How to Calculate Roof Pitch

If your roof has two or more sides that meet in a peak at the top of the structure, then you have what is known as a ‘pitched roof’. Most homes in Britain feature a couple roof. This is where two sets of rafters are fitted symmetrically and meet at the top, often called the apex or ridge. Aside from a single sloped pitched roof you might find on an extension, these are the simplest you’ll find. Beyond these, pitched roofs come in all shapes and sizes, from small and simple to complex and grandiose.

How to Work Out Your Roof Pitch

Unless you work in construction or you’ve worked on a roof before, it’s unlikely you’ll know how to calculate your roof’s pitch. This information is incredibly useful for a range of home projects, but absolutely vital if your next DIY endeavour involves your roof. This applies whether you’re completely replacing the roof covering or installing a brand new roof window. To do these jobs correctly, you will need to be able to figure out the pitch of your roof.

Roof diagram for calculating pitch

How to Calculate Roof Pitch in Degrees

There are a few ways to measure or work out the pitch of your roof. All you’ll need to hand will be a measuring tape, spirit level and calculator. With these tools in hand you just need access to your loft space and to remember some of the maths you learned at school.

  1. First, you need to measure the run of your roof. This is the horizontal length that spans between the peak of your roof and the wall. You can do this with a tape measure or spirit level.
  2. Next, you need to figure out the rise. This is the height of your apex above the structure wall.
  3. Now, divide the rise by the run. This will give you the tangent of the roof. (rise ÷ run = tangent)
  4. Then, divide 1 by your tangent.
  5. Finally, multiply this result by 180/π and you've calculated your roof pitch!

REMEMBER: Roof Pitch = (1 ÷ Tangent (Rise ÷ Run))*180/π

Example: Rise of 8m and Run of 5m.

  1. 8 ÷ 5 = 1.6
  2. 1 ÷ 1.6 = 0.625
  3. 0.625 x 180/π = 35.80986...
  4. Therefore, the roof pitch is 35.81°!

How to Calculate Roof Pitch as a Ratio

The traditional method of measuring roof pitch is to display it as a ratio of X:12. This is done by measuring the number of inches a roof rises vertically for every 12 inches it extends horizontally. This is simple to do, and you’ll need the same tools and access to your roof space as above.

  1. First, place one end of your spirit level against your roof deck. Then measure to find and mark the point 12” along your level. This is the run.
  2. Now, rest your tape measure vertically against your run line, and measure the distance between the top of your spirit level at the mark and the roof deck. This is your rise.
  3. Now you should have your X : 12 ratio based on rise : run.
  4. If you’d like to convert this ratio to an angle you can do so.

Using a Roof Pitch Calculator

A simpler alternative to getting out the pencils and paper, there are numerous options both online and as part of smartphone apps that you can use to work out your pitch. To use these calculators, you’ll need the same rise and run measurements as with manual calculation. Some are more sophisticated and can tell you your roof pitch just with a photo. Our favourite option is VELUX’s Roof Pitch app, which is available for both iPhone and Android.

These calculators do have some drawbacks, however. The photo-based ones can get things wrong if your camera angle or the ambient lighting isn’t ideal. The more primitive calculators can also require so many figures beforehand that you might as well figure your pitch out yourself. However, these apps and tools are getting better all the time, providing an excellent alternative to homeowners that may find it difficult to access their roof space.

Conclusion

You should now have a better idea of how best to calculate the pitch of your roof. There are many ways to work it out, so you just need to find the one that suits you best. Figuring out your roof pitch is useful in so many ways, from replacing tiles to the feasibility of extensions.