Roof Pitch Calculator

Table of contents:
There might be many reasons you need to know the pitch of your roof. For example, you might need to know it to know the rafter lengths you need to buy. No matter what the reason, this calculator can help.

A roof pitch calculator can help you to ascertain your roof’s pitch, as well as how to work it out in a few steps. What’s more, if you only know the angle of your roof, you can also use this roof pitch calculator to understand it in a percentage too. Read on to find out more.

What is a Roof Pitch?

The roof pitch is the slope that the rafters create on your roof. They are either the angle formed by the timber or the proportion of the run of the roof and the rise. 

In most cases, a roof pitch is a ratio consisting of the rise and the run in the form of X:12. Such a formula means that if you had 1:12, your building would rise by one yard for every 12 yards of length. 

How you calculate the roof pitch can also depend on the type of roof you have – just to confuse you further!

High-pitched roof: usually need additional fasteners because their pitch can be as high as 21:12 (a rise of 12 yards for every 21 yards of length). 

Low-pitched roof: require special materials to reduce the risk of leakages and tend to be high-maintenance. The pitch is generally at around 4:12, or 33.3 percent. 

Flat roof: a flat roof is not entirely flat. It has a gentle slope so that water can run off of it. The pitch can go between ½:12 to 2:12. 

Standard roof: safe to walk on, straightforward to build, and has a pitch of between 4:12 and 9:12.

How Do You Calculate a Roof Pitch?

You can use the same formula for a right triangle as you can for a roof pitch. The following methods can help you to establish your rafter length and your roof slope. 

rafter² = rise² + run² 
rise / run = pitch (in percentage) 
pitch = tan (angle in degrees)

If you prefer the ratio of x:12, calculate the pitch, substitute 12 for your run length, and the value will be what you replace x with.

An Example of Using a Roof Pitch Calculator

If you are still not yet sure how you can confidently calculate the slope of your roof, then follow these steps and examples below.

   1.	Measure the horizontal distance between your building’s wall and the roof ridge – also known as the run length. In this example, we will say eight meters. 
   2.	Measure your roof’s rise. In this example, it’s 2.5 meters
   3.	Calculate your rafter length by using those values in the formula below. 
rafter² = rise² + run²
2.5² + 8² = 6.25 + 64 = 70.25
rafter = √70.35 = 8.38 meters

   4.	Use the proportion of rise and run to calculate the roof pitch. 
pitch = rise / run = 2.5 / 8 = 31 percent

   5.	Turn the percentage into an angle 
angle = arc tan (pitch) = arc tan (0.31) = 17°

   6.	Identify the roof pitch in the ratio of x:12
x = pitch x 12 = 0.31 x 12 = 4:12

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