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How Do I Work Out My Roof Pitch?

how do i work out my roof pitch

Do you know the exact pitch in degrees of your home’s main roof or extension? If you are currently planning to convert your loft or considering a ground floor extension with roof windows, it is important you know just how sloped your new or existing roof is.

This has less to do with making sure your pitched roof windows fit and more to do with ensuring you get the right flashing with your new window. It is essential to ensure flashing and window are compatible in order to guarantee a watertight installation.

Flashings are specially designed by manufacturers to match the same brand of roof window, specific roof coverings and certain roof pitches.

The warranties that come with roof windows are often only valid when installed with the right flashing, which makes getting the right pairing vital should anything go wrong.

So just how do you work out the pitch of a roof? Let’s take a look.

How Do I Work Out My Roof Pitch?

pitched roof

There is a formula to measuring the exact pitch of your roof that you do not need to be Pythagoras to use. What you will need, however, is a tape measure/spirit level and calculator.

There are two important things to measure; the run and the rise. The run is the horizontal distance between the peak of your roof and the wall, while the rise is the height of the peak above the wall.

Once you have these two measurements, you'll need to grab hold of a calculator and divide the rise by the run to work out the tangent. The next step is to punch in 1÷ whatever your tangent is to find the angle of your roof pitch.

Here is the equation in full- Roof Pitch = 1 ÷ Tangent (Rise ÷ Run)

Measuring properly will require access to your roof, either externally or from your loft space. Either way, it is important that you are safe when carrying out all measurements to avoid causing damage to yourself or the roof.

Isn’t there an App For that?

roof windows installed in combination on pitched roof

Yes there is. Although some will prefer taking the traditional approach above to work out their roof pitch, for those who would prefer not to do any climbing and leave calculations in the hands of technology, there are mobile applications just for the job. 

Sophisticated apps, such as Pitch Gauge, use the phone’s internal gyroscope and camera to measure angles. All that is required of you is to line up the edge of your roof with a line on your screen to get an approximate measurement. 

Though apps like Pitch Gauge are free with good accuracy and reviews, they do have their flaws. One of these is how the location of where you’re standing in relation to the roof can put the measurements out by a few degrees- a difference that could change what flashing you need entirely.

Roof window specialist VELUX had produced its own app to work out roof pitches. Like Pitch Gauge, the VELUX app uses your phone’s camera to find the pitch, with another option that lets you place your phone directly on the roof to work out pitch.

Are There Different Roof Windows For Different Pitches

velux roof window installed into profiled interlocking tiles

Yes, depending on whether you want a centre pivot or top hung roof window. Though both can be installed on any pitch from 15° to 55°, this is the limit with top hung windows. With centre pivot windows, such as those from RoofLITE and ECO+, any roof pitch up to 90° is perfectly acceptable for installation.

As far as flat roof windows go, most manufacturers design their rooflights to be suitable for pitches of 2° to 15°. Any angle larger and your roof would be classed as pitched and therefore more suitable to a roof window.

All FAKRO and ECO+ rooflights can be fitted onto flat roofs as low as 2°, while VELUX models require a minimum pitch of 5°. VELUX do, however, have flat roof kerbs available that allow pitched roof windows to be installed onto flat roofs.

Read the Sterlingbuild Flat Roof Home Improvements Guide for a bit of rooflight inspiration. For further guidance on finding the right flashing, visit our Flashings Buying Guide.