Replacing an old VELUX with a modern equivalent

Roof windows do not last forever. The timber sash, the glazing seals, the gaskets and the flashing all age at different rates, and the point at which the whole unit is better replaced than left alone is usually obvious once you know what to look at.

The sealed glass unit is the part that tends to go first. When the perimeter seal on a double-glazed pane fails, moisture gets into the cavity and you see misting or a permanent smear between the two panes that no amount of cleaning shifts. That unit has stopped performing thermally, and on an older window the replacement pane is often no longer made in the exact spec, so a new complete window is the sensible route.

Then there is the timber. A pine sash that has been in a bathroom or a kitchen for two decades, taking condensation every winter, can soften at the bottom rail where water sits. Paint that will not hold, dark staining, a sash that no longer closes tight against its gasket: these are signs the frame itself is tired. The handle mechanism and the friction hinges wear too, and a window that drops open or will not stay shut has usually done its time.

One thing to separate out. If your only problem is water coming in around the outside of the window after a storm, that can be a flashing or a lead detail that has lifted, and a single failed seal on an otherwise sound recent window is a roofer’s job rather than a reason to change the whole unit. But a window that is old, misted, draughty and stiff is not a candidate for making do with. That is a replacement, and a replacement is straightforward work: the old unit comes out, a current one goes into the same rafter opening, and the roof is made good around it.

Every VELUX window carries a data plate, and finding it turns a guessing game into a five-minute job. Open the window and rotate the sash until it is roughly horizontal, and look at the top edge of the moving frame. There is a printed plate there with two things that matter: the type code and the size code. Write both down before you do anything else.

The size code is a letter followed by two digits, and it maps to a set pair of dimensions. The letter is the width, the number is the height. These sizes have stayed consistent for a long time, which is the single biggest reason a modern window can drop into an old opening without touching the roof structure. The common ones look like this.

The width letters run C at 550mm, F at 660, M at 780, P at 940, S at 1140 and U at 1340. The height numbers climb in steps: 02 is 780mm, 04 is 980, 06 is 1180, 08 is 1400. Once you can read the code you can see at a glance whether a like-for-like swap is on the table or whether you are choosing to go bigger, which is a different conversation because it means opening up the roof.

The type code tells you what family the window belongs to. GGL is the pine centre-pivot, GGU the same window in a white polyurethane finish that suits damp rooms. GPL and GPU are the top-hung versions, where the sash swings out from a top hinge and gives you a clear head-height view and an escape opening. The digits after the code identify the glazing generation and the operation, and the year of manufacture is stamped on the plate as well. Take a photograph of the whole plate. Whoever specifies your replacement will want every character on it.

This is where a straight copy costs you. The glass in a roof window has moved a long way in twenty years, and the gap is widest exactly where south Essex feels it most: summer heat.

Take winter first, because it is the easier number. An older double-glazed roof window from the 1990s or early 2000s might have a whole-unit U-value up around 2.0 to 2.9 watts per square metre per degree, depending on how basic the pane was. A current double-glazed unit sits nearer 1.2 to 1.4, and a triple-glazed one can reach 0.8 or lower. In plain terms, the modern window loses somewhere around half the heat of the one it replaces on a cold night, and the inner pane runs warmer, so the streaming condensation people put up with on old rooflights largely stops.

Summer is the part almost nobody mentions on a quote, and it is the part that decides whether you enjoy the room. The number that governs it is the G-value, the share of the sun’s energy that passes through the glass and becomes heat inside. Older roof windows were rarely specified for it at all, so a tired unit on a south slope is very often clear glass with a G-value somewhere around 0.6, letting roughly sixty per cent of the solar energy straight in. Modern solar-control glazing brings that down to around 0.3 or lower while keeping the room bright, because the coating rejects the invisible near-infrared that carries the heat rather than simply darkening the glass.

Read the two right-hand columns together. Replacing a 1990s window with a modern clear double glazed one solves your winter heat loss and does very little for the July afternoon, because the G-value has barely moved. Replacing it with solar-control glass solves both. On a north-facing slope, where there is no direct sun to control, the clear unit is the right choice and you take all the daylight you can get. On a south-facing slope facing the water, the solar-control unit is the one worth paying for, and choosing it is the whole point of thinking about the glass before you think about the frame.

Southend-on-Sea faces south across the Thames Estuary, which is unusual for an English seaside city and is the reason a like-for-like swap so often disappoints here. A roof window is not a wall window. It faces up towards the sky, so at midsummer noon it catches the sun almost square-on and collects far more energy per square metre than a vertical window on the same wall would. Point that upward-facing glass at an open southern horizon with a large reflective sheet of water beyond it, and you have built a solar collector into the ceiling.

The old window on that slope was almost certainly never specified for the aspect. When it went in, solar-control coatings for roof windows were either unavailable or an expensive extra nobody was offered, so a south-facing loft bedroom got the same clear glass as a north-facing landing. Replacement is your one chance to correct that without opening the roof again. The rafter opening is already there, the size code is already known, and the only decision that changes the room is which glass goes back into the hole.

