While writing our last post, an interesting question came back to me. It was the same question I had when I received the Holts Roof Gauge after purchasing it on eBay and began to look at it closely. What’s the difference between slope and pitch? 1/4 pitch is 6:12 slope? 7/12 pitch is 14:12 slope? What the heck is going on here? I have been in roofing for 35 years and slope and pitch has always meant the same thing to me.
Well that sent me off to the internet. I like to think I am a good Googler, so I thought real hard on what would be the best search terms to use and came up with "what's the difference between roof slope and roof pitch." I told you I was a good Googler. I hit enter, and voilà, my question was answered.
Hello Mr. NACHI (that’s The National Association of Certified Home Inspectors for those who are not friends). The NACHI website has a super duper good answer to my question. Granted, the NACHI article is geared more towards shingles and Mule-Hide doesn’t make shingles anymore, but the information on slope versus pitch is just as relevant for low-slope roofing too.
Here’s what they had to say:
Slope is the incline of the roof expressed as a ratio of the vertical rise of the horizontal run, where the run is some portion of the span. This ratio is always expressed as inches per foot.
A roof that rises 4 inches for every 1 foot or 12 inches of run is said to have a "4 in 12" slope. If the rise is 6 inches for every 12 inches of run, then the roof slope is "6 in 12."
The slope can be expressed numerically as a ratio. The slope ratio represents a certain amount of vertical rise for every 12 inches of horizontal run. For example, a "4 in 12" slope can be expressed as the ratio of 4:12. A "6 in 12" slope is expressed as 6:12.
Slope is expressed:
- as a ratio; and
- in inches per foot
Pitch is the incline of the roof expressed as a fraction derived by dividing the rise by the span, where the roof span is the distance between the outside of one wall's top plate to another.
Historically the word "pitch" meant a ratio between the ridge height to the entire span/width of the building or ratio between the rafter length to the building width. And back then, the ridge was typically in the middle of the span. This is no longer the case in modern building practices. The ridge can be placed anywhere in the span, from directly middle to either span endpoint.
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."
The pitch can be expressed numerically as a fraction. The pitch fraction represents a certain amount of vertical rise over the entire span. For example, given a roof with a rise of 4 feet and a span of 24 feet, the pitch is "1 to 6", which can be expressed as the fractkion of 1/6. A "12 to 24" pitch in expressed as 1/2.
So what did I learn? I don’t think I can say it any better than the NACHI article:
“The term 'pitch' and 'slope' are often used interchangeably, which is incorrect. They do not mean the same thing. And slope provides more valuable information than pitch, as defined in this article.”
Thank you again NACHI.org and here is a link to the whole article: https://www.nachi.org/roof-slope-pitch.htm