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Roof Truss Calculator

Table of contents

How to use the roof truss calculatorHow to calculate rafter length?How to calculate truss count and roof truss cost?What is on-center spacing?What kind of roof trusses are available?FAQs

There is no house without a roof, and to build a roof, you need proper roof trusses - this is exactly what this rafter length calculator will help you find! 🏠

With this tool, you will find the dimensions for your roof trusses, as well as calculate the adequate number of trusses for your house. On top of that, our roof truss calculator is not only focused on rafter length and truss count. You can also use it to estimate the roof truss cost and even include the approximate costs of installation. Read on to find out how to calculate rafter length, roof truss count, and roof truss cost!

In need of a broader view? Try the roofing calculator!

How to use the roof truss calculator

Our tool has two applications. You can use it either as a rafter length calculator, and get the dimensions of your trusses estimated or as a roof truss count calculator, which will also allow you to estimate the roof truss costs and even include the price of installation.

To start your calculations, you need to choose from the two options mentioned above. In the field above the illustration, pick either "rafter length" or "truss count".

  1. If you picked rafter length:

Start by deciding what you want your calculations to be based on. You can choose the rise of the roof (which is its height) or the roof pitch (which is the slope created by the rafter). If you want to learn more about the way it's calculated, check out our roof pitch calculator.

  • If you picked the rise, all you need to do is input its value, put in the roof's run, and the calculator will return the rafter length in the bottom-most field.

    Please note that the rafter length calculator works both ways. That is, you can also use it to calculate the rise or the run! If you already know the rafter length and want to figure out any of the other dimensions, simply input the two you know, and you will get the third one.

  • If you decided to include roof pitch in your calculations, to get the rafter length, you will need to input the run and the pitch of the roof. After doing that, you will see the rafter length in the bottom field.

    Here, as well, our calculator works both ways. You can input any of the two values to get the third one calculated.

    You can input the roof pitch in any of the three formats (angle, percentage, ratio), and our tool will automatically tell you the other ones.

  1. If you picked truss count:
  • Input the roof length, the on-center spacing (the distance between the middles of two neighboring roof trusses), and the roof truss calculator will return the number of roof trusses needed.

  • Our tool can also figure out the approximate roof truss cost. In the next fields of the calculator, decide whether you want to include the costs of installation in the calculations.

    • If you leave the checkbox unchecked, the only thing left to input is the cost of a single roof truss. The roof truss calculator will return the total expenses in the bottom-most field.

    • If you check the checkbox, you will also need to declare how much per time unit would the contractor installing the trusses charge you, as well as the expected duration of the installation. As soon as you fill in all the variables, you will see the total expenses at the bottom of the calculator.

Don't worry if you're unsure of what exactly such terms mean, such as the roof's run, rise, on-center spacing, etc. We illustrated the variables you need to fill out in the calculator.

How to calculate rafter length?

This rafter length calculator lets you calculate roof truss dimensions using two slightly different sets of variables. It all comes down to what information you already have. Usually, that is either the pitch or the rise of the roof. What is more, the formula used to calculate roof truss dimensions is one you probably know very well - it's the exact same formula you use to describe a right triangle!

The formulas we used are the following:

  1. For calculations based on the roof rise:

rise² + run² = rafter length² ,

meaning that:

rafter length = √(rise² + run²).

  1. For calculations based on the roof pitch:

The formula used to calculate rafter length from roof pitch is, in fact, the same as the one above. The one difference is that we need to calculate the roof rise using the pitch first. So what you need to do first is to get the pitch using the following formula:

rise / run = pitch,

expressed as percents, meaning that:

rise = run × pitch.

We can merge these two equations to get a single rafter length formula based on roof pitch:

rafter length² = (run × pitch)² + run²

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

rafter length = run × √(pitch² + 1)

If you're unsure about the terms such as roof run and rise, refer to the illustration below:

Roof truss calculator - rafter dimensions.
(Image created on the basis of: King post truss 3D created by George Ponderevo, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.)

How to calculate truss count and roof truss cost?

As you know, figuring out rafter length and other truss dimensions is not the only thing you can use this tool for! Our roof truss calculator can also help out with planning out your roof project by estimating the truss count and showing you the costs you can expect for the materials and their installation. To do so, it uses the following equations:

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

rounded up to the closest integer (for example, if the result is 14.5, you need to get 15 trusses). Here, we assume that the on-center spacing equals 24" and the roof length is given in ft.

