Rafter Length Calculator

Rafter Length Calculator

Without a roof, a building is simply incomplete. To make a building remotely complete and enhance it with aesthetics and beauty, you need to install proper roof trusses for that. What’s going through your head right now? Is it on how to decide what certain type of roof trusses is perfect for your house? Is our guess correct? If yes, well then that is where the rafter length calculator comes in. 

The rafter length calculator is superb at making out the dimensions of roof trusses. In no time at all, it can also give you the exact number of trusses you will be needing given the calculations it makes. It can do many more calculations that can help you in the process of constructing roofs. Today, we will do a detailed discussion on what the rafter length calculator exactly is and how we can use it efficiently and get our work done.

What are Roof Trusses?

Many of you who are new to construction and making your new home, might not be familiar with the term roof trusses. 

What are roof trusses

Roof trusses are structural components used in constructing roofs for buildings. They are made up of a series of interconnected triangles that provide support and stability to the roof. Trusses are typically made using steel or wood and they are capable of distributing the load evenly to the building of retaining walls.

Rafter Length Calculator

A rafter length calculator is a tool used in the construction industry to determine the length of a rafter based on the pitch and span of a roof.  It is designed to take into account the shape and size of a roof to provide accurate results. The calculator is essential in ensuring that the roof’s structure is safe and secure.

Read More: Span Calculator

Features of a Rafter-Length Calculator

Some of the features and capabilities of a rafter-length calculator include:

Customizable Inputs

Users can input specific values such as the pitch and span of a roof to get an accurate measurement of the rafter length.

Compatibility with Other Tools

Some rafter length calculators are capable of integrating with other software tools such as CAD software to enhance accuracy and speed up the design process.

Multiple Unit Conversions

Some calculators can convert between different units of measurement such as feet, inches, and centimeters.

How to Use a Rafter Length Calculator?

The majority of the rafter length calculators that you will find on the internet have two main uses. You can use it as

  1. Rafter length calculator
  2. Roof truss count calculator

While the rafter calculator will provide you with the estimated value of the dimensions of roof trusses, the roof truss count calculator will estimate the expenses for you. So when are going to do calculations, you need to select one of the above options. You can either select truss count or rater length at a time.

Selecting Rafter Length

If you have selected the option of rater length,  then your calculations will be done on a certain basis. You can perform calculations based on two things:

  1. Roof pitch
  2. Rise of a roof

Based on the Roof Pitch

If you have decided to perform calculations on the basis of the roof pitch, you will need the roof’s pitch and run value to calculate the rafter length. The tool on which you will enter these values will give you the desired output.

Based on the Rise of a Roof

On the other hand, if you decide to use the rise of a roof in your calculations, then enter its value as input along with the value of the roof’s run. Consequently, the calculator will provide you with the rafter length once it is done performing calculations on it.

Selecting Truss Count

For this one, you will have to enter two values as per requirement. One is the length of the roof and the other is the on-centre spacing(distance in between the middle point of two neighbouring trusses). In return, the calculator will give you the exact number of roof trusses needed for your construction project.

How Can We Calculate Rafter Length?

To calculate rafter length, we have already discussed above that we will need the value of the roof pitch and rise of the roof. If you already have these values calculated for you then it is well and good. However, you can calculate them by using certain formulas.

How can we calculate rafter length

To Calculate the Roof Pitch

To calculate pitch we have the formula:

pitch = rise/run

It is represented as a percentage, which suggests:

rise = run × pitch.

Furthermore, we n=can combine the above two formulas and as a result, we will get a single rafter length formula that is dependent on the roof pitch:

rafter length² = (run × pitch)² + run²

rafter length = √((run × pitch)² + run²)

rafter length = run × √(pitch² + 1)

To Calculate the Rise of the Roof

Here is the formula to calculate roof rise:

rise² + run² = rafter length² 

Stating that:

rafter length = √(rise² + run²)

Calculation of Roof Trusses Count and Roof Truss Cost

Besides calculating rafter length, you can calculate roof truss count and roof truss cost using this single calculator. 

Calculation of roof trusses count and roof truss cost

To Calculate the Truss Count

Truss count can be calculated using the formula:

  • truss count = ((roof length × 12)/24) + 1 

After getting the resultant value, round it to the closest integer to get a whole integer value

To Calculate the Costs

We can use the following two formulas for the cost calculation of our construction project:

If we include the installation costs

  • total costs = truss count * single truss price + cost per time unit of work × duration of work

If we exclude the installation costs:

  • total costs = truss count * single truss price


In conclusion, a rafter length calculator is an essential tool for anyone building a roof. It takes the guesswork out of calculating the length of the rafters needed for a particular roof design, ensuring that the roof is structurally sound and able to support the weight of the roofing materials and any additional loads. Without a secure and safe roof, your house and the people living inside are at continuous risk. This risk can be eliminated by following all the mentioned steps accurately and precisely.

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Can we use a rafter length calculator for any type of roof?

Rafter-length calculators are designed to work with common roof designs, such as gable roofs and hip roofs. However, if you have a more complex roof design, then you may need to consult with a professional. Apart from that, you could also use more advanced software to determine the correct rafter lengths.

What other factors should I consider when calculating rafter lengths?

In addition to the pitch, width, and spacing of your roof, you should also consider the type of roofing material you’ll be using and any additional loads that will be placed on the roof, such as snow or wind. All of these factors will directly have an impact on the rafter length.

Can I use a rafter length calculator for a roof with a non-standard pitch

Some rafter length calculators may be able to accommodate non-standard pitches. But to have assurance, you may need to consult with a professional or you can even use a  more advanced software to ensure accuracy.