Roof windows and skylights, such as those made by Velux, are essential components of modern loft conversions, extensions, and flat roof systems. They transform dark attic spaces into light-filled rooms and provide crucial ventilation. However, their installation involves integrating a hole into the most critical part of your home’s weatherproofing—the roof. If installed incorrectly, the risk of water ingress and structural damage is high.
Hiring a certified skylight fitter who understands the nuances of leak prevention is non-negotiable for a long-lasting, watertight installation.
Structural considerations and building regulations
Before installation begins, the fitter must assess the structural integrity of the roof and ensure compliance with UK Building Regulations.
1. Structural assessment
- Rafters and Joists: The fitter must confirm that cutting a hole for the window will not compromise the load-bearing capacity of the existing rafters. If a rafter needs to be cut, the fitter (often in conjunction with a carpenter) will install a new timber framework, called a trimmer, to distribute the load around the new opening.
- Roof Type: The requirements differ significantly between a flat roof (where the window sits on an upstand) and a pitched roof (where the window is recessed into the tiles/slates).
2. Building regulations compliance
- Part L (Energy Efficiency): New roof windows must meet current thermal efficiency standards (U-values). This means installing modern, thermally broken glazing units.
- Part F (Ventilation): The window must provide adequate background and purge ventilation for the room to meet regulations.
- Part B (Fire Safety): If the window is being installed as part of an escape route from a loft conversion, it must be large enough and open wide enough to qualify as an egress window.
The key to roof window leak prevention: Flashing
The most critical part of any installation is the flashing installation. This is the engineered metal system that directs water away from the window frame and seamlessly integrates the window with the surrounding roofing material.
| Roof Type | Flashing Requirement | Key Risk Factor |
| Pitched Roofs (Tile/Slate) | The fitter must use the exact flashing kit designed by the manufacturer (e.g., Velux) for the type of roofing material (e.g., plain tiles need different flashing than slate). | If the protective layers beneath the tiles (like the roofing felt/membrane) are not properly cut and sealed around the opening, water ingress will occur. |
| Flat Roofs | The window must sit on a timber kerb (upstand) that lifts the base of the window above the roof level to prevent standing water ingress. | The waterproofing membrane (e.g., EPDM, felt, or fibreglass) of the flat roof must be flawlessly integrated and sealed up and over the timber kerb. |
A professional will always install the flashing layers meticulously, overlapping them correctly, and ensuring the final finish sheds water away from the window base and down the roof slope.
Cost to fit a Velux window
The cost varies widely based on the complexity of the roof and the size of the unit. The labour costs are usually higher than for a vertical window replacement due to the specialist nature of the work and the heights involved.
- Labour Breakdown: The quote will typically include access (scaffolding or roof ladders), structural trimming, membrane cutting/sealing, window installation, flashing installation, and internal finishing (plasterboard).
- Window Type: Centre pivot windows are often cheaper and easier to fit than top-hung windows or bespoke electric models.
When selecting a skylight fitter, verify their experience with the specific roof system you have (flat or pitched) and check that their quote includes all elements of leak prevention, particularly the manufacturer-specific flashing kit. A high-quality installation guarantees decades of light without the costly headache of leaks.
With Rated People, you have the power to get it done. Find a trusted, local tradesperson today.
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