VELUX window installation in Exeter

People use skylight, rooflight and roof window for the same thing, but the trade draws a line. A roof window is made for a sloping roof. It has a frame that sits in the pitch, a sash that opens, and a flashing that ties the frame into slate or tile. A rooflight is the wider family, and includes flat glass set on a kerb in a flat roof. VELUX is the best known maker of roof windows in Britain, and for most pitched roofs in Devon it is the product people mean.

Inside, a roof window is a different animal from a fixed pane. The sash pivots or hinges so you can open it, clean it, and let air through. Most are made for rooms people live in: a loft bedroom, a study under the eaves, a stair landing that has never had a window. They are sized to fit between rafters, which is why a standard window often drops in with little structural work, and why a wider or taller one asks for trimmed timbers and a calculation.

The parts are worth knowing because every quote and every fitting instruction uses them. The names below are the ones you will see on a drawing or a delivery note.

Frame

The outer box that is fixed to the rafters and flashed into the roof. It stays in place when the sash is replaced.

Sash

The opening part that carries the glass. On a pivot window it turns about a central axis, and on a top-hung window it swings out from the top.

Sealed unit

The double-glazed panel itself, made of two panes with a warm-edge spacer and a gas-filled gap between them.

Flashing

The metal skirt that dresses over the roof covering around the frame and carries water down and away.

Lining

The boarded or plastered return that connects the frame to your ceiling, and the part you see from the room.

VELUX windows are made in a fixed range of sizes, each identified by a two-letter code and a two-digit number, such as MK04. The letters give the width and the digits give the height class, so the code tells you the frame size without a tape measure. On an existing window, the code is on a label inside the frame, usually on the side of the sash opening, and finding it is the first step in an accurate quote for a like-for-like swap.

On a new window, size is a design choice. Rafters in a Devon roof are rarely in the same place as the catalogue, and rafter spacing varies hugely between a 1930s semi in Heavitree and a Georgian terrace in St Leonards. A frame that fits between two rafters usually needs no structural work, whereas one that crosses a rafter needs the rafter trimmed back and the opening headed off with new timbers. That is building work, and we cover it in the planning chapter below.

It is tempting to go as large as the roof allows. The better test is what the finished room looks like. A tall window in a shallow sloping ceiling may fill with light at the top and leave the lower reveal in shadow. A wider, shorter one can sit better, and we check the proportion against the ceiling slope and the plaster line on the wall below before the opening is cut. You can read more in our guide to VELUX window sizes and codes, and the short answer to how to find your VELUX size code walks through the label on the frame.

It also helps to know what the letters in the model code stand for. For example, the question of what VELUX GGL means is answered on its own page, because it comes up whenever someone is reading an old quote or a delivery note.

If you are weighing up a swap, a few minutes with a tape and your phone camera before the survey makes the quote more accurate.

A roof window is only as good as the junction between the frame and the roof covering. The flashing is a purpose-made kit, and it is matched to the covering: one pattern for flat concrete tiles, another for profiled tiles, another for natural slate, and another for the thicker coverings found on some older roofs. Kit codes begin with ED, and the installer picks the one that matches your covering and the window size. A kit chosen without checking the covering is a common source of problems years later.

The detail changes the work. On a slate roof, the slates around the opening are lifted, set aside, and re-laid over the flashing once the frame is in, and any slates that crack on the way are replaced from a matching stock. On a tile roof, the battens are cut and trimmed, and the tiles are cut back to the flashing line with a clean edge. On an older roof with an uneven covering, such as a mix of handmade tiles on a cob cottage, the flashing needs a little more forming to sit well on the roof as it is, not as it was drawn.

Below the covering, the rafters, the insulation and the underlay all need to be dealt with correctly. The underlay is dressed into a collar that directs any water that gets under the covering down and out around the frame, not into the lining. Insulation is packed around the frame so there is no cold gap between the window and the ceiling, which is where condensation starts. We write these into the finish schedule alongside the lining detail, because they decide how the room behaves in winter.

