Birdsmouth cut on rafters

[Sal(Guest)]

I am framing the rafters over a 16x16 room.  The  walls are 2x6 with 3/4" plywood.
My question is this: what is the Critical Rafter Depth require to remain after the birdsmouth cut is made.
References to HAP vary in their description.  Two thirds of the wood is mentioned.  However, in order to have the 2x6 rafter sit fully on the top plate, the amount of wood is les than the 2/3 mentioned.
Snow loads are not not that big a deal here, and winds are moderate.

I will a rafter tail about 12" and the roof will be sheathed with 3/4" CDX plywood.
Do I worry to much?
Sal

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[John Raabe]

If you don't have it, get a good framing manual such as the one listed first in the book section http://www.countryplans.com/books.html

There, Wagner shows a diagram on p. 127 on how to layout such a cut. "The seat cut on a rafter should be no more than 1/3 the rafter depth".

[RAB]

To add to what John has mentioned.
For slopes up to about 12in12 any seat cut seems to be self governing for the 2/3 rule, however if you have to make adjustments to plate heights or seat cuts for a proper slope or to match existing slopes you might see the two different scenarios that are in the attached image. Also problems can exist when you're using smaller stock - 2x4 etc.
DC – Deep Cut
SC- Shallow cut
Obviously SC rafters strength is not an issue but area of seat cut is – this should be larger than 11/2 inches resting on the top plate (either side is ok)
Red areas are areas you should be aware of.
So long tails - small stock - wide walls - a steep slope - this is where the 2/3 rule is important.
*For many roofs* the size of the rafter is not governed by strength but by insulation depth requirements so the 2/3 rule can be reduced back to the needed size for strength only.
Rafters with long tails can be supported by soffit framing also.

-Engineers and designers – I have simplified this post for the forum users.
RB