How and where to use guide
VELUX Underfelt Collar 55x98cm BFX CK04 1000
Installation location and sequence
The underfelt collar is positioned during the roof window installation process, after the window frame has been positioned in the roof opening but before final flashing components are secured. It sits directly between the window frame and the roof underlay material, bridging the gap where water could otherwise penetrate. On pitched roofs, this collar must be installed on all sides of the window frame to create a complete weathertight envelope. Installation sequence is critical: the collar should be fitted before the roof tiles or slates are replaced, ensuring the underlay sits properly over the collar edges.
- Position collar between window frame and roof underlay
- Ensure collar sits flush against the frame on all sides
- Install before replacing roof covering materials
- Check that underlay overlaps the collar edges correctly
- Verify no gaps or wrinkles in the collar material
Typical applications and roof types
This underfelt collar is designed for standard pitched roof constructions where roof underlay is used as part of the weatherproofing system. It is suitable for both new installations and replacement window projects on existing pitched roofs. The collar works with all standard roof tile and slate coverings, provided the roof structure includes an underlay layer. It is not suitable for flat roofs or roofs without underlay systems.
- New CK04 window installations on pitched roofs
- Replacement of existing roof windows
- Roofs with standard breathable or non-breathable underlay
- Tile and slate pitched roof constructions
- Standard domestic and light commercial applications
On-site handling and fitting tips
Handle the underfelt collar carefully during installation to avoid creasing or tearing the weatherproofing material. Before fitting, check that the CK04 window frame is correctly positioned and level in the roof opening. The collar must be smoothed into place without wrinkles or folds, as these can create water pathways. Ensure the underlay material is properly secured over the collar edges using appropriate fixings or adhesive as per VELUX instructions. Do not cut or modify the collar dimensions, as this will compromise the weathertight seal.
- Inspect collar for damage before installation
- Keep collar dry and protected from debris until fitting
- Smooth out all wrinkles and creases during installation
- Ensure underlay is firmly adhered over collar edges
- Do not trim or cut the collar material
Maintenance and longevity
Once installed, the underfelt collar requires no routine maintenance as it is a permanent part of the roof structure. However, during any future roof work or maintenance, take care not to damage the collar when accessing the roof. If the roof underlay is replaced or repaired, ensure the collar remains in place and is properly resealed with the new underlay. Inspect the collar visually during annual roof inspections to confirm it remains in good condition and that the seal with the window frame has not been compromised.
- No routine maintenance required after installation
- Protect collar during any roof work or repairs
- Recheck seal if roof underlay is replaced
- Include in annual roof inspection routine
- Replace only if damaged or if window is replaced