Fixing and buying guide

What this product is and who needs it

This underfelt collar is a flashing component that creates a weather-tight seal between your VELUX roof window or Sun Tunnel and the roof structure. It is designed for builders, roofers, and serious DIY installers undertaking new build or refurbishment work on pitched roofs. The integrated drainage gutter is critical for managing water run-off and preventing damp problems, making this collar a non-negotiable part of a proper roof window installation.

Compatibility and sizing

This collar is sized 550 x 780mm and is engineered specifically for compatible VELUX roof window and Sun Tunnel systems. Before ordering, confirm that your VELUX window or tunnel model is compatible with the BFX CK02 1000 collar. The collar is constructed from aluminium and synthetic components to withstand typical UK roof conditions. Check your window documentation or contact VELUX technical support if you are unsure of compatibility.

• Collar dimensions: 550 x 780mm
• Compatible with VELUX roof windows and Sun Tunnels only
• Aluminium and synthetic construction
• Designed for pitched roofs

Installation and pre-install checks

The underfelt collar is positioned beneath the roof felt or underlay during installation. Before fitting, ensure the roof opening is correctly sized and prepared to receive the collar, and that all roof structure is sound and level. The transverse drainage gutter must be positioned to direct water away from the window frame and towards the lower edge of the roof. Proper positioning of the collar is critical to the weather-tightness and long-term performance of your installation. Follow VELUX installation guidance carefully, and consider professional fitting if you are unfamiliar with roof window installation.

• Install beneath roof felt or underlay
• Ensure roof opening is correctly sized and prepared
• Position drainage gutter to direct water downslope
• Follow VELUX installation instructions precisely

Pack contents and what to check on delivery

This product is supplied as a single underfelt collar unit. On delivery, inspect the collar for any damage to the aluminium frame or synthetic components, and confirm the dimensions match your roof opening. Check that the transverse drainage gutter is intact and unobstructed. Store the collar in a dry location until installation to prevent any surface damage or corrosion.