Roof pitch

Take a single roof and describe it twice. Say it climbs 6 feet from the eaves line to the peak over a run of 12 feet, so the span from eave to eave is 24 feet.

The Asphalt Roofing Manufacturers Association's Residential Asphalt Roofing Manual gives slope as “the ratio of the rise, in inches, to the run, in feet.” Seventy-two inches over 12 feet makes that roof a slope of 6 inches per foot, written 6:12. The same glossary gives pitch as “the ratio of the rise, in feet, to the span, in feet; pitch is 1/2 of slope.” Six feet over 24 is one quarter, which is half of the 6:12 slope, just as the manual says.

The FRSA-TRI Florida High Wind Concrete and Clay Tile Installation Manual writes the same words with different units. Its slope is “a ratio of the rise in inches to the run in inches,” so the same roof is 72 inches over 144, a ratio of one to two. Its pitch is a ratio of the rise, in feet, “to the run or span (horizontal), in feet,” which lets the same roof be written as 6 over 12 or as 6 over 24. One roof, one set of measurements, and the figures on paper depend on whose glossary is open.

Both conventions divide one length by another, and ARMA's manual defines each length on its own. Rise is “The vertical distance from the eaves line to the ridge,” the line along the peak described in the ridge entry in this glossary. Run is “The horizontal distance from the eaves to a point directly under the ridge,” and the manual adds that it is one half the span. Span is the horizontal distance from eave to eave.

Put those together and the difference between the two manuals is easy to see. In ARMA's version slope divides rise by run and pitch divides rise by span, and because run is half the span, pitch always comes out at half of slope. The tile manual's pitch allows either run or span as the bottom number, so a single roof can carry two pitch figures under that definition. Each manual is consistent with its own terms; the two simply do not share units.

OSHA's construction fall protection standard uses neither manual's wording. It writes incline as vertical to horizontal and, in 29 CFR 1926.500(b), sorts every roof by one ratio: a slope of 4 in 12 or less is a low-slope roof, and anything greater is a steep roof. The shallow side of that line has its own entry at low-slope roof.

That line was drawn for one purpose, deciding which systems the rule accepts at a roof edge, and the details sit under fall protection. It is a definition inside a safety standard. It is not a rule about whether a roof can be walked, and nothing in it says a roof of any pitch can or cannot be cleaned.

ARMA's manual uses slope, not pitch, when it says where shingles go. In general, it says, most asphalt shingles may be installed on slopes from 4 inches per foot to 21 inches per foot, 4:12 to 21:12, using standard application methods. From 2:12 to less than 4:12, most may be used if “special low slope application procedures are followed.”

ARMA's 4:12 and OSHA's 4 in 12 describe the same incline, and they were written for different jobs: one is the floor of standard shingle installation, the other is the top of OSHA's low-slope definition. Eagle Roofing Products, a concrete tile maker, uses a plain phrase with no number at all. Its 2020 cleaning article says that if a professional is “walking on a steep-sloped roof pitch,” a fall restraint apparatus should be used.

None of these numbers is a cleaning instruction. The trade's own cleaning documents set their limits by material, as how soft washing works sets out, and whether a steep or tall roof is handled any differently has its own answer at roof cleaning on a two-story home.