Roof RaftersDate: 02/03/97 at 18:51:26 From: Ken Badalamenti Subject: Roof Rafters Hello, I am a general contractor. I am working with some estimating software and setting up a database to find lumber quantities for projects I bid. My problem is that I am trying to figure out a formula for some roof framing. PROBLEM: I am trying find a formula that will give me the length of a roof rafter by using the pitch of the roof. All sloped roofs have what's called a pitch. The pitch is how far the roof rises for every 12" of horizontal run. Example: a 4/12 pitch means that for every 12" of horizontal run, a roof will rise 4". A 6/12 pitch means it will rise 6". In a right triangle, the length of the hypotenuse would be the same thing as the length of the rafter. The pitch would give you the angle at the bottom of the rafter. A 4/12 pitch would mean that the angle is 15 degrees. A 6/12 would have a 22.5 degree angle. I know I can figure this using the Pythagorean theorem but I am trying to devise a formula so one can use the dimensions already noted on the blueprints without having to figure out the rise. All blueprints give the pitch of a roof and the run will always be 1/2 the length of the wall underneath. I'm fairly sure that this formula will require the use of cosines and/or sines. The unfortunate part is that my estimating software does not have these functions so I am trying to come up with a formula that does not require this. Your help in this matter would be greatly appreciated. If you cannot help, maybe you could at least lead me in a direction that can. Thank you very much, Ken Badalamenti Date: 02/04/97 at 10:17:14 From: Doctor Robert Subject: Re: Not a student but needs help Let L be the length of the underneath wall, and let R be the length of the rafter you need. Let p be the pitch, which is the rise over the run of the rafter. If you imagine a right triangle with the vertical leg of length a, the horizontal leg of length b, and R the hypotenuse. Then: R^2 = a^2 + b^2 = b^2(a^2/b^2 + 1) = (L/2)^2(p^2 + 1) R = L/2 * sqrt(p^2 +1) This should give you the length of the rafter. -Doctor Robert, The Math Forum Check out our web site! http://mathforum.org/dr.math/ Date: 07/25/97 at 10:29:25 From: Doctor Ken Subject Re: Not a student but needs help Here's a note from the archivists. Wayne McLennan, a reader of our archives, just wrote in to say that his definition of pitch differs from the original poster's. According to Wayne, pitch is the ratio of the total rise of the roof to the total span of the building. Thus the pitch is half of what mathematicians call the slope. If this is the case, then the following modifications to Rob's work are in order: R^2 = a^2 + b^2 = b^2(a^2/b^2 + 1) = (L/2)^2((2p)^2 + 1) R = L/2 * sqrt(4p^2 + 1) In my search of the web for information on roof pitch, I found that there are several different standards. Some people seem to communicate pitch by simply noting the angle - for instance, a pitch of 60 degrees. Other people seem to say things like "a 10 inch pitch", meaning that for every 12 inches of horizontal distance, the roof rises 10 inches. And some people express pitch as a ratio (the same way as the original poster), saying things like "a pitch of 2 in 12". See http://newberry.deschutes.org/CDD/planning/old/Maho.htm#5 . -Doctor Ken, The Math Forum |
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