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I am used to 10% waste on gable roofs and 15+% on hip roofs, but what I am looking to figure out is a real life formula. Is it possible to take all of our data collected, such as perimeter, ridge, valley, and concise measurements, to come up with an exact waste factor on paper. It may not always work out that way of course, but I want a justifiable waste factor. I would imagine we could take math and figure out how much waste we are using on the perimeters. Such as if there is 450lf of perimeter, how many shingles will have to be cut. How many will still usable for another end shingle? How about the same theory with valleys? I know technique comes into this. For example, we use starter strips instead of 3 tab. We use the "California Cut" instead of weaved valleys. How about waste by the ridge, whether or not there is ridge vent installed. There are a million variables, but can anyone give their insight on some mathematical ways of figuring this out? Thanks and this is a great site! I'm glad I found it.:thumbup::thumbup:
 

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There is never any waste on my jobs.
We roll up our cigarettes with the stuff we cut off. Doubles the tar intake. Cheaper than smokes...
 

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. . .if there is 450lf of perimeter, how many shingles will have to be cut.
If I correctly understand your question this may take some back and forth, and maybe a small computer program.

In the simple and unrealistic case of one layer of non-overlapping shingles:

for a square shape
with 450 lf perimeter
the area is (450/4)^2 = 12656.25 sf.
This will take 4218.75 shingles of dimension 1' x 3'.

For a rectangle shape of 1:3 ratio
with 450 lf perimeter
the area is 9492.1875 sf
and will take 3164.0625 shingles.

How much shingle overlap?
What other complicating factors are there?

Let's work out one case at a time.
 
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