South Essex has an enormous stock of 1930s semis and Victorian and Edwardian terraces, and the loft conversions in them tend to put windows into whichever slope the room happens to offer. A great many of those slopes face south or west. If yours does, the replacement conversation should start with the compass and not with the catalogue. A window on a north or east slope can be a simple modern swap. A window on a south or west slope wants solar-control glass, and quite often an opening that vents the hot air off the top of the room as well.

The other thing that has moved on since your old window went in is how it opens. A centre-pivot window high in a slope is awkward to reach, and a lot of old ones get left shut all summer because nobody wants to fetch a pole twice a day. Replacement is the moment to put that right.

A modern electric roof window opens and closes at the press of a wall switch or from a handset, and closes itself automatically when its rain sensor detects the first drops. That matters most on a south-facing room, because the fastest way to shed the heat that solar-control glass has not stopped is to open a vent at the very top of the space and let the stratified hot air escape upwards. On high ceilings and over stairwells, an electric opening unit earns its place the first hot week you have it. Solar-powered versions run off a small panel on the frame and a rechargeable battery, so they need no wiring into the room below, which makes them the practical choice when a swap would otherwise mean chasing cables across a finished ceiling.

Two more upgrades come with a new unit. The flashing kits are better than the ones from twenty years ago and are matched to your roof covering, whether that is slate, plain tile or a deep profiled interlocking tile, so the seal into the roof is more reliable than the detail it replaces. And the modern accessory range means blinds, insect screens and awnings clip straight onto the new frame, where an old window often takes nothing at all. An external awning blind, which sits outside the glass and stops the heat before it reaches the pane, is worth knowing about on a south slope because it works alongside solar-control glass rather than instead of it.

Replacing a roof window is notifiable work under the Building Regulations, and it is worth understanding why so the paperwork does not surprise you. Part L covers thermal performance, and it sets a maximum U-value that a replacement rooflight in a dwelling has to meet, which is one more reason a straight copy of a 1990s pane is not an option even if you wanted one: the old spec would not pass. Part K covers safety, and it is the reason any glass fitted overhead uses a laminated inner pane that holds together if it is ever broken. A current window is built to satisfy both as standard.

Replacing like for like in the same opening does not usually need planning permission on a house, because roof windows that project no more than 150mm beyond the roof plane and sit below the ridge are generally permitted development. The exceptions matter locally. Flats and maisonettes, listed buildings and conservation areas fall outside that, and the Leigh Cliff and Leigh conservation areas carry an Article 4 Direction that removes permitted-development rights for changes to roofing and windows, so a planning application is needed there where it would not be a mile away. Treat all of this as general guidance and check your own address with Southend-on-Sea City Council or your local authority before you commit, because the line between permitted and not is drawn property by property.

On the Building Control side, the notification is ours to make. We notify the work to the relevant local authority on your behalf as part of the installation, so the replacement is recorded and signed off without you having to arrange anything. If you enlarge the opening rather than reuse it, that is a bigger piece of work with its own structural and notification requirements, and it is a different quote.

You can settle a replacement in a single survey if you walk into it with the right questions. The old window has already told you most of what is needed. The data plate gives the size and type, the slope tells you the aspect, and the room tells you how it is used. What is left is the specification.

• Reuse the opening or enlarge it. Same size code means the roof structure stays untouched and the job is quicker and cheaper. Going bigger means cutting and trimming rafters, and it is worth doing only if the room genuinely needs more light or a compliant means of escape.
• Match the glass to the aspect. North and east slopes take clear or standard glass for maximum daylight. South and west slopes take solar-control glass. Ask for the G-value in writing, not a vague “energy efficient”, because the window energy ratings used in this country are weighted towards winter and say nothing about summer heat.
• Decide how it opens. Manual centre-pivot is fine within easy reach. High or out-of-reach windows, and any south-facing room you want to vent in summer, are the case for an electric or solar opening unit with a rain sensor.
• Match the flashing to your roof. The kit differs for slate, plain tile and profiled tile, and getting it right is what keeps the weather out for the next twenty years.
• Confirm who notifies Building Control. On our installations we make that notification for you, so the replacement is properly recorded.

As a rough guide to budget, a like-for-like replacement of a standard roof window with a current unit, glass, flashing and fitting included, tends to fall in the region of a few hundred to just over a thousand pounds depending on size, glazing spec and whether it is manual or electric, with larger units and triple-glazed solar-control glass at the upper end. Enlarging the opening or reaching an awkward slope adds to that. Treat those as industry-typical ranges rather than a set price, and get the specification measured against your own roof.

Roof windows on a pitched slope are the core of what we do, and the same orientation-first thinking runs through all of it, from a single bedroom swap to a full pitched-roof installation. If you are ready to replace one or several, our VELUX window installation service handles the survey, the glass specification for each slope and the Building Control notification in one job. When you want a figure for your own roof, ask us for a quote with the size code, the U-value and the G-value written down for every window, so the replacement you buy is measurably better than the one it takes out and not just newer.