Rafter run outer wall


Roof rise


Rafter Width




Birdsmouth Plumb


Wall Thikness


General view



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If you are making repairs to your roof or installing a completely new roof, you will need a rafter calculator to help you figure out rafter lengths. Once you know the appropriate lengths, you will be able to determine the total cost of your roof project.

Calculating rafter length starts with a few basic inputs that you will need to gather before using this tool. You can either get these measurements directly, by going up on your roof, or indirectly, by using your house footprint as a proxy for the variables. Obviously, the results will be more accurate if you can measure directly.

Inputs for rafter length calculator

The good news is that there are some common inputs that can be used for any roof, no matter its dimensions, angle, or shape. These are the inputs that you will need (all of them measured in inches):

  • Run
  • Rise
  • Overhang
  • Wall thickness
  • Birdsmouth plumb

By entering these inputs into the rafter length calculator, you will get a final number, which is the rafter size. Using this number, you will know exactly how much timber you will need. And by getting the prices of wood from your local home improvement store, you will know the exact cost of your roof project.

Example of figuring rafter length

For first-time homeowners, some of these roof inputs might not be immediately obvious, so let’s work through a hypothetical example based on the “General View” diagram. Here, you can see how each of the five inputs above were figured:

  • Rafter run outer wall – This should be measured starting from the center of the roof span, and then extending to the very edge of the exterior wall. Here, the number is 1166.66 inches.
  • Roof rise – This should be calculated from the base of the roof to the highest point of the roof gable (the peak). Here, the number is 941.86 inches.
  • Overhang – The overhang should be measured from the outer edge of the exterior wall to the furthest point of the rafter. Here, the overhang is 320.13 inches.
  • Birdsmouth plumb – If you study the diagram carefully, you will see that there is an indentation in the roof rafter where it meets the exterior wall of the building. The actual indentation or cut is known as the “birdsmouth,” while the birdsmouth plumb is the depth of the cut out. Here, the birdsmouth plumb is 76.37 inches.
  • Wall thickness – It is important to take into account the overall thickness of the exterior wall. Here, the wall thickness is 240.57 inches.

Once all of these inputs have been entered into the calculator, you will see that the appropriate rafter width is 120.33 inches. In addition, using the “Rafters” diagram, you can see the precise dimensions of the roof rafter (including length) from every possible angle.

Conclusions about the roof rafter length calculator

The layout of your home or building will have a significant impact on the final design and slope of your roof. Using this handy online tool, you will have an easy way to determine and figure out the specific rafters needed for the unique slope and dimensions of your roof.