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How much can you notch a roof rafter for the seat and heel cuts at bearing?
Saturday, September 19, 2020
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)
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To learn more about roofs and attics, see these other blog posts:
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