This assumes you have a rafter square
Look for this table on the square's body
" Length of common rafter per foot of run "
For 6 in 12 pitch , look under the 6
The value is 13.42
So the rafter will be 13.42" long for every foot of run
In your example of a 10' wide building , assuming a centred ridge , the run would be half the width , or 5'
13.42 x 5 = 67.1 or 67 1/8"
Draw a plumb line at 6 in 12 at the top of the rafter , this represents the theoretical centre of span
Measure down the top edge of the rafter the 67 1/8" and draw another plumb line at 6 in 12
This represents the exterior of the framed wall
At the wall , you want to cut your bird's mouth , in my picture I laid it out for a 2x6 wall , but it applies the same for a 2x4 ( or any thickness ) wall
For a 2x6 exterior wall , set the square along the plumb line and slide it up until 5 1/2" ( 3 1/2" for a 2x4 wall ) intersects the lower edge of the rafter
This represents your bird's mouth with the rafter flush on the inside edge as well as exterior edge of the top plate
So far we have this :
A rafter laid out on a 6 in 12 pitch , with a theoretical length of 67 1/8 for a 5' span , with a bird's mouth for a 2x6 wall
You will need a ridge board , and it will normally be 2x material and 1 1/2" thick , so you need to shorten the rafter 1/2 the thickness of the ridge board or 3/4"
Make sure to measure this amount parallel to the plumb line and not down the edge ( run ) of the rafter
If you plan to sheet your building with 1/2" plywood and the rafters will overlay the sheathing , you need to remove a further 1/2" from the bird's mouth on the exterior side of the lower plumb line that is representing our exterior plate
Overhangs can be calculated with the same common rafter run per foot
There will be a test later