Fixing and buying guide

What this product is and who needs it

This underfelt collar is a specialist rooflight installation component for builders, roofers, and serious DIY customers undertaking VELUX rooflight projects. It sits between the rooflight frame and the building structure, providing both a weather seal and an insulation layer. If you are installing a VELUX rooflight into a timber-framed roof, this collar is a necessary part of a compliant, weather-tight installation.

Compatibility and sizing

This collar is sized 78x118cm and is designed for standard VELUX rooflight openings. It is compatible with timber structural members graded F16 or higher. Before ordering, confirm your rooflight opening dimensions match this size and verify your rafter or joist timber grade meets the F16 minimum requirement. Check your VELUX rooflight documentation to confirm this collar is the correct specification for your model.

• Size: 78 x 118cm
• Minimum timber grade: F16
• Installation surface: Rafters or joists
• Material: Felted underfelt

Pre-installation checks

Before fixing the collar, inspect the timber structure around your rooflight opening for damage, rot, or movement. Ensure the opening dimensions are accurate and that the timber grade is F16 or above. Check that you have appropriate fixings for your structural application and that the rooflight frame is correctly positioned and level. Review the VELUX installation instructions and any site-specific requirements before proceeding.

• Verify rooflight opening size matches 78x118cm
• Confirm timber grade is F16 or higher
• Inspect timber for damage or rot
• Ensure rooflight frame is level and correctly positioned
• Prepare appropriate fixings for your structural application

Installation and performance

Install the underfelt collar in line with VELUX manufacturer guidance and current site practice. The collar creates a weather-tight seal and insulation layer around the rooflight opening, reducing heat loss and preventing moisture ingress. Secure it firmly to the rafters or joists using fixings appropriate to your structural application. Proper installation is essential for the collar to perform its thermal and weather-sealing functions effectively.