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Mike, it depends on the load. You can cantilever 2×10’s on 16″ center quite a bit if it only supports a bumpout closet or similar protrusion, or a deck to a lesser degree. It only depends on the load and the purpose. You must have assumed he was planning to cantilever a whole story or?
MD
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Replies
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Yep I knew you knew, Mike. You've known a lot more
complicated stuff here than this! Maybe he could reply with
what he wants to support?
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Sorry, been away for a few days.
I am running a floor 4' past the plate. It will be supporting parts of two rooms: bath and a closet. The total width of this cantilever is 14'; the foor is 2X8"'s / 16" OC and spanning total of 12'. I will be backing up the joists with double 2X8's (4X8's). I could use knee braces or I could double up more joists or drop to 12" OC. The rim is a single 2X8". I will install joists crown down to get the lift out to the cantilever. Of particular note is that the walls are not load bearing as they and the roof are carried by ceiling girders -- I should have said this first :-0pps
Opinions ?
*Sounds fraught with danger, whats the crush strength of the 2x8 rim, what loads are you imposing, what makes you think the Bldg. Inspector is going to let this one by without a stamp. Usually in a case like this I rough engineer it and then get on the fax with my PE, sort of like I do the scut work and he cleans it up and shows me where I went wrong.Skyhooks supporting the wall , huh ?Anyway, sounds like afun project.
*Ah ... you guys are no fun ... I asked for ideas, not scuttlebutt ... there are other ways to build besides stick framing.The lower "plate" is actually a 4 X 10 girder bolted through with two 1/2" steel rods at each end of its 14" OC span. The 2X8 joists will lever off this girder. The cantilever will be holding up itself and little else except for some bathroom fixtures and the flooring material. The wall, roof and ceiling loads are spread across a top girder, with horizontal wall framing and the girders attached to vertical, square timbers bearing on the foundation. If you think crushing is a problem, how can wood I beams frequently be cantilevered over 6" (5 1/2")plates without crushing ? The 2X8" rim is the outside rim stock framing in the joists at the end of the cantilever -- it will share no load at all -- serves only to block in the floor cavity.
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Seems feasible, although 2x8s may be a little small. Do you
live in a cold climate? Be sure to insulate this section
really well because the cold air has access to your floor as
well as the walls.
M
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The floor cavity will be packed with cell and sealed underneath ... that's around R 26, I believe. I can always drop from 16 to 12" OC or double-up every second or third joist if the X8's don't give the lift.
*See my comments about my architect designed cantilever above.Tim
*This is all very interesting.I have a question about all of this.I do not do much building of structures, although I have done quite a few over the years. We do not have basements, and very few two story houses. Most of our construction here is one story single family dwelling type construction.Most of it is slabs, but an awful lot is pier and beam, and I am seeing a lot more cantilever construction than ever before.I have always been taught, and recently read the same thing in the latest issue of FHB about the limits of canitlever construction. If building other than normal formula dimension, you need an engineer to design it.What I would like to know is if the recent article (by David Utterback) complete with pictures and measurements is correct. For those who do not subscribe, or do not have the mag for reference, I will tell what the article said.It states that if the cantilever is not designed by an engineer, then you have two choices. First choice is LOAD BEARING cantilever. It states that maximum extension is equal to depth of joist (i.e. 2 x 10 = 9 1/4" extension)Second choice is NON LOAD BEARING cantilever. It states that length of the cantilever cannot exceed four times the depth of the joist (i..e. 2 x 10 = 37" extension)Is all of this correct? Is there more to it than this?I understand about having an engineer involved if in doubt, or if an unusual circumstance is invloved.Thanks everybody. I ask here because you people do this on a regular basis, and I don't. I try to learn anytime I can.James DuHamel
*"Load bearing" is relative, isn't it James? For example, say you cantaliever a few 2X10s 2 feet over the foundation wall. Then, you nail a rim joist across the end of them, and put some cedar on it and call it a cat walk for washing windows. No sweat, right? OK, so use the same set up and build walls around the perimeter and you might call it "bumping out a wall to enlarge the bathroom, right? Well now the joists are "load bearing". But what if you sheet those 2' long end walls - both sides - with plywood, and nail the begeebers out of it. Now you have taken the load off the joists by creating a "box beam". The only way those joists can sag is if that box beam pivots right off the wall, which it can't do, because the roof sheathing ties the top of the whole sheebang into the roof. So what I always think about these rules of cantaliever and spans and whatnot is you have to look at all the framing as a system, and transfer the loads to where you know you can support them. If you just stop and think about this stuff, and look at what supports what, and consider gravity, it really is pretty clear what will work, and what won't. Interesting to think about though. Box beams are beautiful things. Easy to build and very, very strong.
