Gable roof, 4 over 12 pitch, 14′ run ( 28′ wide building ).
AHJ say’s 20 pound live load / 10 pound dead load for Wilmington Delaware.
Also consider these rafters will be notched to sit on top of a ridge beam, most likely a trus joist. ( is it still called a bird’s mouth when it’s on the ridge beam?)
I would like to go with 2 x 10’s but I am having trouble finding span tables that take into account the notch at the ridge?
Thanks for any help,
Bill
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Replies
Just one comment -
When you say "Truss Joist", I assume you mean an I-joist.
No way should an I-joist ever be used for a ridge beam.
When you say "Truss Joist", I assume you mean an I-joist.
And, I was half-picturing a floor joist; something collected as an extra or over-run or the like. This is a shed, isn't it (or am I remembering badly)?
I'm still trying to cipher on wht "over" a ridge beam, rather than the more-typical "up to" . . . That "bird's mouth on top" is rattling around the emptier parts of my head in distracting ways.Occupational hazard of my occupation not being around (sorry Bubba)
At 16" centers, and a 30 psf live load, my book says 2x8 Doug fir #1 will get you a 14.7' span. #1 southern pine rates at 14.4' for the same conditions.
At 24" centers: 2x10 Doug fir #1 = 17.4'; South. pine #1 = 17.1'.
I don't have any values for #2 unfortunately.
Yeah, the notch is still called a bird's mouth. There's a number of ways to do the connection, but I'll let others opine as to the best method.
As far as I know, the bird's mouth does not degrade the span rating.
Edited 6/20/2007 4:26 pm ET by Ragnar17
Span tables don't need to account for the birds mouth if it is less than 1-1/2"
There is no I-joist that will suffice s a ridge beam unless it is only 8-10 feet long and even tht would require some stabiliation to resist defletion for a free under beam.
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There is no I-joist that will suffice s a ridge beam unless it is only 8-10 feet long and even tht would require some stabiliation to resist defletion for a free under beam.
I'm not following that explanation. Why isn't an I-joist adequate for a ridge beam? Is it because it has very weak resistance to lateral bending (i.e. weak axis) and/or twist?
To all,
Sorry for the confusion.
The ridge beam is a Weyerhaeuser Ilevel trus joist product.
A 1.55E Timberstrand LSL, 3 1/2" x 11 7/8" x 30' with a center post down to footing.
I'll be building over a flat roof on my garage next spring. Due to existing conditions on the building I want to use a ridge beam to support the roof.
The birds mouth on top of the beam just seams like a solid, not to fussy way to get-er done with out a ton of hangers and straps.
I am in the planning stages right now, gotta get my ducks in a row, make a drawing, get a permit……..
Thanks again, Bill
( note this is just an example, not my actual drawing ) hope this attachment works
Edited 6/21/2007 6:32 am ET by 43Billh
Thanks for clearing that one up, but it would be far less fussy and time consuming to raise the beam and seat the rafters to it than to cut birds mouths on it.
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Hmmm, thats interesting.
Is it safe to say you would use some type of Simpson Hardware that stradles or hangs from the top of the ridge beam? Regular rafter hangers??
What would you do?
I wouldn't think toe nialing would be very good practice for this situation.
Bill
Is it safe to say you would use some type of Simpson Hardware
Not always 'safe' to guess on what/how Piffin may opine upon <g>
Now, approached correctly, the simpson rafter hardware can be a labor-saving exercise. How so? Well, if you use both the heel connectors and the ridge connectors, the only cuts the faters need at all are a plumb cut at the ridge, and whatever end cut for length your specific roof design needs (which might be plumb, or square, or whatever).
The only "hiccup" is getting used to banging in all the hanger nails. There's a couple of good FHB articles on nailing off hangers, each with a discussion on using pneumatic nailers and types to speed things along.
Only "gotcha" in not cutting a bird's mouth rafter seat is in how much that "adds" to the tabulated span calcs. Personally I write any extra off as a "free" bonus, and call it even (which also means never having to haggle with BIs, too).Occupational hazard of my occupation not being around (sorry Bubba)
Bill,If you put the rafters on top of the structural beam with a birdsmouth and run them along side each other and nailed into each other, you don't need any hardware. Cutting a birdsmouth is nothing and isn't as time consuming as nailing on hardware.If you cut plumbcuts and nail them into the face of the beam, you probably will have to use hardware, not all the time. I've framed this way with and without hardware and just toenailing. It's all up to who designs it. If you frame this way, you should raise the top of the rafters up at least 1" for a ridge ventHeres' a drawing I did showing the rafters on top with each one overlapping each other.Joe Carola
you guy's are great!
Framer;
Your description is exactly what I had in mind. It just seams to me to be very strong, and maybe fun to do!
Piffin;
thanks for making me look from another point of view, and I would have never thought of strapping the rafters together.
