How and where to use guide
VELUX Underfelt Collar 134x140cm BFX UK08 1000
Typical applications and compatibility
This underfelt collar is designed specifically for VELUX roof window and Sun Tunnel systems. It forms part of the complete flashing assembly and must be used with compatible VELUX frames and installation kits. The collar works in conjunction with the roof underlay and external flashing to create a fully sealed penetration. It is suitable for use in new build construction and refurbishment work on pitched roofs with standard UK roof coverings such as tiles or slates.
- VELUX roof windows (standard ranges)
- VELUX Sun Tunnel systems
- New build and refurbishment projects
- Pitched roofs with tile or slate coverings
Installation and positioning
The underfelt collar must be positioned beneath the roof covering and integrated with the roof underlay system before tiles or slates are laid. The transverse drainage gutter is engineered to direct water run-off away from the window frame and down the roof slope. Correct positioning ensures the collar sits flush against the roof deck and that the drainage gutter functions effectively. Installation should follow VELUX technical guidance and be carried out by competent installers familiar with roof window flashing systems.
- Install beneath roof covering, above underlay
- Position transverse drainage gutter to direct water downslope
- Ensure collar sits flush and level on roof deck
- Integrate with underlay before laying tiles or slates
- Follow VELUX installation instructions for your specific window model
Weather-tightness and water management
The integrated transverse drainage gutter is the key feature for managing water run-off at the roof penetration. This gutter channels water away from the window frame and prevents pooling or ingress at the critical seal point. The collar creates a weather-tight barrier between the roof structure and the external environment, protecting the interior from rain penetration and condensation issues. Proper installation of the complete flashing assembly—collar, underlay integration, and external flashing—is essential for long-term weather protection.
- Transverse gutter directs water away from window frame
- Prevents water pooling at roof penetration
- Works with external flashing for complete weather seal
- Protects against rain ingress and condensation
On-site handling and professional finish
The underfelt collar is designed to deliver a visually consistent and professional roof finish. Handle the collar carefully on site to avoid damage to the drainage gutter or seal surfaces. Ensure the collar is not bent or twisted during transport and storage. Once installed and covered by roof tiles or slates, the collar is concealed but remains critical to the weather-tight performance of the installation. Do not attempt to modify or cut the collar; use only genuine VELUX components sized for your specific window model.
- Handle carefully to avoid damage to gutter or seals
- Store flat and protected from site damage
- Do not modify or cut the collar
- Use only genuine VELUX components for your window size
- Ensure installer is competent in roof window flashing systems