How to Calculate an Effective Slope for Draining a Roof

Adequate roof slope will ensure that water drains from your roof, protecting the shingles and subsurface from rot. Low-slope roofs are those with less than a 3 inch rise per 12 inch run, expressed as a 3/12 slope. These roofs are typically found on commercial buildings and do not drain well. In fact, they are often plagued with puddles and valleys during rainstorms. Residential roofs should have a slope between 4/12 and 12/12. 12/12 slopes are very steep; while they may drain water well, the cost to install and maintain them is approximately 50 percent higher than a 4/12 roof.

Level
Measuring tape

Instructions

- 1
  Go into the attic or top floor of the house. Taking measurements from the underside of a rafter will ensure an accurate calculation of the roof slope. Measurements taken from outside, on the top of the roof, could be skewed due to multiple layers of shingles.
- 2
  Place a level parallel to the ground (or attic floor) and measure 12 inches from the rafter. This is your focal point.
- 3
  Move the level until it is placed at the 12 inch mark. Turn it so it is perpendicular to the ground (or attic floor), at that 12 inch point, measuring the rise of the roof line. If you measure 7 inches, for example, the roof rises 7 inches for every 12 inches it runs. This can also be described as 7/12 or 7-12.

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