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How do I ventilate a cold flat roof and which product do I use?

The requirements for a cold flat roof is to provide the equivalent of 25,000mm²/m of ventilation on two opposing sides of the roof in accordance with British Standards (BS5250:2021 Management of moisture in buildings code of practice).

The Glidevale Technical team has come up with an easy solution…

Our FV250 Over Fascia Ventilator product can be used long the eaves, easily fixed to the fascia board. This provides continual ventilation along the length of the roof between each joist.

Where there is a parapet or an external wall abutment to the flat roof, the FV250 ventilator can be used above a 150mm timber upstand battened off the wall by 25mm as shown below. The 25mm batten space creates a clear airpath to the roof void below so reducing the possibility of condensation and mould growth.

Diagram below explains the build-up.

To read more about this product, click here to see the Eaves and Low Level Ventilation brochure. For further information, please contact the Technical Team on 0161 905 5700 or email

Related FAQs

Fitting ventilation to an open eaves

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Glidevale Protect is a division of Building Product Design Ltd.

Other subsidiaries include ventilation systems specialist Passivent and Kingfisher Louvres, manufacturer of aluminium louvres systems.

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