Felt lap vent
A felt lap vent is normally a moulded plastic product which is designed to allow air to pass through the lap between two layers of the felt which lies on top of the rafters, beneath the battons which hold the tiles or slates. It is a retrofit solution for houses which have cold roofs and suffer from moisture build up in the attic or loft space from moist warm air rising from the living spaces below. These houses are normally older constructions where roof felt has been used which is not breathable rather than newer construction that may use a membrane that prevents water ingress but lest moist air out ie breathabe mebranes.
In a cold roof scenario, the insulation and air tightness barrier is normally in line with the ceiling of the upper most rooms (rather than the pitch of the roof). Keeping the uninsulated roof space ventilated can be important to prevent moisture build up, mould and potential damage, particularly where non breathable felt or bitumen products are used. Increased moisture from warm air and activity in living spaces below can build up in the cold roof space above, condense on surfaces and with out good air flow can cause structural damage over longer periods.
Felt laps are one way of introducing air flow into a cold roofs to remove moisture build up, other options are tile, slate, soffit, fascia and ridge vents. In some cases where levels of insulation are increased above the ceiling level this can restrict air flow that traditionally would have come through the sofits or barge board ventilators. Felt laps can be installed higher up the roof pitch, where ever layers of felt over lap, thus not being restricted by the increased levels of insulation and without the need to remove or replace roof surfaces such as slate or tiles.
For further information on the ventilation of cold roofs guidance should be sought in Aproved Document C "Site preparation and resistance to contaminants and moisture" section C2.
[edit] Related articles on Designing Buildings
Featured articles and news
Design and construction industry podcasts
Professional development, practice, the pandemic, platforms and podcasts. Have we missed anything?
C20 Society; Buildings at Risk List 2025
10 more buildings published with updates on the past decade of buildings featured.
Boiler Upgrade Scheme and certifications consultation
Summary of government consultation, closing 11 June 2025.
Deputy editor of AT, Tim Fraser, discusses the newly formed society with its current chair, Chris Halligan MCIAT.
Barratt Lo-E passivhaus standard homes planned enmasse
With an initial 728 Lo-E homes across two sites and many more planned for the future.
Government urged to uphold Warm Homes commitment
ECA and industry bodies write to Government concerning its 13.2 billion Warm Homes manifesto commitment.
From project managers to rising stars, sustainability pioneers and more.
Places of Worship in Britain and Ireland, 1929-1990. Book review.
The emancipation of women in art.
Call for independent National Grenfell oversight mechanism
MHCLG share findings of Building Safety Inquiry in letter to Secretary of State and Minister for Building Safety.
The Architectural Technology Awards
AT Awards now open for this the sixth decade of CIAT.
50th Golden anniversary ECA Edmundson awards
Deadline for submissions Friday 30 May 2025.
The benefits of precast, off-site foundation systems
Top ten benefits of this notable innovation.
Encouraging individuals to take action saving water at home, work, and in their communities.
Takes a community to support mental health and wellbeing
The why of becoming a Mental Health Instructor explained.
Mental health awareness week 13-18 May
The theme is communities, they can provide a sense of belonging, safety, support in hard times, and a sense purpose.
Mental health support on the rise but workers still struggling
CIOB Understanding Mental Health in the Built Environment 2025 shows.
Design and construction material libraries
Material, sample, product or detail libraries a key component of any architectural design practice.