curved rim beam for the cantilever, bringing it back in to the two joists that would be tangent to it. This keeps all the joists 90 degrees to the plate. That's assuming that you have a half round bump out. if the bump out is just a chord, then your joists won't be tangent. You can still laminate a curved rim beam that will help carry the load. Also, if the roof is a turret, you can make a circular bow beam with plywood to distribute the roof load off of the floor load. With 2x8 floor joist, check into seven and a quarter LVL's.
Ask your local building inspector. You will have to run it by him anyway to get your building permit. It is easier to ask now, what will be accepted than being told at a later date to correct the framing.