For many people, finding out the roof’s rafter length can be a bit tricky. Whether you are installing a new roof window or fitting new roof tiles, an accurate and precise rafter length calculation could really make or break your project.

Sadly, if you don’t have the right tools, working at heights on decks and coverings getting in the way could be very difficult as it needs some training. Keeping all the aspects in mind, here in this article we will share some new ways to calculate a rafter’s length. This article includes some tips and tricks and formulas that you will learn eventually.

Before we jump on the mathematical calculations, you need to have some clarity about some technical terms related to a pitched roof. Make sure to have a calculator in hand.

Rise of Roof:   

It is the perpendicular or vertical distance between the wall and the top of the ridges. In a simple way, it is the distance from the top of a stud to the apex of the roof.

Pitch of Roof:

It is the slope of the roof, and it is typically measured in degrees. There are no defined standards of Roof pitch. It depends on many factors like availability of material, culture, climate, and style of roof pitch. Normally, they are kept between 40⁰ and 50⁰, in the FPS system of units it ranges from 4/12 to 9/12.

Pitch of Roof (P) = Rise / Run

Run of Roof:

It is the horizontal distance of one side of the roof. It is the distance from the outer wall’s top to a point directly below the center of the ridge. Basically, Run is the half distance of the span.

Run of Roof = Span / 2

Rafter of Roof: 

Rafter is the section that extends past the wall plate and tends to be tucked by roofline components such as fascia boards. It is the distance between the ridge of the roof to the external wall plate. The rafter of the roof is also known as the overhang.

Span of Roof:

It is the clear distance available between the supports of an arch, roof trusses, or beams.

Methods / Formula to calculate Rafter Length:

Rafter length is the horizontal distance from the supporting wall to the center of the roof. It can be calculated by using various formulas namely rise, run and pitch. Some of them are as follows:

• Calculating Rafter length using Rise and Run
• Calculating Rafter length using Pitch and Run

Calculating Rafter length using Rise and Run:

For this method, you are going to recall your math geometry knowledge to find out the rafter length. If you have a look at your homes, you will find out that the rise, run, and rafter length of them actually resembles a triangle. And as far as right-angle triangles are concerned, if you know the length of two sides, then by applying Pythagoras’ Theorem we can find out the length of the remaining side which in our case will be rafter length.

	a2 + b2 = c2

2.52 + 1.52 = 8.5

For our better understanding, let us assume we have a run of 2.5m (a), and a rise of 1.5m (b), then by using a calculator we get:

√ 8.5 = 2.91

Then simply take a square root of your answer:

There you go, now you have your final rafter length of 2.91m. When needed, you can also add the length of the rafter feet to this value to get final measurements.

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Rafter length can also be calculated by using an angle. Suppose you have a roof length of 20 feet, a rise of 7 feet, and an angle of 20⁰. Now by applying some basic trigonometry, we can easily find out the rafter length. Consider Rafter as Hypotenuse, the opposite side as Run, and the adjacent side as Rise. Therefore

	Cos θ = Adjacent / HypotenuseHypotenuse = Adjacent / Cos θ  Rafter Length = 7 / Cos 20⁰Rafter Length = 17.5 ft

Calculating Rafter length using Pitch and Run:

Any chance, you don’t know the rise of your roof, then the rafter length can also be calculated by using your roof pitch. What if you don’t know your roof’s pitch either? Well, the easiest thing to do would be to refer to your building drawings. There you will find the roof’s pitch for sure. If you can’t find the drawing of your house, then there is no need to worry.

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There is number a of android and iOS application that you can use to calculate roof pitch. These online applications use the internal gyroscope of the phone to provide measurement. At the same time, you have to be very careful while measuring to get an accurate reading.

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There are plenty of low-cost tools and equipment to determine the pitch of a roof. Most of them are easy to use and quick, but you have to work at height, therefore always make sure to take suitable safety measures and have someone accompany you while taking readings.

Once you have succeeded to measure the pitch, make sure to round it off to the nearest five and convert it into a slope factor. A slope factor is a number relative to the degree of your pitch which will be used to calculate rafter length. Once you have figured out your slope factor, then simply multiply it by the Run of the roof to find out the size of the rafters.

Consider the previous example, if our run was 2.5m and the roof pitch was 40⁰ or 1.3054, then all you need to do is to put these two values into the calculator and multiply.

	2.5 × 1.3054 = 3.2m

 Again, if needed, add the length of the rafter feet to this value to get final measurements. Now your rafter length has been sorted out, you can start to plan out the rest of the activities.

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