I have a question for everyone to answer. I work with a guy who says that in the picture, the ceiling joists help hold up the rafter. I told him that it wouldn’t and that assuming that the rafter was nailed to the wall on the left, the weight is pushing straight down. It seems to me that the rafter would pivot toward the ground from it’s connection at the wall and the ceiling joist would pivot toward the ground where it is connected to the wall. I don’t see how the ceiling joist could be a brace to the rafter. It might help a little, but shouldn’t be counted on.
Tell me what you guys think. I know that in the articles I’ve read, a brace shouldn’t be less than 45° to the rafter.
If there are any engineers who frequent this board and can explain this, I would really appreciate it. This guy won’t believe anyone who isn’t an engineer.
Replies
Is this intended to demonstrate an awning roof suspended off the left wall. No supporting wall is shown under the rafter tail.
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In the case I'm referring too, the rafters and ceiling joists land on 6x12 beams over a porch. The guy I work with says that those ceiling joists "help take some of the weight off the beams". That is a direct quote.
I don't think it helps do what he says, and I dont' think that the rafters should be braced in this manner if it was an awning. The ceiling joists in some areas are about 6' long, from the house to the outside of the 6x12.
In an unsupported awwning roof, mini trusses could be designed that would allow the load on the ned of the rafter to be transmitted to the wall via the ceiling joist/bottom chord and placing added pullout stress at the top of the rafter at the wall, making him about 2% right, but with a beam supporting this, the load is borne by the beam and by the wall at left. Half the roof load is on the wall and half is on the beam.
It is only when you take away the beam and the load puts the ceiling joist into compression that it begins to work to bear that load.
But this doesn't count since I'm not an engineer.
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Excellence is its own reward!
Piffin,
I was thinking the same thing you are. I don't see how the joists help to recduce the load that the beam is "seeing". I understand that within reason we could make an awining roof work, but if the joists have to go too far it's presumptuous for a framer to make necessary assumptions.
I'm certainly not comfortable designing a mini truss like that.
Thanks for your reply.
The only thing that ceiling joist is gonna hold up....( in the configuration you drew up)is the soffit panels you attach to it. If your soffit is lightweight ( aluminum, vinyl, 1/4 inch ply) then a 6 foot long 2X4 ceiling joist will do nicely. If you don't want a "flat" soffit ceiling, then omit the joist material and simply attach soffit directly to rafters and/or first attach strapping to rafters and then install soffit. If awning was to be "open rafter design", then just omit the joists, period.
The only benefit the joists have are that they give added sway/racking resistance to your 6X12 beam...but if your rafters are attached to the beam properly in the first place, then there are no worries about beam racking.
I agree with Piffin. The existing wall is taking 1/2 the load and the 6X12 beam is taking the other 1/2. The rafters don't need to be "tied"( collered) in this shed style configuration since ( unlike a gabled roof design) there should be no rafter spread.
Hey Tim, here's a lil experiment you could do to prove your point. Find the center point of the rafter length and drill a hole through the center of rafter face. Now insert a dowel rod or a piece of rope and hang some dumbell weights off of it. BEFORE doing this, first tie off a stringline along the top edge of the rafter....now with the weight added, the rafter should deflect somewhat. Using the stringline as your guide, measure the amount of deflection. Now...using the same rafter and the same stringline and the same weight... temporarily remove the weight, then nail on your ceiling joist, then re-attach the weight to the rafter, and measure the deflection. If the deflection is the same as before...voila! the joist did ABSOLUTELY NOTHING in helping to take the rafter load. If the rafter deflection is less than before then( shock! gasp!) your buddy was right and you and me are gonna eat some crow!
It would be interesting to see. Tell me if you go through with this.
LOL.
Davo
I think your statement: "It might help a little, but shouldn't be counted on" is pretty accurate.
The ceiling joist would probably be in compression, to some degree. And that compression would help hold the rafter up to some degree. The limiting factor would be the connection between the rafter and ceiling joist, and the connection between the ceiling joist and the wall.
You mention braces not being less than 45° to a rafter. The angle doesn't matter, as long as the forces involved are dealt with.
Actually, you're probably over-thinking this. If the beam is sized to hold the rafters, and the rafters are sized correctly, this is all pretty much pointless.
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Boss,
The 6x12 beams are more than strong enough to handle the load that is placed on them. 4x12s would have worked.
I'm not sure why my framing partner is counting on the joists to take some of the weight off of the beams. He thinks that he can reason out load paths. I don't see how someone who has only framed can "reason" the load paths. We get into some pretty strong arguments sometimes. He used to feel and still does sometimes that shearwalls keep the house too tight in an earthquake and that the house should be able to rock and roll with the ground movement. I don't agree. He is trying to reason out the forces and applying his own value judgement to them. I don't see how he or I could do that when we don't have any engineering experience.
Thanks for responding to the question. I was hoping you would since you design trusses I figured you could explain it. Thanks again.
I wouldn't be too hard on the guy - He was partly right. Stuff like this could be debated endlessly. Everybody has a different opinion.
Have you ever heard this line? "If you laid all the engineers in the world end-to-end, they wouldn't reach a conclusion". Tell that one to your framing partner next time he questions something.............(-:
I don't blame anyone for questioning how and why things are done. If we didn't, nothing would ever change. But I also think it's important to remember what's important, and not get hung up on needless details. In this case - If the beam is adequate, and the rafters are adequate, it makes zero difference whether or not the ceiling joists do anything. So there's no point in getting hung up on it.
Maybe that would be the thing to ask him next time - Is this REALLY important?The word "cunnilingus" just doesn't roll off the tongue. [Julie McKenna]
Nails between the joist and rafter can transfer about 100# of load each from the rafter load to the joist. The joist is in bending.
The mistake most of you make is to consider the problem to be determinate. It is in fact indeterminate. You solve the problem by considering the amount of deformation (bending and compression) in each member. That is in general difficult.
If an answer is required, I would suggest either taking out the nails between the rafters and joists or leaving a gap between the rafters and supporting beam. In the first case the joists provide no support. In the second case the joists provide all of the support.
In reality the joists and beam share the support.
Thanks to everyone who posted. My belief in this case is that when sizing the beams, the joists should not be taken into account. I don't believe that the beams should be undersized and assume the joists will take enough weight off the beam to allow smaller beams.
George,
Thank you for replying. I can see how the joists can help, but I just don't think it is safe for framers like myself to make the proper assumptions.