Roof Windows Buying Guide: Velux Sizes, Styles and What to Expect from Installation

Understanding the Velux Size System

Velux windows are sized by a two-part code: a letter indicating the width and a two-digit number indicating the height. The most common sizes for residential Velux installation in the UK are: CK02, CK04, CK06, MK04, MK06, PK06, and SK06. The most popular sizes on standard 3-bed semis in Aylesbury are MK06 (78×118cm) and SK06 (114×118cm). The size you can fit is constrained by your rafter spacing — a 114cm-wide window needs a clear opening between rafters of at least 117cm.

Most retrofitted loft conversions on Aylesbury's 1930s–80s estate housing have rafter spacing of 400–450mm or 600mm (the two most common timber frame spacings). At 400mm spacing, you'll need to cut and insert a trimming rafter to create the opening for anything wider than the CK range. This adds to the cost but is standard practice.

The Three Flashing Options

EDL (Recessed). The standard flashing for most UK tile profiles. Positions the window in the roof plane with a 2–3cm recess. Suitable for concrete interlocking tiles, clay plain tiles, and most slate profiles. The correct flashing for 1970s–80s estate housing in Aylesbury. Included in the base Velux price.

EW (Flat Tile). For very low-profile tiles that allow the window to sit closer to the roof surface. Used on some modern concrete tile profiles. Less commonly specified in Aylesbury.

EDW (Conservation). A specialist flashing that sits significantly lower in the roof plane, designed to minimise the visual impact of the window on the roofline. Required by many local planning authorities for conservation area installations and listed building work. Adds approximately £120–£200 to the installation cost. The correct specification for properties near Aylesbury's town centre conservation area.

Glazing Options

The standard Velux GGL (centre pivot) comes with an Energy performance pack (double glazing with low-e coating, centre pivot opening). The GGU is the polyurethane-framed version with higher moisture resistance — preferred for bathroom and kitchen locations. The GHL (hinged at top) opens to almost flat, improving access for cleaning and ventilation.

For improved thermal performance, the Velux GGL Integra (electrically operated) and GGLS (solar-powered) versions are available. On Aylesbury's pre-1950s properties where loft insulation is often marginal, the triple-glazed option (available on some sizes) can make a meaningful difference to thermal comfort in a converted loft.

What a Professional Installation Includes

Supply and fit of the specified window; trimming rafters to create the correct opening with a lintol above; insulating the reveal with rigid insulation board; internal plastering or dry-lining of the reveal; installation of the correct flashing kit on the outside; and weatherproofing the junction between the flashing and the tile on both sides. Lead soakers may be required at the sides depending on the tile profile. The window is warranted by Velux for 10 years, and our installation workmanship for the same period.