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Measuring A Roof Slope Print E-mail
Written by jeff (North Kingstown, Rhode Island) 
Measuring A Roof SlopeTo start off, roof slopes are usually measured as the inches of rise per foot of horizontal span. In construction and remodeling, it's just an easier figure to work with than the angular measurement.

To figure out the slope of a roof, mark the top of a level 12" from one end, set that end against the underside of a Rafter inside the attic or a Rake Board outside the house and, with the level horizontal, set a ruler or carpenter's square vertically at the mark. The distance in inches between the top of the level and the underside of the rafter or rake board is the rise of the roof in a 12" unit of run.

Below is a list with the heights of angled cuts for five standard sizes of rafters. The angle between the rafter and the horizontal span is shown in parentheses.

  Height of a vertical end cut
Roof Slope
 2 x 4
 2 x 6
2 x 8
 2 x 10
 2 x 12
1" (5°)
 3.5"
 5.5" 7.25"
9.25"
10.25"
2" (10°)
 3.5"
5.5"
7.375"
9.375"
11.375"
3" (15°)
 3.625"
 5.625"
 7.5"
 9.5"
 11.625"
4" (19°)
 3.75"
 5.75"
 7.625"
 9.75"
 11.875"
5" (23°)
 3.75"
 6"
 7.875"
 10"
 12.25"
6" (27°)
 3.875"
 6.125"
 8.125"
 10.375"
 12.625"
7" (31°)
 4"
 6.375"
 8.375"
 10.75"
 13"
8" (34°)
 4.25"
 6.625"
 8.75"
 11.125"
 13.5"
9" (37°)
 4.375"
 6.875"
 9"
 11.5"
 14"
10" (40°)
 4.5"
 7.125"
 9.375"
 11.5"
 14.625"
11" (43°)
 4.75"
 7.375"
 9.75"
 12.5"
 15.25"
12" (45°, or 1:2)
 5"
 7.75"
 10.25"
 13"
 15.875"

rafter_measurements.jpg And here's an illustration of where to cut rafters for a 6" slope.

So why is the slope important? When it comes to the slope of the roof you'd like to assemble, keep in mind that function comes before form: it can make the difference between good, bad and downright ugly. For a shed roof, you'll probably want to keep the slope at 4" per foot of horizontal span or less. If you're constructing a roof for an addition, keep the slope the same as the main one.

Roofing materials can play a part, too. Tile and slate shingles can't be used on a roof that slopes less than 4" per horizontal foot, while wood shingles need a minimum of 3". Asphalt and roll shingles are the most versatile out of the bunch, because they only require 2" and 1" per horizontal foot, respectively.
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