Question: The difference between slope & pitch

I understand that the slope of a roof is measured by the amount of inches it rises by the run measurement. So for every 4" rise for every 12" run would be 4:12.

The pitch is: A roof that rises 8 feet over a 24-foot span was said to have a “1 to 3” pitch. If the rise is 4 feet over a 24-foot span, then the roof pitch was said to be “1 to 6.”

I dont understand how they are getting 1/3" pitch or 1/6" pitch? Why is a pitch described in feet and the slope is described in inches?

TYIA Candi

If you’re referring to a Nachi course, could you supply the link to the specific page or pages?

Yes sorry:

Welcome to our forum, Candice!..Enjoy participating. :smile:

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This may help you visually


1:3 and 1:4 are ratios. 1/3 and 1/4 are fractions but also ratios. The long way of describing them is the denominator (bottom number of a fraction) is 3 or 4 times the numerator (top number in a fraction always 1 for a ratio) 1 inch in three inches is the same as 1 foot in three feet, 1 yard in 3 yards or even 1 mile in three miles. The ratio is always the same…

1 inch in three inches = 12 inches in 36 inches = 1 foot in 3 feet = 5280 feet in 15840 feet.

If you think of 4:12 as a fraction (4/12) right away you realize it is reduced to 1/3.


This is perfect Thank you!

Your welcome and best of luck!
Also don’t be a stranger here, lots of knowledgeable people in here.

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I literally just learned about the forum page this week, very excited to have this connection! :grin:


The Nachi course makes a big deal of a supposed difference between slope and pitch, and muddies the water with examples in different units.

A ratio is not in inches, feet or anything else.

Your 8 feet over 24 feet can be expressed as 8:24 or 4:12 or 1/3 pitch. It’s all exactly the same:

8 ÷ 24 = .333333…
4 ÷ 12 = .333333…
1 ÷ 3 = .333333…

8:24 = 1/3 = 4:12 pitch = 4/12 pitch = 18.43 degrees = 4 & 12 or just “4 pitch”.

To say it again, a ratio (the relationship of two numbers) is not inches, not feet, it does not have a unit at all.

See also Slope vs pitch confusion in roof inspection training

2021 IRC defines “Pitch” as “See Slope".


This is great! Thank you so much!