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Bird’s mouth necessary?

Oak River Mike | Posted in Construction Techniques on August 16, 2009 05:14am

A topic of debate among me and some local framers…

If you are using say 2″ x 10″ for rafters and a Simpson H10 to connect them to the top plate, do you indeed need to cut a bird’s mouth?

What say ye?

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  1. User avater
    basswood | Aug 16, 2009 05:40pm | #1

    I still like to build as if there is not any hardware. Consider it fault-tolerance. I may be overly-conservative, but I like beams under joists rather than flush with joists only on hangers.

    The H10's are designed primarily to hold rafters down from the uplift force of wind. This pulling force puts the metal in tension... using the H10's instead of a birdsmouth loads them with primarily shear forces (preventing the sliding of the rafter off the top plate).

    The seesaw of uplift during wind (tension) and down force of gravity causing spreading of rafters (shear) could eventually cause metal fatigue and failure. I'm not an engineer though.

  2. Piffin | Aug 16, 2009 05:50pm | #2

    at what pitch and snow load?

     

     

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    1. User avater
      basswood | Aug 16, 2009 05:53pm | #3

      Not too much snow in Florida. ;o)

      1. Oak River Mike | Aug 16, 2009 10:23pm | #4

        Agreed, Snow load is Zero.  Pitch is 6/12

        1. Piffin | Aug 16, 2009 11:19pm | #5

          t 1/12 I would not cut. At 6/12 I would definitely cut.Of course, the right answer is to be found at the Simpson website - as interpreted by your local inspectator 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

        2. Piffin | Aug 16, 2009 11:32pm | #6

          http://www.google.com/search?hl=en&safe=offei=1WuIStblC6mutgfDxtXnDA&sa=Xoi=spell&resnum=0ct=result&cd=1q=Simpson+strong+tie+H10+%2B+bird%27s+mouth&spell=1Looks like this was discussed at JLC five years ago 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          1. Oak River Mike | Aug 16, 2009 11:43pm | #7

            Piffin,

             

            Thanks.  But I can't get the link to work.  What did you search under?

            BTW, looks the the Simpson literature refers to full bearing of the rafter and top plate so you might have to cut the BM regardless.

            Mike

          2. Piffin | Aug 17, 2009 12:03am | #9

            "Simpson strong tie H10 + bird's mouth"I still had the tab open 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          3. Oak River Mike | Aug 16, 2009 11:53pm | #8

            Piffin,

            I found it...Interesting discussion.

            Mike

          4. pm22 | Aug 17, 2009 01:41am | #10

            An alternative to birdsmouth is to rip a piece the thickness of your top plate at an angle corresponding to your roof slope. In other words, if your pitch is 4:12, then rip your piece about 27 1/2*. This will give solid support and you don't have to cut any birdsmouths.

            I don't know why nobody thinks of stuff like this. Cutting a birds mouth into a TIJ seems criminal.

            ~Peter

          5. MattSwanger | Aug 17, 2009 02:41am | #11

            they make neccessary hardware for TJI rafters,  very pricey last roof i used them on,  easier to cut my studs with the same angle, and nail a top plate on,  beveled both sides. Woods favorite carpenter

             

          6. Piffin | Aug 21, 2009 02:54pm | #12

            When did he mention TJIs? 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          7. Framer | Aug 21, 2009 03:29pm | #14

            Cutting a birds mouth into a TIJ seems criminal.

            It's not, you're allowed. More blocking involved.

            Scroll down to page 19 where it say Roof Details. Underneath that in picture R5, it says Birdsmouth Cut.

            http://www.ilevel.com/literature/TJ-4000.pdf

             Joe Carola

  3. frenchy | Aug 21, 2009 03:24pm | #13

    There is one other quick and dirty way to use rafters without a birds mouth cut in them.

    Sister a 2x4 onto the top plate and cut the tail at the roof pitch and the head to 90 degrees to the end of the top plate.

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