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Roof pitch: what it means on an inspection

Rafter Tie editorial teamUpdated 3 min read

Roof pitch is how steep a roof is, stated as rise over run. A 6:12 pitch rises 6 inches for every 12 inches of horizontal run. The run is measured level, not along the rafter, which is why a rafter is always longer than the run it covers.

A right triangle with a 12 inch run along the bottom and a 6 inch rise at the right edge, labeled 6:12 pitch, with the slope angle at the left marked as about 26.6 degrees
Rise over run, in inches per 12 inches of run. The angle is a conversion, not the pitch.
Use this diagram

How to read and use a pitch

  1. Read the two numbers. In 6:12, the first number is the rise and the second is the run. The run is nearly always written as 12, so 6:12, 8:12 and 4:12 are quick to compare.
  2. Measure run level. Run is horizontal distance, from the outside of the wall plate toward the ridge, not the distance along the sloping board. Use a level or a plumb line, not a tape laid on the rafter.
  3. Find the rise. Multiply the pitch by the run. A 6:12 roof over a 12-foot run rises 6 feet, because 12 feet is 12 times the 12-inch unit, and the pitch rises 6 per unit.
  4. Find the rafter length. The rafter is the hypotenuse of the triangle formed by run and rise. Add the overhang separately and remember that tail cuts and ridge thickness change the cut length. The rafter length calculator does the arithmetic.
  5. Convert only when asked. A 6:12 pitch is about 26.6 degrees. Many code tables and products are written by pitch, so convert only if the question gives an angle.

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Common mix-ups

  • Pitch versus slope. In the field the words are often used loosely. In a drawing, pitch is usually the rise-in-12 notation, while slope may be a percentage or a ratio. Read the notation before you compare.
  • Pitch versus angle. An angle in degrees and a pitch are the same steepness in different units. A 12:12 pitch is 45 degrees, not 12 degrees.
  • Run versus span. Span is the full width a roof covers. For a symmetrical gable the run to the ridge is half the span. Using the span as the run doubles the rise.
  • Low pitch needs different roofing. Roof coverings have minimum slopes. Look up the covering you have rather than assuming a steep or shallow roof is fine.

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Pitch on the B1

Pitch appears in the roof and ceiling area, 14% of the B1, and in the roof covering rules in Chapter 9, where the type of covering depends on slope. The question is usually a quick calculation or a comparison between two roofs. Practice reading the notation fast, because the arithmetic is simple and the setup is where points are lost.

Practice it

Run three roofs through the rafter length calculator and check each by hand. The pitch, rafter, egress opening and stair formulas fit on one printable page (PDF), free, no email. Then read ridge board versus ridge beam and rafter ties versus ceiling joists, which both depend on run and span. The IRC terms glossary links the rest of the framing terms.

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