Installing a roof light in an existing pitched roof is a significant undertaking that improves natural light in a loft or room below the roof and requires careful work to maintain the weathertightness of the roof covering. This guide covers the main steps for a standard installation.

Planning and Building Regulations

Before starting, confirm whether planning permission is required. Roof lights that project no more than 150mm above the roof plane and are not on a highway-facing slope in a conservation area are generally permitted development. Building Regulations do not apply to the roof light installation itself in most cases, but if the loft is being converted to habitable use as part of the project, a full Building Regulations application is required for the conversion including fire safety, insulation, and structural elements.

Determining the Position and Size

Choose a size of roof light that fits between rafters with minimal or no structural alteration if possible. Standard Velux sizes are designed to fit standard rafter spacings. If a larger roof light or an unusual position is needed that requires cutting through a rafter, temporary and permanent structural support for the interrupted rafter must be provided, using a trimmer and header arrangement designed by a structural engineer or competent carpenter.

Cutting the Opening

Mark the opening position on the roof from inside the loft, drilling marker holes through the tiles to establish the corners. From outside, lift the tiles around the opening area and remove the underlay within the intended aperture. Cut the rafters if required and install any trimmer and header framing. Mark and cut the tile battens. The opening in the structural roof should be the correct size for the chosen Velux frame to sit in; refer to the manufacturer's installation guide for the exact rough opening dimensions required for the specific model.

Fitting the Frame

Fit the Velux window frame into the prepared opening according to the manufacturer's instructions, using the supplied brackets to fix it to the rafters. The frame must be level and square. Fit the EDJ (standard) or equivalent flashing kit around the frame, integrating it with the existing tile courses. The flashing kit is designed to direct water away from the frame and back onto the tiles below. Follow the specific flashing kit instructions for the tile profile being used.

Finishing

Refit the surrounding tiles, ensuring they lap correctly over the flashing. From inside, fit the insulation collar if required, and the internal lining kit or reveal boards to finish the reveal between the roof light frame and the interior ceiling or wall. Test the window operation and locking mechanism before considering the installation complete.

Summary

Fitting a Velux roof light involves preparing the rough opening, fitting the frame to the rafters, and installing the correct flashing kit for the tile type. The flashing kit is the most critical element for weathertightness; always use the kit designed for the specific tile profile. If rafters must be cut, structural support must be provided. Building Regulations apply if the installation is part of a loft conversion.