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Hello all, I have a roof question , just started a house, the main span is
59'2", the front is on a 9/12, the back is on a 8/12, I haven't put a roof on
With the commons on different pitches on forever, what is the easiest way to calculate the rafter lengths.? The front is also 12" higher the the back wall. Thanks.
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Are the walls the same height?
No, the front is actually 24" higher than the back, instead of 12" like I originally posted, any help appreciated. Thank you
((Span - Ridge thickness) / sum of pitches) x pitch.


This is for equal Plate heights w/equal Haps.

You need to subtract the Run on the rear (lower plate) to equalize the plate heights, (or add to the front, taller plate), do your calcs, (derives reciprocal runs) and +/- the run offset use to get back to the original plate heights. Calculate Rafters lengths from Runs.


Remember, the initial results are for the opposing, or reciprocal, Runs.
Hello all, I have a roof question , just started a house, the main span is
59'2", the front is on a 9/12, the back is on a 8/12, I haven't put a roof on
With the commons on different pitches on forever, what is the easiest way to calculate the rafter lengths.? The front is also 12" higher the the back wall. Thanks.
A quick sketch up drawing revealed a few things
Cl of ridge is 26' 5 3/16" from the front
Theoretical rise (not including HAP) is 21'- 9 7/8" off the lower plate line.
The theoretical rafter length for the 9p is 33' 1/2"
The theoretical rafter length for the 8p is 39'- 4 1/8"

I would at least use sketch up to find the ridge line and then you can figure the rafters many different ways after you know the runs.
Hello all, I have a roof question , just started a house, the main span is
59'2", the front is on a 9/12, the back is on a 8/12, I haven't put a roof on
With the commons on different pitches on forever, what is the easiest way to calculate the rafter lengths.? The front is also 12" higher the the back wall. Thanks.
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Is there a way to figure it with just a cm 5 calculator? Thanks for the advice
What, you just want to punch in a couple of numbers and get an answer, without putting any effort into it? How many roofs you cut, anyway?
Travisw601 said:
Is there a way to figure it with just a cm 5 calculator? Thanks for the advice
I have been framing homes on my own since 1998, so, probably a lot more than you have! Thanks anyway
Yeah, not likely. Not even close really.

Look, one guy gave you the formula, and one virtually calculated it for you. One of them is 17 and knows more about figuring roofs than you do. All of this can be done on the CM5 if you follow the steps given.
Come on, nicks 18 now
kiteman said:
Yeah, not likely. Not even close really. Look, one guy gave you the formula, and one virtually calculated it for you. One of them is 17 and knows more about figuring roofs than you do. All of this can bee done on the CM5 if you follow the steps given.
Wow! Time flies.

Nevertheless, he knew how to do this when he was 17.
jlsconstruction said:
Come on, nicks 18 now
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Anyone who brags about how many houses they've framed but doesn't know how to do the math on rafters is full of chit. If you don't know how to do simple math (as far as math goes, this is simple) then hire someone who knows what they're doing
I wasn't bragging about how many roofs I have put on, I was asked how many, and I answered, just have never put one on with the front and back where on different pitches like this one, thanks for the advice guys, I will sit down and try to figure it out again
There is no reason your CM5 will not work for these calculations. When you solve for the Rafter Lengths, it gets a little involved. Take notes!

24" (Rise)/8 (Pitch w/lower height) = 3, Adjusted Span is 3' less from lower plate, 8 Pitch side.

59' 2" - 3' = 56' 2" (Span @ Equal Plate Height)

56' 2" - 1-1/2" (Ridge Thickness (?)) = 56' 0-1/2" (Effective Span)

56' 0-1/2" / 17 (Sum of Pitches) = 3' 3-9/16"

((3' 3-9/16" x 9) (= 29' 8") + 3') = 32' 8" (Effective Run, 8 Pitch side) Rafter Length = 39' 3-1/8"

(3' 3-9/16" x 8) = 26' 4-1/2" (Effective Run, 9 Pitch side) Rafter Length = 32' 11-5/8"

*Assuming Ridge is 1-1/2" thick and HAPs are equal. (Rafters are of equal width ?)


I believe that Nick's dimensions (posted above) taken from his SU drawing are correct and the Ridge Line is where he says it is. However it should not be referred to as a Center Line, it is just the Ridge Line as this is where the planes will meet, and it is not centered, and nothing is centered on it either, for instance, "the ridge thickness" is not divide by 2. It too, is divide by 17 (The sum of the Pitches) and multiplied by each pitch tell you how much each pitch occupies of that space. Removing it from the equation will automatically place the Ridge in its proper place w/o having to bevel the ridge to make it plane at its equally divided Center Line.
Is there a way to figure it with just a cm 5 calculator? Thanks for the advice
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The formula is just a way to figure the ratio between an 8/12 and a 9/12. In other words, how far does the one pitch go on the run to meet the other, after the one on the lower wall gets up to the same height.

Sometimes we get touchy when someone just tries to get an easy answer on their first post without an intro.
Thanks so much, I really appreciate your help, that is going to save me a lot of head scratching , thanks again for the explanation.
Yep, The 8 side is 9/17 of the level span, and the 9 side is 8/17 of the span. If he has never done a Salt Box, he is still Very Green. :laughing:
The formula is just a way to figure the ratio between an 8/12 and a 9/12. In other words, how far does the one pitch go on the run to meet the other, after the one on the lower wall gets up to the same height.

Sometimes we get touchy when someone just tries to get an easy answer on their first post without an intro.
I run into this with guys who mostly only do trusses.
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What's a salt box? I've never heard that term.
Generally, a house with two stories In front and one story in back with an unequal pitched gable roof.
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