Is there a formula for cutting/measuring a birdsmouth?

I can use a framing square and figure out my rafters and birdsmouth cut. Is there a simple formula for figuring it out?

I'll use my current project as an example of some numbers.

10x16 shed. Roof will be on the 10' side. Gable roof. 6/12 pitch. 2x6s for roof. 2x4 for walls with t1-11 siding. Lets say a 8" overhang. Probably not going to have a rim joist or whatever you call that board along the peak of the roof. But if i would have one, i know i just subtract 3/4" at the birds mouth to make the cut.

Any simple way to figure out how long of a board i will need, and where to make the cuts?

 

I know you can figure this out, when I was in the trades, I would make one template, then copy it for all the rest after figuring rafter length...

figuring out the approximate length of rafter you will need is real simple, it is the hypotenuse of a right angle triangle...as far as the birdmouth goes, once you do the right angle triangle thing and measure down from the ridge board ...

we used to install the ridge beam with temporary saddles at the right height, then a couple of measurements and you were good to go...

confused, I am, LOL...

 

more formulas...

right angle triangle formula..

run squared plus rise squared equals hypotenuse squared...

lets use your 6 and 12 as an example...

36 + 144 = 180, then just calculate the square root of 180 (13.416, or about 13 and 7/16)....add about 12 to 14 inches on total length so you have enough material for the vertical drop cuts on either end and your rafter extension/overhang

 

The info is right on your framing square if you have a rafter square

I'll show you when I get to work

 

This assumes you have a rafter square

Look for this table on the square's body

" Length of common rafter per foot of run "



For 6 in 12 pitch , look under the 6

The value is 13.42



So the rafter will be 13.42" long for every foot of run

In your example of a 10' wide building , assuming a centred ridge , the run would be half the width , or 5'

13.42 x 5 = 67.1 or 67 1/8"

Draw a plumb line at 6 in 12 at the top of the rafter , this represents the theoretical centre of span



Measure down the top edge of the rafter the 67 1/8" and draw another plumb line at 6 in 12

This represents the exterior of the framed wall

At the wall , you want to cut your bird's mouth , in my picture I laid it out for a 2x6 wall , but it applies the same for a 2x4 ( or any thickness ) wall

For a 2x6 exterior wall , set the square along the plumb line and slide it up until 5 1/2" ( 3 1/2" for a 2x4 wall ) intersects the lower edge of the rafter

This represents your bird's mouth with the rafter flush on the inside edge as well as exterior edge of the top plate



So far we have this :

A rafter laid out on a 6 in 12 pitch , with a theoretical length of 67 1/8 for a 5' span , with a bird's mouth for a 2x6 wall



You will need a ridge board , and it will normally be 2x material and 1 1/2" thick , so you need to shorten the rafter 1/2 the thickness of the ridge board or 3/4"

Make sure to measure this amount parallel to the plumb line and not down the edge ( run ) of the rafter

If you plan to sheet your building with 1/2" plywood and the rafters will overlay the sheathing , you need to remove a further 1/2" from the bird's mouth on the exterior side of the lower plumb line that is representing our exterior plate



Overhangs can be calculated with the same common rafter run per foot

There will be a test later

 

nice oz. im ready for the test, but i will probably cheat and use the builders calculater app on my phone

 

I'm not against calculators , I have a Construction Master Pro but I rarely use it , I prefer to understand the concept of what I am trying to calculate versus which buttons I push to get an answer

 

Construction master pro has an app for iphone, i think its $20 though. But its awesome to have, and it is pretty much the exact same thing as their calculator which is like $70 i think. I hear what your saying though, i learned how to do rafters/stairs without a builders calculator, its the best way to learn

 

I have his covered, real simple

open wallet, pay contractor

 

Good answer

 

Did the OP fall off his roof ?

 

Not if he never erected it

 

Touché

 

I'm here. Got tied up in another project. Will try to tackle this this weekend.

Those directions were excellent. It's probably worth the money to get one of those squares. I think mine is just a regular square.

To figure out my length of rafter, I would always step the square down and mark it. Sorta like this guy is doing...

http://www.youtube.com/watch?v=_xbIqwQf3JI

That formula you gave will make it easier once i get a rafter square.

 

Step down works but you can get a small compounding error each time you move the square , a rafter square is only about $15-20