Calculating a birdsmouth cut

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Forum topic by BiotinX posted 07-14-2018 04:41 AM 609 views 0 times favorited 3 replies Add to Favorites Watch
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7 posts in 833 days

07-14-2018 04:41 AM

I am building a patio roof and I cannot figure out how to how calculate the birdsmouth cut to save my life. I’m about 20 minutes from saying screw it and just burning my entire house down.

The rise of the patio roof is 15 inches, the span is 10’ 1 1/2”, and the thickness of the beam the 2×6 rafter will rest on is 6 1/2” thick (2 2×6 sandwiching a 4×4).

Would someone PLEASE give me some help on measuring the birdsmouth cut! I’m dying over here!

3 replies so far

View Ripper70's profile


1368 posts in 1511 days

#1 posted 07-14-2018 06:30 AM

Maybe this or this will help…

-- "You know, I'm such a great driver, it's incomprehensible that they took my license away." --Vince Ricardo

View Robert's profile


3598 posts in 2083 days

#2 posted 07-14-2018 01:15 PM

Been quite a while since I was a carpenter, but let me see if I can remember.

1. Calculate rafter ridge cuts according to pitch (rise/run). I use a speed square for this. You can also set a long board on top and set a sliding bevel.

2. Measure from the top edge of the ridge board to the outside corner of the wall plate. This gives you the distance from the corner of the birds mouth to the long angle of the other end of the rafter.

3. The horizontal cut is the same angle as the pitch (I think).

4. The vertical cut is 90° to that.

We used to cut 1/8” or so longer to give a little wiggle room.

Cut a board, try it, tweak it, and use as a pattern.

-- Everything is a prototype thats why its one of a kind!!

View waho6o9's profile


8811 posts in 3180 days

#3 posted 07-14-2018 02:53 PM

“Cut a board, try it, tweak it, and use as a pattern.”

^ This is wise.

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