The two International Residential Code (IRC) standards that apply to cutting a birdsmouth in a roof rafter are:
1) "Notches at the ends of the member shall not exceed one-fourth the depth of the member" (IRC R502.8.1). That would be a max of 1-3/8” for a 2x6, 1-3/4” for 2x8, 2-1/4” for 2x10 and 2-3/4” for 2x12 (measured perpendicular to the bottom of the rafter).
2) "The ends of each rafter or ceiling joist shall have not less than 1-1/2 inches (38 mm) of bearing on wood or metal and not less than 3 inches (76 mm) on masonry or concrete” (IRC R802.6). Rafters on a block wall are usually installed over a nominal 2-inch thick wood plate on top of the wall, making the 1-1/2” bearing the one that’s applicable minimum in most cases.

    So the two controlling factors are the minimum 1-1/2” bearing and depth of the birdsmouth cut not exceeding 1/4 of the depth of the rafter.

    It may not be possible to get bearing over the full surface of the top plate when framing a high-pitch roof, but as long as you meet or exceed the 1-1/2” minimum it’s fine. Conversely, cutting to the maximum depth allowed for a low-pitch rafter can end up with a seat cut that is wider than the top plate, which increases the possiblity of a split in the rafter at the notch. The seat should not exceed the width of the plate it sits on.

    When the rafter extends past the bearing wall for an overhang/eave, the code does allow the overhang to be notched back to a minimum of 3-1/2” depth for up to a 2-foot cantilever, to avoid an excessively tall fascia (IRC 802.7.1.1)

    Also see our blog posts Are roof trusses better than roof rafters (stick framing)? and Why is there diagonal bracing at the roof rafters in the attic? 

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  To learn more about roofs and attics, see these other blog posts:

• If my roof is not leaking, why does it need to be replaced?

• How can I be sure my roofing contractor got a permit?

• How many layers of roofing are allowed on a home? 

• What are the dark lines running parallel to shingles on my roof?

• Can metal roofing be used on a low slope/pitch roof? 

• How can I make my roof last longer?  

• What are the warning signs of a dangerous attic pull-down ladder?

• How can I find out the age of a roof? 

• Should I buy a house that needs a new roof?

• Should I buy a house with an old roof? 

• What are those metal boxes on the roof?

• What does "lack of tab adhesion" in an asphalt shingle roof mean?

• Why do roof edges start leaking?

• Why do my dormer windows leak? 

• Do home inspectors go on the roof? Do they get in the attic?

• Should I put gutters on the house? 

• How much of a roof truss can I cut out to make a storage platform in the attic? 

• What's the difference between an "architectural" and a regular shingle roof? 

• What does a home inspector look for when examining a roof? 

• Do stains on the ceiling mean the roof is leaking?

• How can I tell if the house needs a new roof?  

• Why does my homeowner's insurance want a roof inspection?

• What are the hazards to avoid when going into an attic? 

     Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.