Just got my construction prints for a dormer remodel I’m doing on my house, just a quick question: I have an area where the front of the dormer is bumped out or cantilevered about a foot. All the books I’ve read say you can only bump out as far as the joist is deep. So if I’m using 2×8’s I can only come out 7 1/2″? I see other cantilevers where they come out like 2 feet! Whats the deal ? Also if I’m coming out a foot do you guys frame it allowing for the sheathing or just bring the plates out a full 12″ instead of 11 1/2″ (assuming I’m using 1/2″ plywood). Figuring the measurement using the prints’ scale I’m supposed to be coming out a full 12″
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It goes without saying (but I'm writing it anyway) that any significant engineering should always be done or checked by a licensed prof. engineer or architect... but hey we all know that already.
That being said, not sure what books you're referring to about the depth of the joist, but the 'rule of thumb' that I've always heard is that member can cantilever about 1/4 the length of the supported span. For instance, if a joist is 12 feet long, you could (under the ideal circumstances) cantilever about 3 feet past one of the supports. Like I said though, any unique features or circumstances need to be properly addressed by those who can run the numbers properly.
I'm just finishing the design of a new second floor for my own home and it features a three sided 2'-0" wide cantilever beyond the first floor walls. I know what my framing plan looks like, but I can't wait to see what the TJI shop drawing looks like on that one... Should be an interesting framing job...
Just asking , but if you just received your plans ....are there no details to that set of drawings?
Typically here when we do a cantilever the framing member needs to extend at least double the distance of the cantilever. Example: If your cantilever is 2' then it would also need to extend back into the floor 4' making it a 6' member. But that also vary's depending on the load at the end of your cantilever.
I don't know the area your in but here we need engineering for pretty much everything these days, even a simple dormer.
Howie
1st off... aside from decks, I did't know people were still framing floor systems with 2x8.
as for the cantilever. what you have read is more a general rule of thumb, and that doesn't sound right to me anyway. (perhaps this rule is referring to when joists do not extend past the wall framing, instead joist blocks are used)
I'd have to double check, but if I remember correctly the limit for a basic cantilever is 2'. also your joist system needs to not exceed more than 1/3 out and 2/3 in. meaning a 2' cantilever needs to be at least 4' in. larger cantilever can be had but it needs to be structurally designed that way. So just because you see something, you can't assume that it is built the way you have read somewhere in a book. having said that I realize that, that is why you are here asking.
anyway..I only got a few minutes, but that should at least get you started 'til someone picks this up and gives you some more details.
P.S. as for the framing and sheathing issue, it is not a big deal either way works. but if I want the upper structure to tie in stronger then I will frame it to allow for the sheathing to hang down. otherwise I normally frame to the exact dimension. with the sheathing above flush with rim joist.
Edited 4/4/2006 5:52 pm ET by alrightythen
there are a few basic guidelines that the guys in the field use but every building design is subject to an engineers stamp, then all those guidelines dont mean jack.
in the end the cantilever will be subject to the size of the framing members and the live/dead load that effect it
nicely summarized...the 2x8 floor joists I have issue issue with.
Generally, It is when there are loads from above that your cantilever is limited to the thickness of the joists used.
Say, when that dormer has a shed roof that loads onto that extended floor frame. if this is a gab;led dormer, the roof loads off to the side walls and the main roof. Doubling that outside floor joist takes care of that.
there are a hu7ndred other ways of dealing with situations like this. For instance, the whole of the side wall of the dormer could be made to be a box truss system, but that would be shown in your plans. Or the floor could be laid oput at 12" OC instead of 16" OC, or ...
you get the picture. Anyways, I doubt that an extra 3" is going to be worth breaking an engineer's pencil over.
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