How and where to use guide
VELUX Underfelt Collar 78x118cm BFX MK06 1000
Installation location and preparation
The underfelt collar is installed on the rafters or joists surrounding the rooflight opening. Before installation, ensure your timber structure meets the minimum F16 grade requirement. The opening dimensions must accommodate the 78x118cm collar size. Inspect the timber for damage, rot, or movement and make good any defects. Clear the installation area of debris, dust, and loose material to ensure a clean surface for the collar to sit properly against the structural members.
- Check timber grade is F16 or higher
- Verify rooflight opening dimensions suit 78x118cm collar
- Clean rafters and joists of dust and debris
- Inspect timber for damage or movement before proceeding
Installation and securing
Position the felted underfelt collar around the rooflight opening, ensuring it sits flush against the rafters or joists on all sides. The collar must be secured using appropriate fixings suitable for your structural application and in line with VELUX manufacturer guidance. Work methodically around the opening to maintain even contact and prevent gaps. Pay particular attention to corners and joints where the collar meets the timber, as these areas are critical for weather-tightness. Ensure the collar is not twisted or bunched during installation.
- Position collar flush against all rafters and joists
- Use fixings appropriate to your structural application
- Work systematically around the opening
- Check corners and joints for tight contact
- Avoid twisting or bunching the material
Performance and maintenance
Once installed, the underfelt collar provides a weather-tight seal that prevents moisture ingress and reduces heat loss around the rooflight. The felted underfelt material is durable but should be inspected periodically, particularly after severe weather or if roof works are carried out nearby. Check that the collar remains in contact with the timber and shows no signs of lifting, tearing, or compression. If damage occurs, the collar should be repaired or replaced to maintain the integrity of the weather seal and thermal performance.
- Inspect collar periodically for signs of damage or lifting
- Check after severe weather or nearby roof works
- Repair or replace if tearing or compression occurs
- Maintain weather-tight seal for optimal thermal performance
Limitations and on-site considerations
This collar is designed specifically for VELUX rooflights and the 78x118cm size is suited to standard rooflight openings. It cannot be used on timber grades below F16, as these do not provide adequate structural support and durability. The collar works as part of the complete rooflight assembly and must be installed in line with current VELUX manufacturer guidance and site practice. Do not attempt to stretch or compress the collar beyond its designed dimensions, as this will compromise the weather seal and insulation properties.
- Suitable only for timber grades F16 and above
- Designed for standard VELUX rooflight openings
- Must be installed per VELUX manufacturer guidance
- Do not stretch or compress beyond designed dimensions