Capmac;
I have a palm nailer ( well worn too ). That's just a lot of hardware I would rather avoid. Not cutting a birds mouth at the top plate just gives me the willies..... I'm shore it's perfectly acceptable but i need to sleep at night.
Well I guess I should re-visit my options ( although Framers way looks like a lot of fun ).
Thanks guys!
Bill
What makes you sleep well at night is always fun.And learning new things is too.Boss might be going to force me to have some fun here...;)
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Piffin,
go for it! I love to watch you guy's hammer things out ( I learn a lot too ).
After today I'll be away from the computer for a few days, but i'll look this thread up on monday.
Sleeping at night is very good!
Joe - Excellent advice as usual, but sorry, that drawing looks like it was done by MC Escher.
Thanks, but who's MC Escher.Joe Carola
M.C. Escher
View ImageTrue, there are architects so called in this country, and I have heard of one at least possessed with the idea of making architectural ornaments have a core of truth, a necessity, and hence a beauty, as if it were a revelation to him. All very well perhaps from his point of view, but only a little better than the common dilettantism. --Thoreau's Walden
View Image
Edited 6/21/2007 11:31 pm ET by rez
I don't know what would be wrong with toe-nailing there. I have never used any hangers at top, only tie strap across above beam tying one rafter to the other.Like Joe says, there are lots of ways and my opinion is what works best for me. Doing a birds mount seat at top means five cuts there basicly. One plumb with circ, then two start the seat with circ, then finish those cuts with jigsaw.
Meeting the beam means only one cut with circ saw
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"I don't know what would be wrong with toe-nailing there."
Depends on the loading on the rafters. You may need more nails than you could get in there without destroying the wood.
Since this is a ridge BEAM and not a ridge board, don't forget that you have to support the end of the rafter. It doesn't just "lean" up against a ridge board.
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Help me think through this one then.The top end of the rafter is leaning against the beam and cut at an angle. In order for it to settle downward like a floor joist would at one end to a flush beam, it has to make the ridge beam move sideways - away from the rafter, but that force is resisted by the same rafter on the opposite side of the beam.So that force may want to push out on the wall the rafter sits on at the low end instead, right?
But only if the toe nails at top do not resist pulling out of the connection there.But a toenailed connection is as strong in that direction as a hanger is. The primary increase in strength of a hanger is the downward load, which is pinching things tight at the beam.show me where my reasoning is wrong in a typical roof, not a 2x6 rafter on a 1/12 pitch. A low pitch like that and small area for placing toenails would definitely need hangers.
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"In order for it to settle downward like a floor joist would at one end to a flush beam, it has to make the ridge beam move sideways - away from the rafter, but that force is resisted by the same rafter on the opposite side of the beam.
So that force may want to push out on the wall the rafter sits on at the low end instead, right? But only if the toe nails at top do not resist pulling out of the connection there."
--But in a normal roof, with just a ridge board to tie together the rafters, the sidewalls are held from pushing outward by the ceiling joists, making the roof section perform like a truss. If there are no ceiling joists, the cut ends of the rafters could slide down the face of the ridge beam by pushing the sidewalls outward, and the only thing preventing the sliding down are the toenails. All that downward load held vertically by 4-6 nails? Am I missing something? I have to look at the start of the thread and check; may have to revise or delete this reply.
OK, the OP said later: "I'll be building over a flat roof on my garage next spring. Due to existing conditions on the building I want to use a ridge beam to support the roof." If that implies that without a beam at the ridge the walls could be pushed out, then my remarks above seem OK. If the existing flat roof does tie the walls from pushing out, then just toenailing at the ridge would seem to be stucturally equivalent to that beam being just a 2x ridge board. Either way, it would seem to be stronger to notch the rafters so as to be supported vertically by the beam, with no dependence on nails in shear at the wall or the beam face.
Edited 6/21/2007 2:57 pm ET by DickRussell
"the only thing preventing the sliding down are the toenails."Here is where I would disagree with that - there ios a friction developed that increases as it attempts to slide down. Say the pitch is 6/12. Say you are using a 2x12 rafter for example. That means the base offcut is about 5" so there is a bearing base of five inches on that beam for download purposes, but we are not talking download here since the beam is catching that. We're saying that the toenails must prevent pulling away from the ridge. I think they do that. Far better than the four or six joist hanger nails in the 2x12 would.Note that I also strap up over the ridge top of rafters. You need to overcome the shear of those nails
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Maybe we're not talking about the same connection at the top end of the rafter. Earlier you said: "The top end of the rafter is leaning against the beam and cut at an angle." I presumed by this you meant the way a rafter would be cut to tie to a simple ridge board.
Now you are saying: "so there is a bearing base of five inches on that beam for download purposes, but we are not talking download here since the beam is catching that." True, if there is a cut so that there is wood-wood bearing on the top edge of the ridge beam, then the toenails into the side face of the beam aren't providing support vertically.
