How to calculate for roof rafters

Updated February 21, 2017

Roof rafters are one of the most important parts of any type of roof. They are generally made of 50 to 100 mm (2 by 4 inch) lumber and are used to form the pitch of a roof. They come from each side of the roof and meet in the middle to form what looks like a V. Knowing the size that the roof will be is the key element when calculating how many roof rafters will be necessary.

Obtain a roof calculator from a local hardware store or use a roofing website to enter in the measurements for the rafters.

Determine the measurements for the roof and record them. Find out the measurement from the front of the roof to the back and write it down. Record the distance between the rafters and the thickness and width of the rafters and write down all three numbers. Code may vary on these measurements , but typically rafter spacing is 405 mm (16 inches) on centre and sometimes 610 mm (24 inches) apart. Rafters are generally 50 to 100 mm (2 by 4 inch) pieces of lumber that are cut to the proper length. The 50 to 100 mm (2 by 4 inch) lumber measures 37 mm (1 1/2 inches) thick and 87 mm (3 1/2 inches) wide. The thickness is the measurement from front to back of the rafter and the width is the measurement across.

Record the measurements for determining the rafter length and enter them into the rafter length calculator to determine the length of the rafters that will be needed. Write down the width of the roof, ridge width, overhang and pitch and enter them into the rafter length calculator. For example, for a roof width of 760 cm (25 feet) with a 255 mm (10 inch) ridge width, a 75 mm (3 inch) overhang and a 150/300 (6/12) pitch, a rafter of 4 metres (13 feet 9 1/2 inches) will be necessary.

Calculate the measurements to get the correct amount of linear feet, which will determine how many rafters will be needed. The amount of rafters necessary will vary depending mostly on the width of the roof. The wider the roof and farther the measurement across it is, the more rafters will be needed to complete the project. The pitch of the roof will also determine the amount of rafters needed. This is measured by the amount it will rise vertically over the horizontal distance. Most roofs will be 150/300 (6/12), meaning 150 mm (6 inches) of rise vertically for every 300 mm (12 inches) horizontally.

Things You'll Need

  • Roof rafter calculator
  • Pencil
  • Paper
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About the Author

Alexander Callos began writing in 2005 for "The Lantern" at The Ohio State University and has written for various websites, including Bleacher Report, Top Ten Real Estate Deals and Columbus Sports. He has published articles for CBS Sports, and other websites. He graduated in 2007 from The Ohio State University with a bachelor's degree in public affairs journalism.