The pitch of a roof is determined by the slope of the rafters in relation to the horizon. This angle is measured not in degrees, but in inches of rise per foot of run. The only tools needed to determine the pitch of any roof is a ladder, a torpedo level, a speed square, and a straightedge.
Measure the Pitch of a Roof
Use a ladder to get into a position somewhere above the bottom 16″ of the roof. Leaning it against a gutter and allowing it to extend up several feet is ideal. Place the straightedge on the shingles in a direction perpendicular to the gutter. Use the torpedo level to make a plumb mark on the straightedge. Measure the roof pitch with a speed square.
The pitch can also be measured if only a level mark can be made. This is the case on the fascia boards in gables where overhanging shingles prevent an accurate measurement with a speed square on the plumb mark. This time make a level mark with the torpedo level and measure the angle in degrees. Subtract that number from 90. Use the speed square to align the degree with the corresponding pitch.
For the most part, roofs are built on pitches based on whole numbers. There are other slopes however. These are known as bastard pitches. It is more difficult to measure a bastard pitch; it is better to calculate it with this calculator.
Translated to Math
Slopes concerning roofs are always measure based on the number of inches the roof rises in one foot of run. This is why the second number is always twelve. For example; a roof with a pitch of 8/12 rises eight inches vertically for every twelve inches it travels horizontally.
In math, the horizontal or “X” coordinate is always entered first. Then the vertical or “Y” coordinate follows it behind a comma. The same 8/12 pitch is represented “12,8” which is completely backwards from carpenter math.
What it Means
It is very important for construction professionals to realize what pitch roof they are dealing with. For framing, roofing, and siding contractors, this means workers will be walking on the slope of the roof. Practically anyone without a fear of heights can walk a roof rather easily up to a pitch of about a 7/12. More experienced and athletic workers can handle slightly steeper pitches. It is a good rule of thumb to make preparations for toe boards on anything steeper than an 8/12.
It is also much better for any of the construction professionals to know the exact pitch of the roof so they may more accurately make bids on their jobs, mark angles on framing members and siding, and estimate materials to a higher degree of accuracy.