Hello,
I am designing an addition to my home and my problem is that I do not know what type of joist design I should use on the roof. I would like to place my ceiling joists on
TOP of the top plate and extend the joists horizontally 2 feet from the exterior walls. The rafter would then meet the joist at the end and sit on TOP of the joist. I have not seen this design. What I have seen is the rafter sitting on top of the top plate with a bird mouth cut to sit on the top plate, and then the joist is nailed next to it.
Is this design of the joists possible?
Replies
There are lots of possibilities that would allow what you want. But they all depend on stuff like loading, local codes, if you have a ridge beam, etc.
Like Boss said, the specifics depend on your local code and loading situation, but something similar to what you are talking about is routinely done. It's called a raised heel, and it's done when there's little or no roof overhang to increase insulation at the critical juncture of wall and roof.
Mike
Thank You for your response, however, my intent is not to increase insulation. It is simply to form the underside or soffit. I think this is ALOT easier to form than if the rafters extend out without the joist providing horizontal support past the exterior walls.
Anyways, I just realized also that if the joists extend out with the rafters on top, then the cantilevered portion of the joists creates an unstable situation. I believe this is why it is not commonly used. Maybe 2 feet is acceptable, but it would depend on the loading and size of room, etc.
"... my intent is not to increase insulation. It is simply to form the underside or soffit."
That can be done with a simple 2X4 board for a soffit return.
Bumpersticker: Next time you wave at me, use more than one finger please.
Basically what you're talking about is a truss (so, it's done all the time, just not when stick framing.) The cantilever condition would be dealt with by putting in a vertical support over the wall and up to the rafter. That carries the rafter. The tension is relieved by tying the rafter at the end of the joist so you end up with a little triangle.
The horizontal 2 x 4 makes the flat soffit most effectively probably. Unless you're dealing with really steep roof pitches and a deep soffit where that would start to really lower the soffit in relation to the windows and such.
I seem to have a problem with birds mouths. Just have a hard time cutting into those rafters....just don't like it....nope...not at all. (but that's the way "everybody else" does it.)
luv experimenting. get to do it alot on my own house (so if it falls down it's my own fault.)
I spent some miserable hours some years ago trying to fix a place that had been built like that.
The problem I was working on was not related to the design of the framing but to avoidable leaks. The way the framing was built just made it harder to fix in that the roof load was on top of the pieces I had to replace and there was no room to work easily.
In that place, the bulders had laid two pieces of something like 2 x 8 side by side at the outer ends of the cantilevered joists and then cut the heels of the rafters to a horizontal line from full thickness to nothing. The heel of the rafter was inside the wall line so the roof load was not cantilevered much.
If the rafter heels are outside the line of the walls, you will have to increase the size of the joists to take that load and that seems to me to be a job that calls for an engineer. There's two easily avoided costs. Rafter birdsmouth cuts are not hard to do right. Soffits are not hard to build.
If it's a design issue, of course, then that's a different story.
Ron