*Hey Jim,Absolutely right about loadbearing.What I get a little confused about around here is the Inspectors version of what is legal, and what ain't.The cantilevers are getting to be more and more prevelant, but I always see footings and piers on each corner of the area that juts out from the foundation. Seems inspectors are making the builders use these piers for support. I am almost positive that they don't know a lot about cantilever building, and in most cases these supports are not necessary.I have been wondering if it isn't just overkill.I was curious if these rules of cantilever construction are good, solid engineering rules, or if it really is up to the local inspectors to decide.Whenever I do build, I use the entire framing of the structure as a support system. With any system, the weakest link is the weakest framing member. I use Simpson Strong tie connectors a lot (we're in a high wind code area), and they sure make a difference.Thanks for the info Jim.James DuHamel
*I'm not a PE nor have I done much structural design work. I do have a BS in Engineering. (and there is no better way to describe it than a BS, as you are about to see.)The cantilever rule can't be as simple as just one rule of thumb. The math involved is pretty complicated. I would say that it needs to be decided on a case by case basis. However, a cantilever of 2x the joist height will work in almost all cases. Unless your building something unusual, I would trust 2x. Anything more, and I'd want some math done. Actually, with the cost of engineering reviews, I'll just ad a couple of pier footings and save myself some money.If an inspector sees something unusual but looks good, I can't blame them for saying "no". It's their ass. I'd probably want an engineer to be the one to take the blame, even if I knew it would be fine. The idea of the cantilevered span being 1/2 the anchored portion is not right!!! That means you can cantilever a 2x6 10', as long as you have 20' back under the house.Maybe the anchored portion should be twice the cantilever. I'd buy that. As they say: Of course, I could be wrong.
*A friend of mine built his dream home (true story, did alot of work himself, many of us were skeptical). Not sure who designed the plans (I think he worked with someone to create a custom design or modify an existing design), but all the framing and building was certified and done to specs and code. The front door opens into an open two story cathedral room. Makes a big first impression. He's rightfully proud of the results.Also on the front wall is a bump out bay. Nothing extravigant, maybe 5-6 ft wide and cantilevered about 1 1/2 ft. The bump out bay goes right to the roof and is almost entirely walled in windows. No floor loads, just the walls, glass, and roof.The cantilever was created using 10" joists which spanned maybe 8' from the sill plate to a support beam. I can't recall exactly, but I think the joists where continuous over the beam and carried to the next support.Not long after construction, he noticed the floor in the bay sloped slightly down from the main floor. On further inspection, he also discovered that the joists which made up the cantilever had also lifted up off the beam a good 1/8 to 1/4". The cantilever loads had been properly designed, but nobody considered the uplift force at the other end of the joist. If the floor plan had called for interior walls over those joists, then it probably would have balanced out just fine. His floor plan as it was, is just a light weight empty floor space.Point is, it's more involved than just formulating a joist over a sill and looking at what's sitting on the end. You also have to consider what's sitting on the other end as well.BTW, the beam in my friend's house was a steel I beam with a 2x sill on top. The joists had been toenailed to the sill, but the sill wasn't anchored to the steel. When the joists lifted up, they took the sill with them. It was initially thought that the sill had warped, causing the uplift. Don't know what (if any) solution he came to.
*I've designed a lot of floor truss cantilevers, so I'll relay what I typically do in hopes that it will help some. Ryan Cruzan is correct in saying that "the anchored portion should be twice the cantilever." That's probably the reason that Scott Mazur's friend had problems with his floor system. Non load bearing cantilevers I generally allow to go 3 or 4 times the depth. (Interior balconies and such. For load bearing cantilevers, I'd never suggest going more than 2 times the depth. I've seen cantilever situations go bad more than once, so I'd urge you to be a little on the conservative side.
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The conventional formula for cantilever of 2x dimensional lumber is that one can overhang the beam 1/2 the span of the anchored portion. Any ideas what this formula would be for nailed 4x beams.
For example, a 2x10 anchored at at one end and then again at the 10' point can be cantilevered 5'past the fulcrum (ie; the plate). What cantilever can a 4X10 have past the plate ?
*What you mention as a formula is really a rule of thumb. It applies to 2x joists with no concentrated load on the cantilever. If what you want to know about is 4x joists with no concentrated load, then the same rule of thumb would apply. But, as soon as you start dealing with single use beams, i.e. headers, girders, etc. or other than uniform loads, it's another ballgame, best delt with by an engineer.
*I agree with Mike. My first thought was "depends on what the beam is holding".
*I don't know about your application, but the only rule of thumb I know for cantilever 2x joists is that the max is the depth of the joist plus some Gee Whiz. So a 2x10 floor system @ 16"oc. would be a foot. Concentrated loads can make you look real dumb with cantilevered beams, you frame it up, put some flooring in , and some plaster, and some roofing and next thing someone is asking you if you own a level ?
*Mike, it depends on the load. You can cantilever 2x10's on 16" center quite a bit if it only supports a bumpout closet or similar protrusion, or a deck to a lesser degree. It only depends on the load and the purpose. You must have assumed he was planning to cantilever a whole story or?MD
*Well MD, I know that 'n u no that, but I can't figure from Tedd's post what kind of load he's trying to support, so I guess what I was trying to say was............DEPENDS, but if in doubt, consult an engineer, structural.Hey ...have you seen some of the software out there , like "Beamcheck" ?