To calculate the costs, we use the following two formulas:

Including installation costs:

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

Excluding installation costs:

  • total costs = truss count × single truss price

If you're unsure about the terms, such as the on-center spacing, refer to the illustrations below:

Diagram of the roof with marked length.
Trusses with marked dimensions and on-center spacing between them.
(Image created on the basis of: King post truss 3D created by George Ponderevo, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.)

What is on-center spacing?

One of the values needed by the roof truss calculator is the on-center spacing. This is not a very well-known term, but it can prove useful in more contexts than just when discussing rafter length, roof truss cost, and the like. Why don't we spend a minute learning about it?

On-center spacing (often abbreviated into OC or o.c. in construction) is a measure commonly used in architectural designs and construction drawings. It stands for the spacing between the exact centers of two framing members (in this calculator, roof trusses).

For example, if a plan states that there should be "20 inches OC between fence posts" and those posts are 6 inches wide, this means the space between posts will be about 14 inches (20 - (3 + 3)).

One of the most prominent uses of OC spacing is the stud framing. Framed walls tend to be covered by sheet goods, for example, plywood or drywall. A single sheet is typically 48 inches wide (size 4' × 8'). Framing a stud wall at 16 or 24 inches OC will result in the edge of a vertical sheet falling over the center of a stud, as 48 is divisible by both 16 and 24. Thanks to this, the edge of the sheet is securely supported and backed by about ¾ inch of wood, which we can use for nailing. Thanks to this, the installation of sheet goods with OC spacing is reliable and relatively simple.

OC spacing is also useful when you use materials that have varying thicknesses, such as milled lumber, which does not come with precise dimensions. Thanks to using this measure, you can ensure the accuracy of layouts that use such materials.

What kind of roof trusses are available?

If you're using this rafter length calculator, you're probably in the process of building a house, so you might be interested in what kind of trusses there are on the market! When it comes to different truss types you can use for roof construction; there's plenty to choose from, depending on your needs. Here are the four most common ones:

Roof truss calculator: roof truss types.
(Image credit:
  1. Fink roof truss - generally the cheapest option. Its main advantage is low weight and the fact that you can build it relatively fast (even as quickly as in one day).
  2. Traditional cut roof - especially useful for complex roof shapes.
  3. Attic truss - this design comes with space that you can use for storage. Unfortunately, they tend to be on the expensive side.
  4. Panelized roofing - best for simple-shaped roofs. It is made out of large pre-insulated sheets that are laid across the beams. The materials can be more expensive than regular trusses, but the simplicity of the design reduces installation costs.

What is a roof truss?

A roof truss is a structural timber frame that creates a rigid structure and supports your roof. They are also used to connect the space above the room. They usually occur at regular intervals and are bound by horizontal beams.

What is the formula of roof truss?

The formula is truss count = ((roof length × 12)/24) + 1. This equation will help you determine how many trusses your roof needs. For example, if your roof is 30 feet long, you will need 16 trusses.

How can I build a simple roof truss?

The easiest way to build a simple roof truss is as follows:

  1. Measure your roof to determine how big a truss you need.
  2. Select the type of roof truss.
  3. Design your roof truss and make a detailed sketch with the dimensions and angles marked.
  4. Cut your wood truss pieces to size using a jigsaw.
  5. Fit together pieces in the desired shape.
  6. Combine truss components using adhesive and plates.

Do trusses have to be evenly spaced?

Yes. Typically, roof trusses should be spaced 24" apart in the center. Trusses can be 12" or 16" apart. However, trusses are designed to the individual specification of a particular house. Consider local conditions. For example, if you live in a hurricane-prone area, you can move the trusses to 16" in the center.

How many trusses do I need for a 40ft long roof?

You will need 21 trusses. Use the formula truss count = ((roof length × 12)/24) + 1 and enter the length of your roof:
truss count = ((40 × 12)/24) + 1 = (480/24) + 1 = 20 + 1 = 21.

Roof truss calculator - truss count calculation (truss count)

© Omni Calculator

Roof truss calculator - truss count calculation (roof length)

© Omni Calculator

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