Can ceiling joists be hung directly from roof rafters?
Scythen
| Posted in Construction Techniques on
I’m trying to design a remodel where some ceilings would be raised. The rooms currently have an 8ft ceiling and gable roof above, could new ceiling joists be hung directly from the roof rafters? The ceiling joists would be similar to collar beams although some would not span the entire width. The room its self is 18ft across and the space above would be unfinished attic with limited access.
Replies
raising ceiling joist
Yes and No. There are a number of other factors that need to be considered, however simply stated:
Yes you can raise them or as you say hang the from the rafters. How much depend on your particular situation.
No you can't just raise one segment of the ceiling joist. In order for it to tie the roof framing system together it must be continous. An engineer may be able to design an alternative system if you can't run them continous.
Scythne
You should be able to raise the joists one third of the distance UP from top plate to ridge, with no problems.
The problem is that the current ceiling joists are preventing the rafters from "doing the splits" and spreading apart.
As a general rule, you can raise a conventional ceiling joist up about 1/3rd of the way to the peak and still (if it's well-fastened) have it prevent "rafter spread". But of course, general rules often have specific exceptions, and rafter spread can cause nasty damage to the entire structure.
Great responses, thanks!
Sounds like there are three main points I should take from the discussion.
First, raising the joists is most likely possible as long as they are not raised more than 1/3rd the way to the peak. This should not be an issue as I only intend to rise the ceiling about 2ft and the peak is about 8ft above the ceiling.
Second, where the ceiling does not span the entire distance the joists should probably still span the entire distance to maintain structural integrity and avoid rafter spread.
Finally, it's probably advisable for me to consult a structural engineer to be sure the whole thing is structurally sound.
Any suggestions on how to find an engineer for a project like this. I did a few internet searches and haven’t come up with anything that seems like a fit. Most seem more oriented for commercial or large scale projects and this is a really small project, I don't have an architect or formal plans.
The system would have to be designed in order for it to work.
You'd need to consider the size, grade, and species of rafters. Plus your rood loading and how far up the rafter the ceiling joists are fastened. Someone would have to design the connection between the rafters and ceiling joists.
It's not a matter of just slapping something up there and nailing it so it looks good.
Ron
The couple few times I've done this, Building Inspection reviewed and approved my design which pretty much detailed the connection point and method. Joists were through bolted with 2 or 3 half inch bolts each end. The joist size, was just the original span rating (at the plate level) using 30lb load. The rafters were code complient.
Certainly not engineered, but approved by the plan examiner. At the time, they needed no eng. stamp on this. Had to have been at least 10 yrs ago, county inspection in a couple, city in one. If I'd have gone over the 1/3 raise from plate, eng. backup and stamp would have been required.
I'm sure this wouldn't fly now-the approval w/o stamp part-the rating and method, might probably have been to or beyond what could be spec'd.
I wonder if good full service local yards might have a handle on a referral for this (much like they send plans to LVL etc. suppliers).
Here I would go to a local architect and they could send the info to an associate engineer of theirs. The stamp would be the key.
Any time I hear of people thinking about this I have visions of ceiling joists taked onto grossly undersized rafters with a handfull of nails.
I know the common thinking is that you can move a ceiling joist up 1/3 of the way on the rafter. But that adds a lot of stress to the rafter that they most likely weren't designed for. And there's a lot of force at the connection that needs to be designed for.
So I'm a little leery of it. I know it can be done if properly designed. But I think people tend to just throw around rules of thumb and don't put a lot of throught into it.
Yessir, they certainly do ............
not put alot of thought into it.
Have seen it forever.
Matter of fact, just corrected a few attempts just recently.
Thank you for the caution!
The International Residential Code gives some guidance on this. If you are raising the ceiling joists 1/3rd of the distance from the height of the plates to the ridge than the allowable span of the rafters is reduced by 1/3rd. So you would probably need 2x10 rafters for typical lumber spaced at 16" o.c. for a building 18-20' wide. For you case (raising 2' with an 8' high roof you need to reduce the rafter span by 24%. Thats not as bad, but unless the rafters are oversized its not going to work. You could always sister the rafters or add a ridge beam, but thats adds alot of work/cost.