How To Design Rafter Roofs

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Roof design
Roof design (Image: Copyright © 2009 English Explorers LLC)

A rafter roof system takes into account the size and shape of the building, roof pitch or slope and materials chosen. The design can be hand drawn or computer aided. Rafter roofs are made from many different materials and have various shapes and styles. Generally the larger and stronger the rafters, the farther apart they can be spaced. The geographic location of the building will also affect the design.

Things You'll Need

  • Pencil and paper
  • Drafter's ruler or scale
  • Drafting software and printer

Draw the layout of the building by showing the outside walls. Place dimensions on your drawings. Illustration shows one rectangle-shaped building and another L-shaped building.

Building layouts showing outside walls.
Building layouts showing outside walls.

Determine if you want a gable or a hip roof system. A gable roof will allow the end wall to extend all of the way up to the ridge of the roof. An extra vertical triangular section of wall will be used to fill the area. A hip roof has sloping ends and sides all around the building. The gable roof will allow more room in the attic area. Depending on the pitch or slope of the roof an extra room or storage area can be placed in the expanded attic area. See illustrations for comparison.

Side and front views of gable and hip roofs.
Side and front views of gable and hip roofs.

Locate the valleys and ridge boards for each section of the roof. The ridges are usually located at the midpoint of two parallel outside walls. The valley rafters will have 45 degree angled cuts on each valley end of the rafters. Locate any dormers that will exist. Dormers will require ridges and valleys where they intersect the main roof. An asymmetrical roof will require a much more complicated rafter system.

Building layouts showing ridges and valleys.
Building layouts showing ridges and valleys.

Determine the materials that will be used for the rafters and roof decking or covering. Wood of many shapes, sizes and types, steel, or even aluminum could be used. The type material used will determine the spacing of each rafter and the maximum lengths that can be used.

Show side views of each separately pitched roof. Locate the birdsmouth cuts for each rafter based on the amount of overhang and soffit area desired and the distance from the ridge board. The birdsmouth cuts are horizontal and vertical cuts made at the location where the rafters rest on the outside wall. See illustration for details. An overhang that is too long will interfere with the windows and doors located in the walls.

Birdsmouth cut location.
Birdsmouth cut location.

Determine the pitch of the roof by consulting local building codes. Areas that have more ice and snow will require steeper pitched roofs. Smaller buildings with shorter rafters can handle less pitch. The appearance of the finished building will need to be considered. The dimensions of overhang or soffit area are also a factor. The same pitch may not be used for the entire roof. A split-pitch roof can have one pitch for the front side of a common roof and a different pitch for the back side.

Tips & Warnings

  • Familiarity with carpentry and use of a framing square will greatly aid this project.
  • Consult local building codes for acceptable building materials and practices used in your region of the country.

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