Free · Roofing calculators 🏠

Roof Truss Count and Rafter Geometry

Calculate ideal right-triangle rafter length, roof pitch and endpoint-inclusive truss count with quoted costs.

  • Formula & worked example
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

The rafter mode gives a 5 m ideal triangle from a 4 m run and 3 m rise. The separate truss mode counts both ends and rounds up the spacing allowance.

Rafter = √(run²+rise²); pitch = tan(angle); trusses = ceil(length/spacing + 1).

Worked example

Enter these known values and leave the other values blank.

I want to calculate...
rafter length
Run
4 m
Rise
3 m
Rafter length
5 m

Assumptions and limitations

  • Rafter length is an ideal right triangle from horizontal run to ridge rise. It excludes overhang, ridge deductions, birdsmouth cuts, connections and material thickness.
  • Truss count includes both end positions and rounds the continuous spacing estimate up. This is a quantity model, not a structural sizing or building-code check.
  • Clear an unknown before solving backward. Currency controls represent the chosen denomination without exchange rates; prices must use the selected denominator units.
  • Whole package, board, roll or truss ceilings use exact decimal arithmetic. An amount just above a whole-number boundary needs the next whole unit; numeric display rounding does not lower the purchase count.

Common questions

What is included in this estimate?

Rafter length is an ideal right triangle from horizontal run to ridge rise. It excludes overhang, ridge deductions, birdsmouth cuts, connections and material thickness.

References

Enforce the documented geometric domains and positive whole counts. Use exact decimal ceilings for whole material quantities; the captured mathjs ceiling snaps sufficiently near integers and can undercount immediately above a boundary.

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