How to calculate a roof's pitch

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How to calculate a roof's pitch
You will need to know the roof's pitch to calculate renovation costs. (David De Lossy/Photodisc/Getty Images)

You'll need to know your roof's pitch, or degree of slope, in order to estimate repair costs. Fortunately, this is a relatively easy task that doesn't require a slide rule or knowledge of advanced calculus. The roofing trade expresses pitch as a simple ratio of how high your roof rises within a 30 cm (12 inch) run, or length. Here are some easy methods to calculate your roof's pitch.

Skill level:
Easy

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Things you need

  • 45 to 60 cm (18 to 24 inch) carpenter's level
  • Grease pencil
  • Ladder
  • Tape measure or ruler

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Instructions

  1. 1

    Measure 30 cm (12 inches) in from one end of the level. Mark the bottom of the level with a grease pencil at the 30 cm (12 inch) point.

  2. 2

    Place the ladder against the side of your house to calculate from the roof surface. Climb up with the marked level and the tape measure or ruler. Choose a spot at least 30 cm (1 foot) up from the edge of the roof, and place the end of the level against the surface of the roof, holding it so out so it is perfectly horizontal. Measure down from the marked point on the bottom of the level to the surface of the roof, and note the measurement.

  3. 3

    Calculate from a barge rafter at the gable end of your house if your roof surface is uneven or there are lots of layers of shingles. Place the ladder at the gable end, and hold the level against the bottom of a barge rafter (one that supports the roof overhang) with the 30 cm (12 inch) mark pointing up. Make sure the level is truly horizontal, and use the tape measure to measure the distance in from the mark on the level to the bottom of the rafter.

  4. 4

    Work from an inside attic rafter if you want to limit your climbing. Place the end of the level, with the 30 cm (12 inch) mark facing up against the bottom of a roof-supporting rafter. Make sure the level is truly horizontal, and measure up from the 30 cm (12 inch) mark to the underside of the rafter. Note the measurement in inches.

  5. 5

    Take the measurement in inches that you got using Step 2, 3 or 4 and insert it into the pitch ratio "x/12." For example, if the distance you measured was 15 cm (6 inches), your roof's pitch is a moderate 6/12 -- the roof rises 15 cm (6 inches) for every foot of run. If your measurement was 30 cm (12 inches), your roof's pitch is a very steep 12/12, equivalent to a 45-degree angle.

Tips and warnings

  • Save yourself the trouble of juggling level and tape measure by using an inexpensive pitch-and-angle calculator like the Empire Level 36 Magnetic Polycast Protractor. Just place the flat side against the underside of a gable or attic rafter, and the dial will give you your roof's pitch expressed in degrees, with a level of accuracy to within 1 degree. You can use a gadget like this to quickly calculate all kinds of angles, and it can serve as a level, too.
  • If you know four simple measurements-the gable span (width across the gable end), the gable height (from the bottom of the roof to its peak on a gable end), the rafter length (length of a rafter from peak to bottom on a gable end) and the gutter length (length of your roof from one gable end to another)-you can perform all kinds of calculations, including pitch and area, using the free online calculator at Roofing Child's Play (see Resources).

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