The roof slope, which is often called the "roof pitch," can be expressed in many different ways. In the United States, pitch is usually expressed as the rise of the slope over 12 inches. A roof that rises 4 inches over a 12-inch run would have a pitch of "4:12." Sometimes 1 foot is used instead of 12 inches, and in this case, the roof will simply be called a "4 pitch" roof. In other parts of the world, the slope angle, written in degrees from horizontal, is used. Some roof styles, like the gambrel, will have more than one slope angle.
Place a ladder near or against the building so you can have easy access to the roof. Climb the ladder and bring along a level and ruler or tape measure.
Place the level on the roof lengthwise. The level should be at least 1 foot in length. If it is longer, then there should be a 1-foot mark from either end. The end you measure the 1 foot from should be on the high side when you place it on the roof.
Lift the low side of the level until it is horizontal. It is horizontal when the bubble in the level is centred between its indicator marks.
Measure the distance from the 1-foot mark on the level to the surface of the roof, keeping the ruler or tape measure perpendicular to the level.
Determine the roof slope in whatever form you require. If the distance from the level to the roof surface is 5 inches, you could write the pitch as "5:12" or "5 pitch." To calculate the slope angle, take the inverse tangent--arctangent--of the ratio. For example, 5:12 = 5/12; atan(5/12) = 22.6 degrees