On Aug 13, 8:00 pm, "Leif Thorvaldson" wrote:
"dave12345" wrote in message

oups.com...



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...

=====An easy way to figure them is to get the free download at:http://www.download.com/Easy-Rafters...-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



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.