Two roofers measuring the pitch of a roof under construction

What is Roof Pitch and How to Calculate It

Roof pitch describes the slant of your roof, and it is an essential element when it comes to building integrity and safety. In other words, roof pitch is how steep a roof is, or its slope.


The pitch of a roof is usually measured in inches and written as a ratio. The vertical portion is called the rise, while the horizontal span is the run. The pitch of a roof is determined by how many inches the slope rises for every 12 horizontal inches. So, if a roof pitch reads as 8:12, for every 12 inches across, the roof rises 8 inches. These measurements are mathematically reduced, so a pitch of 8:12 would commonly read 2:3.


The purpose of roof pitch is to allow for rain and snow run-off, so if you live in an area where there is more snow or rainfall, it makes sense to have a roof with a steeper pitch. If water or snow pools and accumulates over time, the roof can become damaged – causing it to leak or even collapse. It is necessary to calculate roof pitch to estimate the materials that are necessary to complete the roof, as well as to understand the type of materials that would be best suited for it.


There are no strict categories for roof pitch, but there are generally recognized groupings. Flat roofs are not truly flat and have a slight angle to allow water to drain – so they typically have a pitch between 1/2:12 to 1:12. Low-slope roofs have pitches between 1:12 to 4:12, while medium-slope roofs range from 4:12 to 9:12. Most roofs fall into the medium range, with roof patches of 6:12 – or 1:2. Any roof with a pitch above 9:12 is considered to have a steep slope.

How to Calculate Roof Pitch

So, how do you calculate the pitch of your roof? You can think of the calculation as the same formula that describes a right triangle, and you can use the Pythagorean theorem to calculate the length of the slope and the rafter. Here is the formula:

Rise ÷ Run = Pitch


To get the inputs for this, you must first measure the rise of your roof. Next, measure the run, which is the horizontal length between the wall of the home and the roof ridge. Divide the rise measurement by the run, and you have calculated the pitch! Once you have these measurements, you can also enter them into a roof pitch calculator online to get the exact value and determine other measurements like the angle.


Measuring is much easier said than done, though. If you do not have access to measure your roof from inside the building, you will have to use a ladder to measure it from on top of the roof. The best way to calculate roof pitch is to have a professional like those at External link opens in new tab or windowPreferred Roofing Supply come out and measure it for you! Our experts will be able to tell you exactly what your roof pitch is so you can accurately purchase materials and supplies.