How and where to use guide

Typical applications and installation locations

Use this duo flashing when installing two VELUX roof windows of 78×140cm dimension with exactly 100mm spacing between them. It suits both side-by-side and vertical stacked configurations. The flashing is designed for pitched roofs with clay, concrete, or slate tiles up to 120mm profile height. Common applications include loft conversions where multiple windows are needed for light and ventilation, dormer windows in extensions, and residential properties requiring coordinated roof window layouts. The prefabricated design means it arrives ready to fit without requiring custom metalwork on site.

• Loft conversions with paired roof windows
• Extensions and dormer installations
• Vertical stacked window arrangements
• Side-by-side window configurations
• Pitched roofs with standard tile profiles up to 120mm

Installation steps and on-site guidance

Begin by preparing the roof opening to the correct dimensions for both windows with the 100mm gap. Remove existing tiles and underlayment in the installation zone. Position the flashing unit so it sits flush against the roof structure, ensuring the lower section sits properly on the tiles below the windows. Secure the flashing according to VELUX instructions, typically using the provided fixing points. Bed the flashing into mortar or roofing sealant as appropriate for your tile type. Replace and rebed the roof tiles around the flashing perimeter, ensuring they sit flush and create a watertight overlap. Check that the 100mm gap between window units is maintained throughout installation. Allow any sealant or mortar to cure fully before exposing the roof to heavy rain.

• Prepare roof opening to correct dimensions for both windows
• Remove tiles and underlayment in the installation zone
• Position flashing unit flush against roof structure
• Secure using provided fixing points
• Bed flashing into mortar or roofing sealant
• Rebed surrounding tiles to create watertight overlap
• Maintain 100mm gap between window units
• Allow sealant or mortar to cure before exposure to rain

Compatibility and limitations

This duo flashing is specifically engineered for VELUX roof windows measuring 78cm width by 140cm height. It is compatible with roof tiles and slates up to 120mm profile depth; do not use on roofs with deeper profile tiles as the flashing may not seal correctly. The 100mm gap is fixed and cannot be adjusted, so this flashing suits only installations where windows are spaced exactly 100mm apart. If your roof has a different tile profile or your windows require different spacing, you will need an alternative flashing configuration from the VELUX range. The flashing is not suitable for flat roofs or low-pitch applications.

• Designed for 78×140cm VELUX windows only
• Maximum tile profile 120mm; not suitable for deeper tiles
• Fixed 100mm gap cannot be adjusted
• Not suitable for flat roofs or low-pitch applications
• Requires standard pitched roof installation

Maintenance and durability

Once installed, the flashing requires minimal maintenance. Inspect the installation annually, particularly after severe weather, to ensure tiles remain bedded properly and the sealant has not cracked or deteriorated. Clear any debris, moss, or lichen from the flashing surface and surrounding tiles to maintain water drainage. If mortar or sealant shows signs of cracking or separation, rebed the affected area promptly to prevent water ingress. The prefabricated construction means there are no joints or seams to maintain beyond the perimeter seal. Keep gutters and roof drainage clear to prevent water pooling around the flashing. In areas with heavy snow or ice, ensure the flashing and surrounding tiles are clear of obstructions that could trap moisture.