Birdsmouth Question (s)

edfogel51

Greetings!!

2 questions please on birdsmouths...I just did a search, the question was posed but not answered...

1) do birsmouths provide any structural advantage to the roof, or are they just used to answer various architectural problems?

2) framer on my garage just toenailed 4 #10, thru seat into collar...said that was sufficient. If you aren't nailing thru the overhanging part of the birdsmouth back into the outside of the plate, then what need is there for there to be the down cut of the birdsmouth? (except for cosmetic things)

thanks,

ed

jmic

Ed.
IMO
#1. The birdmouth hooked against the exterior walls along with nailing into ceiling joists and top plate will help keep ridge from sagging and walls from bowing out.

#2. I would put (2) 12d's toenailed into top of wall and (4) 12d's through rafter into ceiling joist.
Joe

Cole

Nothing I can add.

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Tom R

Your question's pretty much already been answered above, but also if you were to nail where you are saying there's a good chance it would split the wood right where the grain crosses the corner of the birdsmouth.

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specwood

The birdsmouth doesn't keep the ridge from falling and the walls from bowing out. The collar ties and ceiling joists do. The best birdsmouth cuts in the world won't make for a self supporting ridge. Don't forget that a birdsmouth is not engineered into trusses, although the truss clips do make up for that a bit firming up the connection between the wall and truss.

The birdsmouth cut mostly aids in assembly. Imagine trying to make sure the ridge is where it is supposed to be without a birdsmouth cut. As long as the walls are braced straight and parallel to each other, and the rafter length is correct, the ridge will fall perfectly into place when the birdsmouth is tight against the wall.

As far as nailing, I would avoid anything larger than 12d (the wood splits too much) and 10d nails are probably ok too as they are not much smaller.

We hand nail ceiling joists to the rafters with 16d's. This is an important connection when weight gets added from above.

Metal connections between the rafters and wall may be required in certain areas.

wbsbadboy

This is what I was tought the birds mouth cut was for....Back in the days of yore before there was expanded aluminum panels for the lathers the birds mouth cut was so that the lath and plaster would be above the level of the soffett and have adaquet nailing for the lath. The soffett would then lock the top of the lath in and keep it weather tight. You need to think about this a bit to really see the logic in it. Now we have stucco that is pretty weather proof in its self. Back in the early 40's thru the mid 50's many homes were coated with plaster. A good coat of some lead containing oil based paint (thank goodness for latex lol)and the plaster was pretty water tight. But if the water got in behind the plaster and lath you were screwed.
I was able to see this type of damage first hand while helping on a historic home here in my home town. A particulerly nasty storm had blown part of the roofing material off and after only 3 days of rain about a 3' by 6' area of the origional plaster and lath had to be replaced. The upper part was litteraly mush when I demoed it out.

JustaFramer

A birdsmouth on rafters 2x10/12 that gives full bearing is more structural that one that has a bunch on meat hanging out on the backside of the rafter.

edfogel51

Ok, so if I have read all six posts correctly, none have given a structural reason for cutting a birdsmouth, (lots of other good ones, but none structural). The reason I'm inquiring is that my framer left me with 7 1/2 inch fascia, said that was the best he could do. I asked him about eliminating the 1 1/2 inch drop, (basically extending the seat out to face) he said that couldn't be done.

Before I got out the sawzall and started cutting, I thought I would ask for further confimation. (Seems that would be better than scabbing something on to extend the rafters, since the roof deck is on already)

Thanks to all for some quick help

K2

The birdsmouth makes the load a vertical load instead of pushing at an angle on the walls. there are other ways to accomplish this load transfer. I'm not sure I understand your question but maybe some of the other guys do.

edfogel51

Rob,

I was of the understanding that proper collarties/ceiling rafters tied together the opposing rafters into a triangle, and if properly attached to each other would become an integral unit, and all the load would be vertical, none lateral.

So as not to be a know it all, I was checking on the board to see if there was any structural reason for making the notch. If it was nailed, there might be some sense, but as mentioned above, it apparently is not nailed.

Also, thanks for your thoughts on the other thread, makes sense, and I'm going to go that way.

ed

K2

Ed, i would consider the birdsmouth a customary practice and not necessary from and engineering standpoint. Generally, when there is not an engineer involved I stay with tradition. Surely your rafters nailed to joists makes your load a vertical load and an engineer could give you details on how many nails, bla, bla,bla, but imo your fine. <P>

I think on a vaulted rafters, and timber frames, birdsmouths or seat cuts are probably more inportant but even there, other options are available.

Tom R

Sure, I'd say as long as it's not a cathedral ceiling go ahead and extend the seat cuts, - - but keep in mind too that fascia isn't going to look so wide with a gutter on it.

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K2

OK, I'm lost. Extending the seat or level cut out to the end of the rafter is going to do what?? Seems to me you'd have a point so i must be missing something. Seems to me a guy could put a 2x10 subfacia on the existing and have a 1x12 facia with a level soffit unless it is too low for windows etc.

jmic

We talking Contemporary here?
Joe

K2

Oh if your going to use that word then i know I'm lost. I still like plaid bell bottoms. Too bad you guys missed the 70's. They let us get away with stuff. From what i remember.

Tom R

If I'm understanding what he's talking about, Rob, is he wants to make a horizontal (level) cut along the bottom of his rafter tail so he can 'narrow' his fascia board, - - true, if the pitch was steep enough and the overhang short enough it would come to a point, - - but in his case (with his pitch and overhang) he's probably saying that 'continuing' his seat cut will give him about a 1 X 6 fascia (or whatever he happens to be looking for). I would imagine he has a fairly low pitch and/or a fairly short overhang.

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Tom R

I remember the 70's but I don't remember 'plaid' being the pants of choice, - - in fact, come to think of it, - - there were 'no pants' of choice!!

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K2

Thanks Tom. That's what contemporary means. Now the whole thread makes sense. i thought Ed was looking for wider facia. They don't let me out much. RT.<P>
Plaid might have been late 60's. but i thought we wore plaid to the disco's with the platform shoes.

jmic

I was there for the 70's and the 60's.
Joe

Teetorbilt

I remember 'poodle skirts', jukeboxes and soda jerks. Getting 'processed', pegged pants, Gant shirts. Being able to eat the fish that you caught......

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