Calculating Roof Trusses

A quick and simple way to calculate roof rafter length.

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How To Calculate Roof Trusses

At Leach Roof Trusses, we’ve been supporting all manner of builders on a range of projects on the Sunshine Coast since 1978. Whether you’re a DIYer putting up a much-needed granny flat, an owner-builder creating your dream dwelling or are project managing a new residential or commercial development, our custom-built, expertly engineered roof trusses are an essential part of your renovation project.


Pythagoras’ theorem is used when building an open gable roof (which just happened to be one of the simplest types of roof to build). The roof trusses work to distribute the roof load and mean that all additional support comes from the roof. Each truss is essentially a pair of right-angled triangles placed back to back, and so understanding and applying Pythagoras’ theorem makes calculating roof trusses a straightforward affair.


Keen to learn more? We assist clients throughout the Sunshine Coast and surrounds, including Gympie, Noosa, Maroochydore, Caloundra, Caboolture, Morayfield, North Lakes and Brisbane.

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Overview of Roofing Terminology

Knowledge is power, so let’s start with a few key terms you’ll come across when calculating roof trusses, and how they relate to Pythagoras’ theorem.


  • Span: the distance between the outside walls that will support the roof
  • Run: half the distance of the span and the basis for the right angle triangle in Pythagoras’ theorem
  • Rise: the height of the roof
  • Rafter: a beam of wood angled up from the ground that forms the hypotenuse in Pythagoras’ theorem
  • Overhang: most roofs have an overhang of 12 to 18 inches which must be factored into calculation
  • Pitch: the amount of slope a roof has, often expressed as a ratio. Ideal pitch varies according to roofing material


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Calculating Rafter Length

You can calculate rafter length by using rise or pitch.


Using Rise to Calculate Rafter Length

  1. Measure the roof span
  2. Use Pythagoras’ theorem—a2 + b2 = c2, taking a as the span, b as the rise and c as the rafter length
  3. Example: A roof span of 20 feet, with a rise of 7 feet needs rafters that equal the square root of 400 + 49, a value of 21.2 feet.


Using Pitch to Calculate Rafter Length

If you’re not sure of the rise, you can use the pitch measurement in a similar way. Imagine you need the pitch of the roof to be 4/12, which equates to a right-angled triangle with a base of 12 inches and a rise of 4 inches.


  1. The hypotenuse can be determined by calculating the square root of a2 + b2 = c2, so 144 + 16 = 160.
  2. The square root is 12.65 inches, which we can convert to feet giving us a hypotenuse, and rafter length of 1.06 feet.


Knowing the rafter length means you can work out the rise (b) is equal to the square root of c2 -a2, where c is the rafter length and a is the span.

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Trigonometry Calculator – Angle Calculator

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Calculating Roof Trusses Trigonometry Tool

At Leach Roof Trusses, we like to make things as simple as possible and our trigonometry tool is a real boon for a quick calculation. Understanding how it all works makes using trig a whole lot easier and our right angle trig calculator just needs a couple of numbers plugging in while it does the rest.


So, if the math above is just a bit much for you, check out our easy to use tool here.

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Frequently Asked Questions

  • How do I calculate the length of a truss?

    Measuring the span of your roof is the first step in determining the length of truss you will need. The span is the distance between the outermost supports, such as the ends of a beam or wall. To calculate the length of a truss, simply measure the span and add an extra foot for each support. For example, if your span is 10 feet and you have two supports, your truss should be 12 feet long.


    Keep in mind that the length of a truss also depends on the type of roof you have. For example, gable roofs generally require longer trusses than hip roofs. If you’re unsure about the length of truss you need, consult a professional contractor or engineer.

  • What is the pitch of a truss?

    The pitch of a truss is the slope of the roof, and it’s typically expressed as a ratio (such as 4:12). To calculate the pitch of a truss, you need to measure the rise and run of the truss. The rise is the height from the peak of the truss to the bottom chord, while the run is the horizontal distance between the two endpoints of the bottom chord.


    To get the ratio, simply divide the rise by the run. For example, if the rise is 12 inches and the run is 4 inches, then the pitch would be 3:1 (or 3/4). While most roofs have a pitch between 2:12 and 12:12, it’s not uncommon for pitches to be outside of this range.


    For instance, some shed roofs have a pitch as low as 1:12, while others (such as gabled roofs) can have pitches as high as 20:12. In general, though, most homes will have a pitch between 4:12 and 8:12.

  • What is the best way to install roof trusses?

    Roof trusses are an essential part of any roofing project. They provide support for the roof, and can also be used to create unique architectural designs. When installing roof trusses, it is important to work with a qualified contractor who has experience with this type of project.


    They will be able to advise you on the best way to install your particular roof trusses based on the specifications of your project. In addition, they can also help you to select the right type of trusses for your needs and ensure that they are installed correctly. With their help, you can be sure that your roof trusses will provide years of support for your roof.

  • What are some common mistakes made when calculating or installing roof trusses?

    When it comes to roof trusses, there are a few common mistakes that are often made. One of the most common is failing to take into account the weight of the materials that will be used for the roofing project. This can lead to the trusses being too weak to support the weight of the roof, which can cause them to collapse.


    Another common mistake is not properly securing the trusses to the rafters. This can allow wind and weather to damage the trusses, and can also lead to them collapsing. Finally, many people fail to account for the expansion and contraction of the trusses due to temperature changes. This can cause the trusses to warp and become misshapen, which can also lead to them collapsing.


    When it comes to roof trusses, there is no room for error. A mistake in measuring or installation can result in a collapse, which can be both dangerous and expensive to fix. For that reason, it is always best to let an expert contractor handle the job. An experienced contractor will have the necessary skills and equipment to ensure that the job is done correctly. In addition, they will be familiar with the building code requirements for your area, so you can be confident that your roof trusses are up to code.

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