How To Calculate Your Rafter Length

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How To Calculate Your Rafter Length

For many homeowners, working out their roof’s rafter length can be a bit of a challenge. Whether you’re fitting new roof tiles, installing a new roof window, or planning a full DIY loft conversion, an accurate rafter length calculation could make or break your project. Unfortunately, whether due to lacking the right tools, working at height or your roof deck and coverings getting in the way, this is rarely as simple as it seems.

With that in mind, the team here at Roofing Megastore has taken the liberty of doing the legwork for you. Not only will you discover a few new ways to calculate your rafter length, but we’ve also included some helpful tips and tricks, along with a bit of jargon-busting to make everything clearer. Just make sure you’ve got a calculator handy!

Jargon Busting

Before we teach you how to calculate rafter length, it’s important to make sure you understand the common roofing terminology we’ll use, and you’ll need to know when taking measurements.

Rise: The total vertical height of the roof. This means from the top of a stud wall to the apex of the roof.

Run: The Horizontal length of one side of your roof. For apex roofs, this will be from the outside edge of a perimeter wall to the centre of the house. For flat roofs (roofs with a single slope), it would be the horizontal measurement of the entire run of the roof.

Pitch: The slope of your roof. This is typically measured in degrees. While there aren’t any ‘set pitches’ most traditional UK homes tend to have pitches in between 40° and 50°.

Rafter Feet: These are the sections of rafter that extend past your wall plate and tend to be covered by roofline components such as fascia boards. They are also known as an overhang.

How To Calculate Your Rafter Length

Method 1: Calculating Rafter Size Using Your Rise and Run

For this first method we have to take a look back at our GCSE maths, specifically geometry. As seen below, the rise, run and rafter length of our roofs actually make up the three sides of a right angled traingle. So, if we know the length of two sides of this traingle there, we can use Pythagoras’ Theorem to work out the length of our rafters:




Diagram showing the rise, run and pitch of a roof

If we assume a run of 2.4m (a), and a rise of 2m (b), then using a calculator we get:


2.4² + 2² = 9.76


Then simply find the square root of your answer:


√ 9.76 = 3.12


This means your final rafter length 3.12m. If needed, you can also add the length of your rafter feet.

Method 2: Work Out Rafter Length Using the Pitch and Run

If you don’t know or cannot work out the rise of your roof, the rafter length can also be figured out using your roof pitch. But what if you don’t know your roof pitch either? The easiest thing to do would be to consult your building plans. Though, as it is unlikely that most of you have these just lying around, there are a few options you can try. For more information, consult our guide to calculating roof pitch.



There are a number of apps you can use to calculate roof pitch. Our favourite option is VELUX’s Roof Pitch app, which is available for iPhone. These apps use the internal gyroscope of your phone to provide your measurement but should be used alongside a straight edge to ensure you can get an accurate reading. You may be tempted by the more sophisticated looking apps which use your phone camera to measure, however, these should be taken with a pinch of salt. As the app will be measuring what is on screen rather than the reality, where you are standing can have a significant impact and lead to big inaccuracies.


Dedicated Tools

There are a number of low-cost tools out there which can help you determine the pitch of your roof. Most of these will be quick and easy to use, but may require you to work at height, so always make sure you take appropriate safety measures and have a someone else on hand to help out.

With your pitch determined, it can then be rounded to the nearest five and translated into a ‘Slope Factor.’ This is a number relative to the degree of your pitch which you’ll use to calculate rafter length. For ease, we’ve listed these factors here:























Once you know your slope factor, simply multiply it by the run of your roof to work out the size of your rafters. Using the previous example, if your run was 2.4m and your roof pitch was 40° (or 1.3054) you’d put the following into your calculator:


2.4 x 1.3054 = 3.1m


Again, if necessary, add the length of your rafter feet to this number to get your final measurement.

Next Steps

Now that your rafter length has been calculated, you can start to plan out the rest of your project and order your roofing supplies. But don’t worry, we can help with that too. With a vast range of over 40,000 products, our library of “How To” guides and our award-winning customer service, you’ll have all the support you need to get your project underway ASAP.

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