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Old 09-17-2008, 02:29 PM   #1
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rafter length


Who wants to help me with my math??? I need to order rafters for a barn I'm building- 30' wide with an 8:12 pitch. I've gotten this far: 15' run with a 10' rise. (run sq. + rise sq. = x sq. = rafter length) so 225 + 100 = 325 sq. = 18.027756. So 18' and how many inches? I will have a foot of overhang so looks like I should order 20' lumber. The foot of overhang I plan on "walking off" with my framing square. I understand I can figure entire length this way but too much room for error in my opinion. Thanks for the math lesson!!!
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Old 09-17-2008, 03:28 PM   #2
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rafter length


Check this link out...it from a book titled Measuring, Marking and Layout: A builders guide by John Carroll and is a snipet from google...it'll explain it in excruciating detail! And the good old standard from Stanley(be cautious it will download a PDF of the rafter square manual) pretty poor quality...but readable.
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Old 09-17-2008, 04:24 PM   #3
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Originally Posted by ugabulldog View Post
Who wants to help me with my math??? I need to order rafters for a barn I'm building- 30' wide with an 8:12 pitch. I've gotten this far: 15' run with a 10' rise. (run sq. + rise sq. = x sq. = rafter length) so 225 + 100 = 325 sq. = 18.027756. So 18' and how many inches? I will have a foot of overhang so looks like I should order 20' lumber. The foot of overhang I plan on "walking off" with my framing square. I understand I can figure entire length this way but too much room for error in my opinion. Thanks for the math lesson!!!
8" pitch with a 15' run including a one foot over hang requires a 19'-3" rafter minus 1/2 the thickness of the ridge. your rise would be 10' -0". Your total lenght of your rafter would be 18"-1/2" plus 1'-2 1/2" for your overhang. = 19'-3"
Good luck BOB
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Old 09-17-2008, 04:30 PM   #4
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rafter length


A2+B2=C2 216.33307 inches minus 1/2 the thickness of your ridge board.
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Old 09-18-2008, 03:22 PM   #5
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thanks and thanks........... I had an old framer tell me once that instead of subtracting 3/4" (half the ridge board) he only subtracted 1/2" so the rafter would set tighter against the ridge. He showed me this after my rafters/ridge fits had a gap at the bottom. Any comments on that??
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Old 09-18-2008, 06:24 PM   #6
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thanks and thanks........... I had an old framer tell me once that instead of subtracting 3/4" (half the ridge board) he only subtracted 1/2" so the rafter would set tighter against the ridge. He showed me this after my rafters/ridge fits had a gap at the bottom. Any comments on that??
That doesn't make any sense. If you subtract 1/2 the thickness of the ridge and cut the rafter the right length and your walls are parallel the rafter will fit perfect without any gaps. If you had a gap, you must have something wrong with the length of your rafter or the plumbcut wasn't right, or the walls bowed.

With a 15' run and a 2x ridge, you effective run is 14' 11-1/4". 8/12 pitch, your rafter length to the outside of the top plate will be 17' 11-7/16". Your 1' overhang, add another 1'2-7/16".
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