Calculating your rafter length is straightforward once you know the Roof Rafter Length formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Roof Rafter Length Calculator.
What is Roof Rafter Length?
The Roof Rafter Length calculation tells you your rafter length from a few simple inputs. The figure you are solving for here is the rafter length.
The Roof Rafter Length formula
The core formula is:
Rafter length = √(Horizontal run ^ 2 + Vertical rise ^ 2) + OverhangHere is what each input means:
- Horizontal run — a value measured in m. Example: 4 m.
- Vertical rise — a value measured in m. Example: 2 m.
- Overhang — a value measured in m. Example: 0.3 m.
How to calculate it step by step
- Write down the horizontal run (for example, 4 m).
- Write down the vertical rise (for example, 2 m).
- Write down the overhang (for example, 0.3 m).
- Apply the formula above to get your rafter length.
- Double-check the result with the Roof Rafter Length Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Horizontal run | 4 m |
| Vertical rise | 2 m |
| Overhang | 0.3 m |
| Rafter length | 4.772 |
| Sloped length (no overhang) | 4.472 |
With horizontal run of 4 m, vertical rise of 2 m and overhang of 0.3 m, the rafter length works out to 4.772.
Example 2
With horizontal run of 8 m, vertical rise of 2 m and overhang of 0.3 m, the rafter length works out to 8.546.
| Result | Value |
|---|---|
| Rafter length | 8.546 |
| Sloped length (no overhang) | 8.246 |
Example 3
With horizontal run of 2 m, vertical rise of 2 m and overhang of 0.3 m, the rafter length works out to 3.128.
| Result | Value |
|---|---|
| Rafter length | 3.128 |
| Sloped length (no overhang) | 2.828 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Roof Rafter Length Calculator does it instantly, for free, with the formula and a worked example built in.
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