How to measure the slope of a roof

When making Swift boxes to fit neatly under sloping eaves or a triangle in the apex of a gable, it is necessary to determine accurately the slope of the roof. We here describe 2 methods of doing this.

Roof slopes are always some ratio to 12, so find the nearest ratio to 12. We have implemented projects with slopes 12:4, 12:5, 12:10, 12:12 (45°), 10:12, 9:12 and 8:12

Smart phone
Smartphones these days usually have an inclinometer app, which may be embedded in a compass app. By carefully aligning the edge of the phone with the roof line one can read the angle directly.
Use the diagram below to find the nearest angle, and then the ratio to 12.

For example, if the angle measured is 23°, the nearest angle is 22.6° corresponding to a slope of 12:5.
A measured angle of 53° has a nearest value of 53.1° corresponding to 9:12

Photography
Stand well back from the slope you are measuring, and take a photograph making sure that the camera is pointed at right angles to the wall. If there were a mirror on the wall, then the camera would appear in the middle of the picture. It is of course important to hold the camera horizontally. This is less critical for the apex of a gable where you can see both sides of the roof and one can usually see some feature, such as a course of bricks as a horizontal reference.

Angles in the picture should now be exactly the same as angles of the roof.

If one measures from a photograph the horizontal and vertical components h and v, then :

For slopes less than 45° use the ratio 12:(12*v/h rounded)
For slopes more than 45° use the ratio (12*h/v rounded):12

For example if h is 239mm and v is 161mm, this is a slope less than 45°, so calculate 12x161/239 = 8.08. 8.08 rounds to 8, so the slope is 12:8

[Alternatively, you could put the expression =atan(161/239)*180/PI() into a cell in an Excel spreadsheet, and then use the angle (33.97°) in the diagram above. The nearest angle is 33.7° corresponding to 12:8]