How to Calculate Rafter Lengths
New house construction and room additions call for installation of new roof rafters. Unlike rafters for flat roofs, rafters for pitched gable and hip roofs rise at an angle from the outside walls and meet at a ridge.
As a doityourself builder, you can expand your knowledge of basic roof construction by calculating the length of the rafters for a particular roof section or room. While it's possible to do the calculations by applying mathematics of triangle geometry, professionals generally use a method that saves time and ensures the rafters are cut to the correct lengths.
Things You Will Need
 Measuring tape
 Marking paint (optional)
 Rafter cutting guide or online rafter length calculator
Tip
Alternatively, use an online rafterlength calculator to determine the length of a rafter based on your previous determinations.

Determine the direction the rafters rise from opposite outside walls to the ridge. Typically, this is shown on the roof construction plan in the approved building plans. Alternatively, measure the room or section of the building each way to determine which outside walls are longer than the walls at each end. For example, if a room or section of a building is 20 feet long by 14 feet wide, the lower ends of opposing pairs of rafters attach perpendicular at the 20footlong walls, and the upper ends rise to a ridge above the center, or 7 feet from those outside walls.

Determine the pitch of the roof, which is the angle that rafters rise from the outside walls to the ridge. Roof pitch is designated by a set of numbers, such as 4/12 or 6/12. The first number represent the amount  in feet  that a rafter rises. The second number is the benchmark for typical pitched roof calculations. For example, a rafter installed at a 4/12 pitch rises 4 feet for every 12 feet from an outside wall.

Refer to the measurements of the shorter opposing walls of the room or building. In this example, the shorter walls are 14 feet long. Divide this in half and the result is 7 feet. Using the appropriate roofpitch table in a rafter cutting guide  such as 4/12  determine the length of each rafter based on the correlating measurement shown for 7 feet.

Add the measurement for the overhang  the eaves  on the outside wall to the result obtained in the rafter guide to determine the overall rafter length.
The Drip Cap
 New house construction and room additions call for installation of new roof rafters.
 Determine the pitch of the roof, which is the angle that rafters rise from the outside walls to the ridge.
 In this example, the shorter walls are 14 feet long.
 Using the appropriate roofpitch table in a rafter cutting guide  such as 4/12  determine the length of each rafter based on the correlating measurement shown for 7 feet.
References
Resources
Writer Bio
William Machin began work in construction at the age of 15, while still in high school. In 35 years, he's gained expertise in all phases of residential construction, retrofit and remodeling. His hobbies include horses, motorcycles, road racing and sport fishing. He studied architecture at Taft Junior College.
Photo Credits
 volgariver/iStock/Getty Images
 volgariver/iStock/Getty Images
More Articles