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Birds Mouth

Geovan3's picture

Does the birds mouth cut into the rafter before setting it on a wall or beam reduce the strength of the rafter?


We are adding a shed roof to the back of the woodshop, 2x10, 2' OC, 13' span, 36" overhang, 5/8" OSB. Someone told us to set the rafters on top of the beam and fill the voids with the the tail cut offs.


Thanks, George


 

 

(post #90745, reply #1 of 13)

Don't worry about the birdsmouth reducing the rafter strength. You say the rafters are 2x10 stock, the slight amount taken out from the birdsmouth won't make any real difference.

(post #90745, reply #2 of 13)

Didn't say how much of a bird's mouth he was making did he? It is variable based on pitch and rafter size. I was taught that if it was going to leave less than 2" you're probably doing it wrong. It is what ever the depth of the width of the top plate gives you for the seat cut. The upper cut is the seat where the rafter hits the top plate and the other cut is the heel cut. The seat is the width of the top plate unless it will leave less than 2" between the intersection of the heel and seat cuts and the top of the rafter....

Just what I learned framing roofs for churches along time ago. Is that still right? We did some that actually didn't have tails too, most notably doing my pet peeve, blind valleys....

(post #90745, reply #4 of 13)

" blind valleys.... "

How dare you talk dirty on church buildings!!!

;-)

.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #90745, reply #3 of 13)

I don't want to sound too negative about a person who may be a friend, but "someone" shouldn't be giving out such advice without knowing what he or she is talking about.

To not cut in the birdsmouth reduces the strength in the following ways;

less friction at seat for less stability.

load bears on outside edge of plate, increasing tendency to roll.

no nailing area provided therefore no way to tie rafter down against wind without excessive hardware and cost.

filling with endgrain cuts - they will split apart when trying to toenail through them. They won't stay 9 of ten times.

cutting birdsmouths is less than easy for a beginner so I suspect that "someone" is looking for an easy way out of doing it right. That is never the right attitude to bring to the job.

.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #90745, reply #5 of 13)

what that guy was saying, about filling the void with blocking, I don't see where it bears on the outside of the plate , ever look at the tail of a truss?

Peace Pot  Microdot

 

(post #90745, reply #6 of 13)

a tiny wedge of end grain cutoff will split with the grain and fly apart when you put nails in it so it won't be there anymore and the load will be on the outside of the plate, is what I'm saying. No, I havent seen what you are talking about. In trusses I've seen, the grain is parallel to the floor on the bottom chord so it doesn't fly apart into chips about an inch long.

.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #90745, reply #9 of 13)

yes, of coarse, a little care is needed with attachment, I wouldn't discourage the dude, he's probably setting himself up for more work. ME?  I like over cutting the notch(yeah, lets let the tail fall off), and smack it out with my hammer, anything left "chi" "chi" with the chisel. The handsaw can stay in the truck. lol.

Peace Pot  Microdot

 

(post #90745, reply #7 of 13)

I don't think it reduces the strength of the rafter at all, as long as the birdsmouth doesn't extend inside the plate.

If you cut too much out, you can definitely decrease the strength of the overhang, though.





A hangover is the wrath of grapes.

(post #90745, reply #8 of 13)

I won't get into the amount cut away vs. the rafter strength, cuz I didn't see a pitch mentioned, but I would like to make one safety observation.  When cutting bird's mouths, it's standard to cut to the points with a circular saw, then to finish the cut with either a handsaw or a recip saw.  We always called the two cuts 'heel' and 'plate' cuts.  To finish the heel cut with the hand saw you should  make the cut with the rafter standing on edge, the better to observe both sides of the cut, making it neater.  Here's the simple little safety fact for the plate cut;  DO NOT finish this cut with the rafter still on edge!  Lay the rafter flat on the sawhorses.  Although you can no longer see both sides of the cut, the waste will drop out once the cuts meet, and you won't get 5 stitches in your left index finger when the handsaw skips out of the plate cut.  Yes, that's my finger, and yes those are my stitches.   It seems I'm never so experienced that I can't watch myself bleed. 


 


"If left is wrong, then right is the only thing left, right?"

(post #90745, reply #10 of 13)

I would like to thank all for your comments. By the way the roof is rather flat with a 3/12 pitch which will be a shallow heel cut.


Iwill ignore the neighbors advise and cut the bird mouths, your advise is sound and reasonable.


Have a good evening, George


 

 

(post #90745, reply #11 of 13)

What is the minimum plate cut depth? Is it always the width of the top plate, except when it weakens the overhang too much? Is there a formula for this, or is it just common sense?

(post #90745, reply #12 of 13)

no it is NOt always the width of the top plate.. a good  MINIMUM is 1.5"....max is whatever.. there are other things you are trying to accomplish with the birdsmouth.. like the amount of overhang for the soffit... or adjusting the pitch to match an existing roof tie-in..


 


 so.. the birdsmouth is a variable


Mike Smith   Rhode Island : Design / Build / Repair / Restore

Mike Smith   Rhode Island : Design / Build / Repair / Restore

(post #90745, reply #13 of 13)

I used to hear a max of one third of the rafter depth - not that that influences this case with 2x10s - but it could be a factor with a 2x4 and a three foot overhang.

.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...