Replacing an Old VELUX Window

Windows rarely fail all at once. They give hints, and the hints are easier to read once you know what you are looking for. A quick look from inside on a bright day, and again after rain, tells you a good deal before anyone climbs a ladder.

The most visible sign is moisture trapped between the panes of glass. Condensation or a permanent mist inside the sealed unit means the seal has gone and the gas or dry air that was sealed in has been replaced by damp air. The glass will no longer insulate as it should, and the mist does not clear. When that is the only fault, the choice is between a new sealed unit and a whole new window.

Other signs are on the frame and sash. Timber that has softened, paint or lacquer that has lifted, a sash that no longer closes flush, hinges that have worn or a window that sticks all point to age. Water stains on the lining or the plasterboard around the window suggest that moisture is getting past the frame or the flashing. Draughts around the sash on a windy day, or a cold patch on the wall beneath the window, show that the seals have hardened.

Age and the unit itself

An old window may still work, but it will probably not match current standards for heat loss. Once a unit is beyond repair, or the frame has rot, replacement is the sensible route. A survey separates that from a fault that sits only in the flashing or lining.

Water stains near a roof window do not always mean the window is at fault. Condensation on the inside of cold glass can run down the frame and soak the lining, which looks just like a leak. A cracked or slipped tile above the window can let water in nearby. Flashing that has lifted or corroded can let rain track beneath the covering. Each of these has a different fix.

This is why a replacement starts with looking, not with ordering a unit. On the roof we check the flashing, the tiles or slates above and either side, and the head and sill of the frame. From the loft we look at the timbers around the opening, the insulation and any signs of dampness. We also look at the lining and the internal finish. When we find that the window is sound and the fault is elsewhere, we say so at the survey, before any quote.

Chasing a leak and want the likely causes first? The page about a leaking roof window takes them one by one.

When the new window is the same size as the old, the existing opening is reused. The work follows a clear order. The old sash and frame are removed from outside, along with the flashing. The roof covering around the opening is lifted back far enough to fit the new flashing kit. The new frame is set in the opening and fixed to the rafters and trimmers. The flashing goes on in the correct sequence, so that water is led over each course and away from the frame. The covering is relaid. Finally, the lining inside is made good and the window is tested.

The size comes from the label on the old unit, or from a tape, and is matched to the maker's code. How that code works is explained in the size code guide.

The roof stays covered

A good replacement is planned so that the roof is never left open to the weather. The old window is not removed until the new frame, flashing and a protective covering are ready to follow, and the work is timed with the forecast in mind. A simple single rooflight is a day or two at roof level.

Replacing a window is often a good moment to ask whether the old size still suits the spot it is meant to light. A window put in years ago may be too small for a room that has changed use, or in a position that no longer matches where you stand. A new opening, whether larger or in a different place, gives a different result from a straight swap.

A larger opening means cutting back the roof structure and fitting new trimmers, so the loads from the rafters are carried around the opening. That alters the roof structure, so the work needs approval, from the local authority or a registered building control approver. Where it has to be notified, we do the notifying. A smaller opening means closing part of the old hole and re-forming the covering around the new frame.

You can also change the type of window, for example from a manually opened unit to an electric opening one, or to a fixed unit where opening is not needed. Electric windows suit places you cannot reach, such as over a stairwell, and some can be linked to a rain sensor that closes them. A window in a bathroom may benefit from a different glazing finish. These options are worth raising at the survey.

The glass in a new roof window is a sizeable step up from an older double glazed unit. Current windows are made to meet the energy limits for roof windows, they have better seals and they often include laminated or toughened inner panes for safety. Many can be specified with extra noise reduction for rain, which can matter in a loft bedroom.

Ventilation matters as much as the glass. Most roof windows can be opened to a venting position that lets air through while the window stays weathertight, and some have a small trickle vent along the top of the frame. Setting the sash a little ajar at the top lets warm air leave high in the room, which makes a noticeable difference in summer.

Blinds and the finish

Blinds and roller shutters are often fitted at the same time as the window. Roof window blinds are made to match a particular window size, so the code on the unit guides the order. Heat control in a south-facing roof is much easier with an external blind or a reflective inner blind than with glass alone. The lining inside is usually finished in white to bounce daylight further into the room.

A like-for-like roof window swap falls under Part L as a controlled fitting. Table 4.2 of Approved Document L, which covers existing dwellings, sets a ceiling of 1.4 W/m²K for the new roof window (or band B on its energy rating), while a rooflight or lantern has a 2.2 W/m²K limit. Certification through a competent person scheme is open to this kind of replacement.

A new opening, or any change to the roof structure, has to be notified to the local authority or a registered building control approver first. We deal with that for you.

Planning and protected buildings

Swapping a window that is already there is not normally a planning matter. A brand new rooflight on a house is usually permitted development, with conditions kept in the planning guide. Householder rights do not extend to flats and maisonettes. Most alterations to a listed building call for listed building consent. Where an Article 4 direction removes permitted development, as it can in a conservation area, ask the council first; in the city that is Derby City Council.

Escape windows

A window serving a bedroom in the roof may be the escape route. A roof window works as an escape route only up to 4.5 m above ground; the opening has to clear 0.33 m² and be 450 mm across both ways. For a loft conversion over two storeys, the answer is a protected stair.

The process begins with a free survey. We look at the window, the roof and the room, and we talk about the spot you want lit. The quote is written, with a fixed figure that holds on the day. If you go ahead, we agree a date, protect the room and carry out the work. The guarantee on our workmanship is written and lasts 10 years.

Comparing a like-for-like unit with other options? See the alternatives page. If several windows in the roof are tired, doing them together saves disruption, as this page on a set of roof windows explains.

Tell us about the window and attach a photo in the survey form. You can also reach us on WhatsApp, 07880 284133, or at info@skylarkskylightsderby.co.uk.