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Is there a formula for cutting/measuring a birdsmouth?

Discussion in 'Garage / Workshop' started by kris77, May 14, 2013.

  1. May 14, 2013 at 4:08 AM
    #1
    kris77

    kris77 [OP] Born in the Backwoods

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    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?
     
  2. May 14, 2013 at 4:19 AM
    #2
    bldegle2

    bldegle2 OldPhart

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    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...
     
    Last edited: May 14, 2013
  3. May 14, 2013 at 4:43 AM
    #3
    bldegle2

    bldegle2 OldPhart

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    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
     
  4. May 14, 2013 at 7:18 AM
    #4
    OZ-T

    OZ-T All of those moments....will be lost.....in time

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

    I'll show you when I get to work
     
  5. May 14, 2013 at 1:59 PM
    #5
    OZ-T

    OZ-T All of those moments....will be lost.....in time

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    This assumes you have a rafter square

    Look for this table on the square's body

    " Length of common rafter per foot of run "

    [​IMG]

    For 6 in 12 pitch , look under the 6

    The value is 13.42

    [​IMG]

    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

    [​IMG]

    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

    [​IMG]

    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

    [​IMG]

    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

    [​IMG]

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

    There will be a test later
     
    Last edited: May 14, 2013
  6. May 14, 2013 at 3:44 PM
    #6
    meatman

    meatman I deal with dead animals

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    There will be a test later[/QUOTE]
    I FAIL! lol now if you were talking about the disassembly of a dead animal then i would pass:eek: Great instructions tho and will save for future refrence
     
  7. May 14, 2013 at 5:21 PM
    #7
    evanmb31

    evanmb31 Well-Known Member

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    :spy: nice oz. im ready for the test, but i will probably cheat and use the builders calculater app on my phone
     
  8. May 14, 2013 at 5:32 PM
    #8
    OZ-T

    OZ-T All of those moments....will be lost.....in time

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    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
     
  9. May 14, 2013 at 5:41 PM
    #9
    evanmb31

    evanmb31 Well-Known Member

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    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
     
  10. May 14, 2013 at 5:44 PM
    #10
    127.0.0.1

    127.0.0.1 AKA ::1

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    I have his covered, real simple

    open wallet, pay contractor
     
  11. May 14, 2013 at 5:47 PM
    #11
    OZ-T

    OZ-T All of those moments....will be lost.....in time

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    Good answer
     
  12. May 15, 2013 at 5:11 PM
    #12
    OZ-T

    OZ-T All of those moments....will be lost.....in time

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    Did the OP fall off his roof ?
     
  13. May 15, 2013 at 5:23 PM
    #13
    xJuice

    xJuice My spoon is too Big!

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  14. May 15, 2013 at 5:24 PM
    #14
    OZ-T

    OZ-T All of those moments....will be lost.....in time

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    Touché
     
  15. May 16, 2013 at 4:47 AM
    #15
    kris77

    kris77 [OP] Born in the Backwoods

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    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.
     
  16. May 16, 2013 at 7:29 AM
    #16
    OZ-T

    OZ-T All of those moments....will be lost.....in time

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    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
     
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