Roof Window / Velux

A roof window (frequently called a Velux after the dominant brand, much like “Hoover” for vacuum cleaners) is a window fitted into a sloped roof surface. It brings natural light and ventilation into rooms directly beneath the roof — loft conversions, attic bedrooms, top-floor bathrooms, and stairwells. Roof windows are one of the most transformative additions in a renovation, turning dark, unused roof spaces into bright, liveable rooms.

Roof window vs. skylight vs. dormer

These terms are often confused:

• Roof window — sits within the plane of the roof slope. Opens for ventilation (usually top-hung, centre-pivot, or bottom-hung). The most common choice for loft conversions.
• Skylight — a fixed (non-opening) glazed panel in the roof. Provides light but no ventilation. Simpler and cheaper but less functional.
• Dormer — a structural extension that projects vertically from the roof slope, creating additional headroom and floor space. Much more expensive and complex than a roof window, but adds useable space.

Types of opening

• Centre-pivot — the sash pivots on a central hinge, rotating 180 degrees for easy cleaning. The most popular type for standard installations.
• Top-hung — hinges at the top, opening outward from the bottom. Gives an unobstructed view when open and more headroom at the bottom of the window.
• Conservation style — designed to meet the requirements of listed buildings and conservation areas. A central glazing bar and flush exterior profile replicate the look of traditional roof lights.
• Balcony — the bottom section folds out to create a small juliet-style balcony. A premium option that adds drama to a loft conversion.

Sizing and placement

Roof windows come in standardised sizes. Building regulations typically require that the glazed area of roof windows in a habitable room equals at least 20% of the floor area. Placing windows higher up the roof slope lets light penetrate deeper into the room. Your architect or designer will recommend the size and position based on the room layout, roof pitch, and structural spacing of the rafters.

Installation considerations

• Structural — the roof structure may need trimming (cutting and reinforcing rafters) to create the opening. This is standard practice and your contractor or carpenter will handle it.
• Insulation — the area around the window must be carefully insulated to prevent cold bridging and condensation. This is a common weak point in poorly installed roof windows.
• Flashing — a weatherproof flashing kit (specific to your roof tile or slate type) seals the window into the roof. Correct flashing is essential to prevent leaks.
• Double glazing — all modern roof windows come with double-glazed units as standard. Triple glazing is available for better thermal and acoustic performance.
• Blinds and shading — roof windows receive direct sunlight and can overheat rooms in summer. Integrated blinds (blackout, roller, or venetian) are available from the manufacturer and are worth including in your specification.

Planning permission

In the UK, roof windows generally fall under permitted development (no planning permission needed) unless the property is in a conservation area or is listed, or the windows would be on a roof plane facing a highway. In the US, a building permit is typically required. Check with your local authority and include any permissions in your renovation timeline.