The simplest method for doing this on an existing roof is this:
Take a torpedo level, tape measure, and a straight edge up on
the roof. Find or mark 12" on the straight edge and set one
end on the roof. Level the straight edge with the torpedo
level and then measure down from the 12" mark to the roof.
Try to go straight down from the straight edge (bottom edge).
Whatever number you get closest to, that is your pitch. If
it is 5", then it is a 5/12 pitch. If it is 8", it is an 8/12
If you have cathedral-style or slanted interior cielings, you can do
this same method inside (but, like, upside-down). Also, if you don't
have 12 feet to work with, use 6 and double your results. If you can't
do it inside, and don't want to go on the roof, there are other
"math"-ish ways as well.
And, as an aside, this isn't really math. It is just measuring. "5:12
pitch" (or 5/12 pitch, or 5-in-12, or whatever) just means it goes down
5 feet for every 12 feet across, or equivalently 5 inches for every 12
inches. A 4-in-12 pitch would just be 4 feet down for every 12 over.
Robert Allison wrote:
Doesn't always work, if the roof was framed with trusses the pitch on
the outside can be different.
Don't know how exact the OP needs to be, but it is better to measure
the pitch on the rake board so the overlap of the shingles doesn't
throw you off. It is also better to use at least a 2' level to get the
measurement more accurate.
Hold a framing square against the edge of a gable roof with the long
leg on top and the short leg hanging down at the downhill side. Slide
the square until the 12" mark on the inside of the square lines up with
the edge of the roof, then hold a level on the top of the square.
Whatever reading you get on the inside of the short leg is the direct
You can also determine pitch by using a folding rule:
That requires a conversion table, but you don't have to go up on the
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