Felt Lap roof Vents Pack of 25 Prevents roof condensation and moisture drops. Easy to fit just slots between the felt laps. Can be installed from inside of loft. The 3 felt clips secure the product in place even if the felt is degraded of loose. Can be fitted between 400mm joists and 600mm joist openings. Provides 3000mm² air flow per unit. Dimensions: 275mm x 190mm. Meets building regulations. The Manthorpe Felt Lap Vent is a refurbishment product designed to increase the amount of ventilation into existing roof spaces. Usually fitted retrospectively to properties with additional loft insulation recently installed. However the felt lap vent will help to combat the problem of condensation build up in any roof space with a lapped felt / membrane construction.