How to Measure Roof Pitch

How to measure roof pitch with a level

You need two measurements: a horizontal run and the vertical rise over that same distance. A level and a tape measure can establish this small triangle along an accessible, straight rafter.

Use a place where you can work from secure footing. An attic can provide access to a rafter, but only where the space and flooring are suitable. Do not step onto ceiling material or climb a roof to obtain a reading for this calculator. If the framing is not safely accessible, use measurements from the plans or ask someone equipped to inspect it.

Step 1: establish a horizontal run

Choose a measured distance along your level. In imperial units, 12 inches makes the conversion convenient. In metric units, 300 mm is a practical example; it does not have to equal 12 inches.

Hold the level horizontal with one end at the rafter. Check the bubble. The distance along the level is the run, not the distance along the sloping timber. The run versus rafter length diagram shows why these differ.

Step 2: measure the vertical gap

At the other end of the chosen horizontal distance, measure vertically between the level and the same straight edge of the rafter. This gap is the rise. Avoid measuring diagonally with the tape or switching from one edge of the board to another.

Both measurements must describe the same triangle and use the same units. If possible, repeat the reading at another accessible point on the same straight roof section to check for a measurement error.

Step 3: convert the reading

Open the roof pitch calculator, select Rise / run, and enter the measurements:

The metric example uses the same ratio, so it describes the same slope. Enter fractions as decimals: 6 1/2 inches becomes 6.5.

If you already have an angle or percentage from a drawing, select Degrees or Percent instead. Enter 8.7 for an 8.7% slope; do not enter it in the degrees field. For a quick lookup, use the printable roof pitch chart.

Step 4: add full roof dimensions only if needed

The short level reading is enough for pitch and angle. It is not the full roof run. To estimate a rafter length, open Estimate rafter length and separately enter the horizontal wall-to-ridge distance in feet or meters.

For example, a 6:12 slope over a 12-foot full run gives a theoretical sloped length of about 13.416 feet. The rafter calculator can include a horizontal overhang. Neither tool calculates birdsmouth depth, ridge-board deductions or structural capacity.

Assumptions and corrections

This method assumes you are measuring a straight roof section whose rafter follows the slope you want to describe. Irregular, curved or compound framing needs a different measurement plan. A pitch reading alone does not determine whether a roof covering or framing member is suitable.

Read our calculation methods for formulas and rounding. Send us a correction if a label or worked example is unclear.