The guide to VELUX flashing kits sets out the differences between kit types in plain terms, and is worth reading before you compare quotes, because two quotes that look identical on the glass can allow different flashing.

Most roof windows on a house are permitted development, which means you do not need to apply for planning permission. The right sits in the General Permitted Development Order, Schedule 2, Part 1, Class C. It holds while the window stands no more than 150mm off the roof plane and its top stays below the roof's highest point. A VELUX window is built to sit close to the covering, so it normally clears the first test. A side-facing slope brings one more condition: the glass must be obscure and the sash fixed, unless the part that opens is higher than 1.7m above floor level.

There are exceptions, and they matter in this part of Devon. On a listed building, external work of any kind needs listed building consent first. Householder permitted development is not available to flats and maisonettes. Conservation areas allow Class C, but a local Article 4 direction can remove it, which is worth checking for streets in Exeter, Topsham and Exmouth. National parks, including Dartmoor and Exmoor, are Article 2(3) land, where some rights are restricted. Where it varies, we tell you to check with the local planning authority, such as Exeter City Council, East Devon District Council or the Dartmoor National Park Authority, and we help you word the question.

Building Regulations are a separate matter, and they apply even where planning does not.

Building Regulations are a separate matter, and they apply even where planning does not. Adding a roof window to a roof that is already built counts as building work. A rafter is cut or trimmed, which alters the structure, and the window itself has to satisfy Part L, the conservation of fuel and power. Building Control approval is needed. A full replacement, frame and glass together, is notifiable as well, since windows are controlled fittings and the new one must meet Part L. Changing only the sealed glass unit and keeping the frame falls outside notification. Where notification is required, we submit it.

Two further points apply to loft rooms. A roof window can serve as an escape window if it meets the size and position rules in Approved Document B, so it is worth checking before a bedroom is added, and the rules vary with the storey and the escape route. An opening window can also help with purge ventilation under Part F. These are points to settle at the survey, with the room's layout in front of us, not rules of thumb to apply from a distance.

A replacement is simpler on the roof and more variable inside. If the new window is the same size as the old one, the opening in the roof already exists and the rafters are already trimmed, so the work is mostly swapping the frame and renewing the flashing. We cover the options in our answer on whether a VELUX window can be replaced like for like.

The old lining is where the surprises are. A window fitted twenty years ago may have a lining that was never insulated, a flashing that is the wrong type for the covering, or a plaster edge that has been damaged by years of condensation. A replacement is the right time to improve these, because the lining is disturbed anyway. Rather than re-fixing the old plasterboard, we can form a better reveal with insulation behind it and a clean bead at the edge.

There are two kinds of replacement, and the difference is notifiable work. Changing the sealed unit alone, in a frame that is sound, is not notifiable and does not disturb the lining. Changing the frame and sash is notifiable, because it is a new controlled fitting that must meet Part L, and the frame and flashing go in new. You can also change the size or style at the same time, such as from a pivot to a top-hung, which turns the job into a new opening with all the work that implies.

How long a window lasts is a fair question to ask before you decide, and the answer depends on the glazing, the frame and the exposure. Our short answer on how long VELUX windows last explains what ages first. If your concern is a misted sealed unit rather than a whole window, our page on misted and failed rooflight replacement sets out that route.

The other products we fit are on their own hubs. If you are weighing a VELUX against a different roof window, see roof window installation, or for powered options electric and opening rooflights, and for the thermal side energy efficient rooflights.

The note covers one room on one page, so it stays short enough to read in a minute.

You can, legally, as the homeowner. In practice, the work is awkward: it means opening the roof, trimming rafters in some cases, and weathering the flashing correctly. It also needs a Building Control notification and a roof sound enough to work on. If the lining and ceiling will be left unfinished, you will still need a plasterer. We install and finish the whole opening together, so there is one person responsible for the water path and the plaster.