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
Future Homes Standard Essentials launched
Future Homes Hub launches new campaign to help the homebuilding sector prepare for the implementation of new building standards.
Building Safety recap February, 2026
Our regular run-down of key building safety related events of the month.
Planning reform: draft NPPF and industry responses.
Last chance to comment on proposed changes to the NPPF.
A Regency palace of colour and sensation. Book review.
Delayed, derailed and devalued
How the UK’s planning crisis is undermining British manufacturing.
How much does it cost to build a house?
A brief run down of key considerations from a London based practice.
The need for a National construction careers campaign
Highlighted by CIOB to cut unemployment, reduce skills gap and deliver on housing and infrastructure ambitions.
AI-Driven automation; reducing time, enhancing compliance
Sustainability; not just compliance but rethinking design, material selection, and the supply chains to support them.
Climate Resilience and Adaptation In the Built Environment
New CIOB Technical Information Sheet by Colin Booth, Professor of Smart and Sustainable Infrastructure.
Turning Enquiries into Profitable Construction Projects
Founder of Develop Coaching and author of Building Your Future; Greg Wilkes shares his insights.
IHBC Signpost: Poetry from concrete
Scotland’s fascinating historic concrete and brutalist architecture with the Engine Shed.
Demonstrating that apprenticeships work for business, people and Scotland’s economy.
Scottish parents prioritise construction and apprenticeships
CIOB data released for Scottish Apprenticeship Week shows construction as top potential career path.
From a Green to a White Paper and the proposal of a General Safety Requirement for construction products.
Creativity, conservation and craft at Barley Studio. Book review.
The challenge as PFI agreements come to an end
How construction deals with inherited assets built under long-term contracts.
Skills plan for engineering and building services
Comprehensive industry report highlights persistent skills challenges across the sector.
Choosing the right design team for a D&B Contract
An architect explains the nature and needs of working within this common procurement route.
Statement from the Interim Chief Construction Advisor
Thouria Istephan; Architect and inquiry panel member outlines ongoing work, priorities and next steps.