How do you read a roof pitch diagram chart?
Short answer. A roof pitch diagram draws the roof as a right triangle: a horizontal run of 12 inches along the bottom, a vertical rise up one side, and the sloped rafter line as the long side. Read the rise number against the 12 inch run, so a triangle with a 6 inch rise is a 6/12 pitch.
The three sides of the triangle
- Run: the horizontal distance, always shown as 12 on a pitch diagram. On a whole house, the run is half the span, from the outside of the wall to the center of the ridge.
- Rise: the vertical height gained over that run. On the diagram it is the number before the slash.
- Rafter or slope line: the hypotenuse. Its length per foot of run is what roofers call the pitch multiplier, which is why sloped roofs have more surface than their footprint.
The small triangle on drawings
House plans and truss drawings show pitch with a tiny triangle symbol next to the roof line: a horizontal leg marked 12 and a vertical leg marked with the rise, such as 8. That symbol means 8/12. If a plan shows two different symbols, the house has more than one pitch, often a steep front gable and a lower main roof.
Sketch your own
- Draw a horizontal line and label it with half the width of the house.
- Draw a vertical line at one end for the height of the ridge above the top of the wall.
- Divide the height by the half width and multiply by 12. That is your pitch.
For example, a 28 foot wide house has a 14 foot run. If the ridge is 7 feet above the wall plate, 7 divided by 14 is 0.5, and 0.5 times 12 is 6, so the roof is 6/12.
What the diagram leaves out
A pitch diagram ignores overhangs, decking thickness and the shingles themselves, so measured heights on a real house run slightly higher. For the full picture, see roof anatomy explained and how roofers measure a roof.