Gable Roof Calculator: Area, Slope, Volume, Rakes & More

What Is a Gable Roof?

A gable roof is a very common type of roof. Gable roofs have two sloping sides that rise to meet at a central ridge, creating a distinctive triangular shape on each end of the structure. These vertical walls are known as gables or gable ends.

Unlike a hip roof, which has four sloping sides and no vertical ends, a gable roof has only two sloping roof planes. The sloped edges run between the ridge and the eave edges, which are the lower horizontal edges of the roof planes. The edges of the roof above the gable ends, where the roof meets the gable walls, are called rake edges, or simply rakes.

Gable roofs are popular for their simplicity, efficiency, and the fact that they cost less to build than most other roof shapes. Gable roofs require fewer framing components and simpler cuts, which typically results in lower labor and material costs. They offer excellent water drainage, allow for good attic ventilation, and typically create usable space under the roof. The triangular gable ends also make it easy to install vents or windows. Gable roofs can be more vulnerable to wind loads compared to hip roofs. Gable roofs are widely used in residential construction across a variety of climates and architectural styles.

What This Calculator Does

This gable roof calculator helps you determine the important dimensional attributes of a gable roof based on your inputs:

• Total roof surface area: combined area of the two sloped roof planes.
• Roof footprint: the horizontal area covered by the roof.
• Attic volume: an estimate of the triangular prism-shaped space enclosed beneath the roof. Read more about gable roof attic volume.
• Perimeter: total length of all roof edges, including horizontal eaves and sloped rake edges.
• Total eave length: the combined length of the two horizontal eaves (equal to 2 × roof length).
• Total rake edge length: the combined length of the four sloped rake edges along the gable ends. There are two rake edges on each end of the roof. The length of one rake edge is equal to the roof surface slant height (the sloped distance from eave to ridge).
• Ridge length: the full length of the ridge where the two roof planes meet (equal to roof length).
• Roof height: vertical distance from the level of the eave to the level of the ridge; in the calculator this is either entered directly as an input or calculated from a slope input.
• Slant height: the sloped distance from the eave to the ridge along the roof surface.
• Roof Slope (X/12 and degrees): shown in both common “X in 12” and angle (°) formats.
• Pitch multiplier: slant height divided by horizontal run, useful for estimating roof surface area, rafter length, and material quantities. Read more about roof pitch multipliers.

How to Use It

• Choose your measurement system: Select USA (feet and X-in-12 roof pitch) or Metric (meters and degrees). The calculator accepts decimal inputs in both modes.
• Select input mode:
  • Choose Height Input if you know the vertical rise of the roof from the height of the eave line to the height of the ridge.
  • Choose Slope Input if you know the roof slope, expressed as X-in-12 (USA) or in degrees (Metric).
• Enter the roof’s base width and length:
  • Width = the distance across the gable end (typically the shorter dimension).
  • Length = the distance parallel to the ridge (typically the longer side of the roof).
• Note: If you enter a width greater than the length, the calculator will show a warning since this would usually indicate a slope, ridge, etc. running perpendicular to the standard direction and may not reflect the intended layout for a standard gable roof.
• Enter either the roof height or the slope, depending on the selected input mode.
• Click Calculate to generate the results.
• Click Clear to reset all input fields, error messages, and results.

Important Notes

• In Slope Mode, the calculator determines the roof height based on the entered slope and base width. In Height Mode, the slope is calculated from the width and vertical rise.
• In USA Mode, enter only the rise portion of the slope (enter 6 for a 6-in-12 slope, etc.). You can use decimal values such as 7.5.
• All inputs must be positive decimal numbers. Do not use fractions, inches, or millimeters. Use decimals only (e.g. 10.25 feet or meters).
• The ridge length is equal to the full roof length.
• The roof pitch multiplier is calculated using the slope and is used to convert horizontal run into actual roof surface area or rafter length for estimating materials.
• This calculator is designed for regular gable roofs with two equal sloped planes and two vertical gable ends. It does not account for irregular or multi-section gable roofs.

Gable Roof Calculator

This is not a framing calculator.

Don’t forget to include any eave overhang in your roof dimension inputs; roofs typically extend farther out than the exterior walls of the building, so you can’t just use the width and length of the walls.

About the Author

Jack Gray spent 20 years as a principal roof consultant with the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. Mr. Gray has worked in the roofing industry for over 25 years, with training and practical experience in roof installation, roof inspection, roof safety, roof condition assessment, construction estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management. He was awarded the Registered Roof Observer (RRO) professional credential in 2009. He also served as an infantry paratrooper in the 82nd Airborne Division and has a B.A. from Cornell University.