Ridge Height Calculator
Works out how high a ridge sits above the wall plate and above ground. Useful for planning applications, scaffolding and checking whether a loft conversion has the headroom.
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The formula
rise = span ÷ 2 × (rise ÷ run) ridge height = wall plate height + rise
Half the span is the run of one rafter, and the pitch is rise over run, so multiplying gives the rise directly. Doubling the span doubles the ridge height at the same pitch, which is why wide buildings end up with either very tall roofs or very shallow ones.
What to watch out for
Two places this figure matters:
- Planning. Ridge height above ground is usually the dimension a planning constraint is written against, and permitted development rights often turn on it.
- Loft conversions. Usable headroom is the rise less about 250 mm for the ridge board, rafter depth and ceiling. Most guidance wants 2.1–2.3 m over a useful share of the floor area.
The rise scales with the span. A 10 m span at 6/12 gives a 2.5 m rise; the same pitch on a 6 m span gives only 1.5 m, which is why narrow houses rarely convert without raising the roof or adding a dormer.
Frequently Asked Questions
How do I calculate ridge height?
Multiply half the span by the pitch expressed as rise over run. An 8 m span at 6/12 gives 4 × 0.5 = 2 m above the wall plate.
How much headroom do I need for a loft conversion?
Commonly 2.1–2.3 m over a reasonable proportion of the floor area, though requirements vary by jurisdiction. Subtract about 250 mm from the ridge rise for the structure and ceiling before judging it.
Does a steeper roof give more loft space?
Considerably. Going from 6/12 to 10/12 on an 8 m span raises the ridge from 2 m to 3.3 m, which is often the difference between an unusable loft and a convertible one.