rafter layout

Can someone tell me if this is a feasable way to layout commons.
lets say its a 10/12 with a 20' span.
i look on my framing square, and it says "COMMON RAFTER LENGTH PER FOOT RUN" for a 10/12 it says 15.62.
so i multiply 15.62 by half my span which is 10.
gives me 156.2 which is about 156 1/4"
i subtract 3/4" for my ridges, gives me 155 1/2"
now is this 155 1/2" the measurement from my birdsmouth to the top of my rafter?
also how do i determine the depth of my birdsmouth and do i need to factor this into my math somewhere?
if anyone can understand what i just wrote, i appreciate your help...
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dave12345 wrote:

If you mark a vertical line across your rafter at the birdmouth (the vertical cut), then from the point where that mark intersects the top of the rafter to the top of the vertical cut at the ridge it should be 155-1/2".
I usually make my birdmouth as deep as it needs to be to completely seat across the to plate. So if you have a 2x4 top plate, you want the seat cut (horizontal cut) of the bird mouth to be 3-1/2". If you have a 2x6 you might want to make it 5-1/2", but I don't like cutting that deep into the rafter, so I keep mine less. I just want a good bearing surface for my rafter on the top plate.
If you don't want to have to cut a bunch of slots for your sheathing, then you should apply the sheathing to the walls first, then add an additional 1/2" to your span for the thickness of the sheathing, then install the rafters so that they seat across the plate and the sheathing.
You should cut two rafters without taking out for the ridge (meaning that the thickness of the ridge should NOT be subtracted from the calculation). Take them up and try them at both gables of your building and in the middle if it is a long building. They should fit well at all points. When you are satisfied that they will work, then cut off the additional 3/4" for your ridge and use them for patterns to make the rest of your rafters.
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I agree totally with Robert, but I like to try to keep a full 2x4 (3 1/2") above the birds mouth to carry the rafter tails. If the bearing is getting cheated too much, it may require some horse trading. That steep pitch is harder to work around. The ceiling joists spiked into the sides of the rafters establish much of the tie.
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=====>An easy way to figure them is to get the free download at: http://www.download.com/Easy-Rafters/3000-6677_4-10520659.html
Althought it is a good exercise to use the square to keep one's hand in. I like the rafter program because I am generally using my computer cad program to design my building and it is handy to have rather than go down to the shop and get the square and mess around with it.
Leif

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Everyone has given excellent responses so far. I figure the rafter length with a scientific calculator and trig. 10/12 inverse tangent gives you the angle (make sure it's in degrees and not radians. the span multiplied by the cosine of that angle gives you the length. I work from plumb cut to plumb mark at the outside of the wall plate. I make my birds mouth as wide as the wall plate if possible, but no deeper than 1/3 of the rafter depth.
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