I am cutting rafters for a shed roof. The rafters will be 16o.c 2x6. The rise is 3:12 (or 3 inches for each 12 inchs of run)
My question concerns the birdsmouth cut. If I cut a 1 and 1/2 inch heel cut and a 3 and 1/2 inch seat cut, I am in compliance with the code for Height Above Plate for the rafter. If I make this birdsmouth cut, doesn't this alter the pitch of 3:12? Would I also have to alter the rise in some way to account for the birdsmouth cut out of the rafter?
This is where experience counts. You have correctly identified a problem that often results in roofs that are 2-3/4" per foot or other odd slopes. The easiest way to deal with this is to simply measure
1-1/2 less for the top of your ridge, because that is how far down the exterior bottom of your rafter is when it seats on the plate.
Example: 3/12 pitch roof with a 20' span = 30 inch rise
If you mark a vertical line at the vertical cut of your birdsmouth, you will find that it is approx. 4-1/8" to the top of the 2x6 rafter from the plate (or from the seat cut). So 30" + 4-1/8" = 34-1/8" to the top of the ridge from the top of the plate.
Since your birdsmouth is 1-1/2", you can measure from the bottom of the top plate to the bottom of the rafter at the ridge for your 30" and then add your rafter for the top of the ridge. (A 2x6 rafter with a plumb cut of 3/12 will measure approx. 5-5/8" along the plumb line.) I usually cut a pattern to mark the top of the ridge.
Of course the simplest and commonest solution is to just ignore it. The change in pitch over any reasonable run, e.g. 12 ft for example isn't enough to screw up anything.
That is what most people do. That is why you see so many rafters that have gaps at the ridge and the birds mouth only sets on the plate at one point.
I pay a little more attention to detail. The curse of being both a framing carpenter and a cabinet maker. It also leads to everything fitting better in the long run, and all the way through the building process.
Just a quick story related to cutting birds mouths...
Many years ago my brother-in-law's family took the summer off to build a "cabin" on some land they owned - a 2-story, three bedroom cabin. They sent out invitations a year earlier, inviting friends and family from across the country to come down for whatever time they could during that summer, camp out and help raise the house. I spent almost three weeks helping out and it still ranks as the best vacation I ever took.
Anyway, over the course of the summer, people of various skill levels came and went, pitching in wherever they could. On the day we were raising the rafters, I was cutting the birds mouth after birds mouth with a circular saw when I was approached by a new-comer, asking me how he could help out. I took this as an opportunity to take a break and started to show "Cousin Jeb" how to layout the birds mouth and cut them.
Suddenly out of nowhere, appeared a family member who knew who this guy was, and who sent him off to perform some menial task. The family member then came back to me with this information: "Be careful what jobs you give him. He's dangerous with hand tools, deadly with power tools!"
I'm glad I never actually handed him the circular saw!
Any gap at the ridge or poor fit at the plate would be due to piss poor layout work, not ignoring a very minor, nit picking, point in the calculations. There are points in construction where such finicky calculations are needed. In rough framing (and rafters are just that), it is not only not needed, it is wasted time. I have been kickign around both amatuer and pro constuction for most of my life off and on (55 years) and have never seen anyone worry about that much less do it. The carpentry couse I took conducted by professionals, did not even mention it.
You are right. Most people don't pay attention to details like that, especially framers. But the ones that care about their work, do pay attention to it.
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