About this calculator
Work out the eaves and high-level ventilation for a cold pitched roof using the common BS 5250 provisions, and count vent strips.
How it’s calculated
Pitches of 15° and above get 10 mm continuous ventilation at each eave. Below 15° it rises to 25 mm. With traditional HR felt, or on steep pitches, add 5 mm continuous high-level ventilation at the ridge.
Worked example
With these inputs:
- Total eaves length (both sides)
- 20 m
- Ridge length
- 10 m
- Roof pitch (degrees)
- 35
- Underlay
- Traditional felt (HR)
the result is:
- Eaves ventilation
- 10 mm continuous
- High-level (ridge) ventilation
- 5 mm continuous = 50,000 mm²
Tips from the trade
- Baffles or rafter trays keep insulation from blocking the eaves.
- Don’t mix ridge vents with gable or powered vents on the same attic. They short-circuit each other.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and the Building Regulations before ordering.