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Pitched Roof Ventilation - Tile & Slate Inline Vents

Manthorpe Felt Lap Vent G630 - Roof Vent

Prevent Loft Condensation with Manthorpe Felt Lap Vent G630

Manthorpe Felt Lap Vent G630 - Roof Vent

£3.14 £2.62 (ex. VAT)
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Increase roof ventilation and prevent loft condensation problems with the Manthorpe felt lap vent

Manthorpe felt lap vents provides an effective solution to increasing the amount of free air ventilation to your roof space and thus preventing condensation. Typically retro-fitted in older refurbished properties that may not have an effective roof ventilation system or tile vents installed, and which may have had additional loft insulation. The Felt Lap Vent helps to combat the problem of condensation build up due to warm air in any attic or loft with a lapped felt/membrane construction, by increasing ventilation. Please ensure that you work safely at all times and get advice or help from a professional trades person if you are unsure.

Felt Lap Vent Installation

Simple to install even for a DIY Enthusiast, the vent simply slides between the two layers of underlay felt beneath your existing roof tiles or slates. They can be installed from inside the property so no need to remove any of the existing tiles, timber battens or felt.

Important loft lap vent installation notes

You should ensure that the opening of felt laps on the property will not adversely increase the wind uplift load on the tiles/slates or the driving rain resistance of the roof, for more information see BS 5534:2003, 5.8.1 &

Roof Felt Vent Key Information

  • 3,000mm² airflow for each G630
  • Can be fitted at joist centres ranging from 400mm to 600mm
  • Easy to fit, the tapered lead in helps to slot the product between the felt laps
  • The 3 felt clips help secure the product in place even if the edge of the felt is degraded

Loft condensation FAQ's

Why is loft condensation a problem?

Moist air condensating on a cold surface, causes the formation of droplets. These will fall onto whatever is in the loft space and make them damp; stored items, electrics, insulation, etc. Left unchecked, items will become damp and moldy and will ultimately degrade.

What are the causes of loft condensation?

Condensation occurs when moist, humid air meets a cold surface. When the dew point temperature is low enough, and the air contains enough moisture, condensation droplets will form.

How can you stop condensation in a loft space?

The normal solution to condensation, is to reduce humidity of the air and increase the temperature of the surface. In a roof space, this is best achieved through ventilation. By increasing the air flow, we transfer the air outside, replacing it with less humid air, and this reducing condensation. Ideally, this would be achieved using a combination of sofit vents and tile vents. However, that can be costly, so the loft felt lap vent is an ideal solution in certain circumstances.

Does insulating the loft stop condensation?

Insulating between ceiling joists can help up to a point, but too much insulation can actually contribute towards the problem. Insulating between rafters would help prevent the surface reaching the dew point, but is of course, an expensive project, and still needs to be done in conjunction with adequate ventilation.

Can I use a dehumidifier?

A dehumidifier would help, but only as a temporary solution, as it is more cost effective to solve the issue with adequate roof ventilation.

The manthorpe lap vent is available for direct delivery by our network partners or collection from our depot in Maidstone, Kent. 

If you require further information or assistance, then please do contact us by Telephone 01634 869823, email: or by the live chat service. We are generally available Monday to Friday 07.00am to 5.00pm.

Q. Are these easy to fit - DIY?

A. Yes, very easy. A competent DIYEr should have no trouble. 

Q. How do they fit the roof felt?

A. They simply slide into place in the overlap of the existing roofing felt.

Q. Arevthey screwed into place?

A. No fixings required. The integral clips hold the vent in place.

Q. Are these fitted from the outside?

A. NO, these are fitted from the inside of your loft space.

Q. What is the minimum space required between timber rafters?

A. You will need a minimum rafter space distance of 400mm.

Q. How many vents will i need?

A. This depends on how big your loft space is and how many roofing felt overlaps are exposed. 

  • Material:
  • Manufacturing Process:
    Injection Moulded
  • Size:
    275 x 20 x 194mm
  • AirFlow:
  • Minimum Rafter Centre:
  • Pack Size:
    1 x Lap Vent
  • GTIN:
  • Brand:
    Manthorpe Building Products Ltd
  • Product Code:
  • Weight:
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Product Code G630

275 x 20 x 194mm



50 per box



Injection Moulded