Manthorpe G630 Felt Lap Vents

These Manthorpe G630 Felt Lap Vents will help stop condensation in the loft.

Manthorpe Felt Lap Vents are refurbishment products designed to increase the amount of ventilation into existing roof spaces with a lapped felt/membrane

Attic or loft condensation is normally spotted either in the winter months or after installing new loft insulation. All homes generate moisture from everyday activities like cooking and showering. When this moisture condenses in a cold loft it can eventually lead to costly problems like rotten roof timber. For more information read our guide on condensation.



Condensation in the loft is usually caused by inadequate loft ventilation. This can often be remedied quickly and easily if your loft has a traditional lapped membrane or roofing felt behind the tiles. G630 Lap vents are fitted inside the loft by simply clipping individual vents onto the edges of the felt or membrane close to the eaves.

Each felt lap vent creates an air gap delivering up to 3000mm2/m of ventilating airflow. When combined with existing roof space ventilation, fitting evenly spaced lap vents on both sides of the loft will fix most loft condensation problems.

Made from an injected molded polypropylene they are a lightweight solution that is fully recyclable.


How Many Vents Do I Need?

The number of felt lap vents can be worked out using the following formula:

Selecting the amount of airflow, then dividing the roof length by the spacing (in meters) indicted below.

Free Flowing Airflow Felt Lap Spacings
  Millimeters Meters
5000mm/m² 600 0.6
10000mm/m² 300 0.3
15000mm/m² 120 0.12

For Example

For a roof that is 7m requiring airflow of 10000mm/m²

7/0.3 = 23.333 or 24 felt lap vents evenly spaced.

For roof spaces that require more than the 15000mm/m² the spacings half for each additional 5000mm/m²

Please note these are designed as a remedial solution to roof ventilation and do not meet the standards for permanent roof space ventilation on their own, and felt lap vents may adversely affect wind uplift of slates and tiles. For more information, please see our blog on the changes to BS 5534:2014 or see BS 5250 Code of Practice for Control of Condensation in Buildings.  

Further Reading

For further information on what condensation is, why is it caused, and other ways to reduce it please see our articles on our resource centre

What is Condensation?

What causes Condensation?

Ways to reduce Condensation






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