I'll have to do a drawing to demonstrate my thinking on this, I guess. Sorry
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"The top end of the rafter is leaning against the beam..."
Nope - It's SUPPORTED by the beam. Same as if the rafter was a floor joist hanging off the side of an LVL floor beam.
"In order for it to settle downward like a floor joist would at one end to a flush beam, it has to make the ridge beam move sideways..."
...Or the wall move out. If no ceiling joists are present, the rafters are what keeps the wall plumb. The rafters do NOT have compression in them.
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Now, you just said that the ridge beam is supporting the rafters. So if they are supported by the beam and securely connected to it, then the walls cannot move ot. The only point we are really focusing on is whether or not the connection can handle what forces are applied to that joint.
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"The only point we are really focusing on is whether or not the connection can handle what forces are applied to that joint."
I thought it was first necessary to make the point that the rafters didn't "lean against" the ridge beam. You're the one who said that - Not me.
Once you figure out what the reaction is at the connection, then you can figure out the number of nails needed to attach the rafter to the beam.
It could be that toenails are O.K. But it should be figured out, not just slapped up without any thought being put into it.
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>> Doing a birds mount seat at top means five cuts there basicly. One plumb with circ, then two start the seat with circ, then finish those cuts with jigsaw. <<Piffin,The top birdsmouth is only three cuts. The birdsmouth is one seatcut and heel cut, no jigsaw. You can over cut it and not have a problem.I've never once used a jigsaw for as birdsmouth whether it was on the top plates or the ridge beam. The third cut is the reverse angle at the top.>> Meeting the beam means only one cut with circ saw <<If he does that, he might have to go back and nail straps or some kind of hangers on every rafter, and that takes a lot of time. Maybe he doesn't have to use any hardware.Joe Carola
I never over cut my notches. I'd much rather use hardware if it came to that.
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>> I never over cut my notches. I'd much rather use hardware if it came to that. <<Piffin,When you cut a birdsmouth on the top plate for rafters, you use a jigsaw and never overcut?Joe Carola
right
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Alright, as long as you feel comfortable, but you don't have to.Still love you though......;-)Joe Carola
First, because an I-joist is designed to support the minimal loading that a normal joist would support so it would only be able to resit minimal loading just like a 2x10 or 2x12 would. The added comment refers to the fact that he lack of massing in cross section provides less resistance to deflection as it loads.
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First, because an I-joist is designed to support the minimal loading that a normal joist would support so it would only be able to resit minimal loading just like a 2x10 or 2x12 would.
I know this is moot now, but I was assuming that something other than a common I-joist would be used at a ridge location -- maybe something like a "4x12 I-joist". Maybe there is no such thing.
Ragnar,They have 3-1/2" x 11-7/8" I-joists. Even if you could use an I-joist for a ridge(It could never be for a structural ridge) why would you want to? You would have to pack it out in the middle to have solid nailer for the rafters.Joe Carola
Joe,
I just was wondering about using an I-type ridge beam after the OP seemed to suggest that approach. I've never seen it done but I was just wondering why it couldn't be done.
You would have to pack it out in the middle to have solid nailer for the rafters
Why would this be the case? If you just ran the rafters across the top flange and then lapped and nailed each rafter face together, why would there be a need to pack out the web?
The attachment is one option (among several) presented in a book I have. Is it a bad way of framing in your opinion?
View Image
Ragnar,Usng it for a ridge beam, it wouldn't have to be packed out. I thought you were talking about face-nailing the rafters to it. They do make an I-joist header though. Someone posted a link way back about it. There was a thread some time ago when someone asked about using left over I-joists as headers. You can't do that because they not designed for that.My friend did that about 8 years ago and I think the next day the I-joist was sagging 3/4".That picture you show I wouldn't do it that way unless it was draw that way I would have no choice but I would definitely call the Architect and ask if it was a mistake. I like the full bearing birdsmouth and nailing the seatcut to the top of the beam and then nailing along side the other rafter.Joe Carola
Thanks for the input, Joe.
Anytime, Ragnar.Joe Carola
Ragnar,
In addition to what Joe said, in the drawing, your cross-section of the beam shows a solid beam ,not an "I" joist, also, how would you toenail the rafter to the ridge? You only have a small point of contact between the two(ridge and rafter). There's not a lot of resistance to any twisting or lateral movement of the ridge.
Geoff
Geoff,
The picture I posted was just a quick scan from a book, and was only only meant to convey the approach where no bird's mouth is used. Sorry for any confusion.
I think you're right in critiquing the fact that there is very little actual contact between the ridge and the rafters. It seemed to be a weak characteristic of the design to me, as well, but I wanted to get Joe's (and others') opinion on it since my framing experience is minor.
Thanks for your response, too.