Pitch Gauge and Roofing Calculator

Roof slope affects which materials can be used, how they must be installed, and how much material is needed. A pitch gauge (also called a roof pitch finder or slope gauge) measures the slope of a roof surface or rafter, and a roofing calculator, whether a chart, a construction calculator or software, converts slope and plan dimensions into areas and material quantities.

How slope is expressed

• Rise per run: in North America, slope is usually stated as inches of rise per 12 inches of horizontal run, written as 4:12 or 4-in-12. Metric practice may give millimetres of rise per metre of run, or a ratio such as 1:3.
• Degrees from horizontal are common in Europe, Australia and many other regions.
• Percent (rise divided by run, times 100) is used in some contexts, especially for low slopes.

Strictly, some carpentry sources distinguish "pitch" (rise over total span) from "slope" (rise over run), but in everyday roofing the terms are often used interchangeably. As a worked example, a 4:12 slope is about 18.4 degrees and a 12:12 slope is 45 degrees. Building codes set minimum slopes for many coverings, so an accurate reading matters.

Measuring tools

• Mechanical pitch gauges with a bubble level and pivoting arm, placed on the roof or a rafter, read slope directly.
• Digital inclinometers and angle finders give readings in degrees or as a ratio.
• Framing (steel) squares and levels: holding a level horizontally and measuring the drop over 12 in (or a set metric run) gives rise per run.
• Smartphone apps, drones and aerial measurement services estimate slope and area from images.

Calculating quantities

Plan (horizontal) area is multiplied by a slope factor to find actual roof area; steeper roofs have more surface area than their footprint. In North America, materials are commonly sold by the square, equal to 100 sq ft (about 9.3 m²) of roof coverage. Allowances are added for waste at hips, valleys and cuts, plus ridge and starter pieces. Manufacturer coverage figures for each product, including laps and exposure, govern the final quantities.