Replacing a VELUX window is a smaller job than people expect, and a bigger decision than it looks. Out comes the old unit, in goes the new one, and the rain stays outside. In between sits a handful of choices that decide how the room feels for the next few decades.
A skylight, or rooflight as the trade calls it, is the one window that looks straight at the sky. Side windows face fences, walls and neighbours. When yours is misted, stiff or stained, replacing it is a chance to put that clear view back in good order.
This guide follows the job in the order it happens: reading what you have, choosing between a straight swap and an upgrade, getting the flashings right, meeting the rules and finishing the inside. It is written for homes around Milton Keynes, and it links to the VELUX windows hub for the wider picture.
Start by reading what is in the roof today
Before anything is ordered, the existing window is read like a label. Its type, its size code and its age tell us what a like for like swap would involve. The size code sits on the frame or the sash and is explained in full in VELUX size codes explained. Width comes from the letter and height from the digits, so a quick look gives the starting point.
Next we look at the glass. Double glazing that has lost its seal shows up as a grey mist or beads of water between the panes. That is condensation inside a sealed unit, and it cannot be wiped away because it is not on a surface you can reach. A frame that has gone brittle, cracked or discoloured tells a similar story about the materials.
Then we look at the room below. A bedroom that overheats, a kitchen that steams up or a loft room that never feels fresh each point to a different upgrade. The window that suits a landing is not the window that suits a loft bedroom, so the room comes into the conversation early.
Like for like: the straight swap
A like for like replacement means the new window takes the old one's place without changing the opening. The roof covering around it stays, the rafters stay, and the size matches. It is the simplest route and the quickest, because nothing structural moves.
The process runs in a set order. The old window and its flashings are lifted out, the surround is checked for damp or rot, and the new frame is fixed to the rafters. Flashings are fitted over the frame and woven into the roof covering, then the sash is hung and tested. Inside, the lining is closed up and finished so the reveal looks tidy.
A straight swap suits a window that fits the room well but has simply worn out. It also suits a roof where the covering is sound and the owner wants as little disturbance as possible. The fitting itself is one or two days of work on a standard rooflight, and we plan the weather around it so the opening is never left exposed overnight. The question of timing has its own answer in how long a VELUX installation takes.
Not every window is a candidate. If an older model has been discontinued, the nearest current size may differ by a few centimetres, and that small difference is handled with a matching flashing kit rather than by cutting rafters. The survey confirms which case you are in.
When the opening itself changes
Sometimes the better answer is a different window altogether: wider, taller, or moved up or down the slope. That is a new opening rather than a swap, and it changes the work. Rafters may need trimming and doubling, the roof covering is cut back and relaid, and the lining inside is rebuilt to suit.
If you are wondering whether a window can simply grow, read can a VELUX window be made bigger. The short version is that it depends on the spacing of the rafters and what sits above and below the window.
Changing the opening is worth it when the room has outgrown the old window. A loft converted into a bedroom, a landing that was once a dark corridor or a kitchen where the old window sits behind a worktop can all gain from a better position. A new opening also lets us put the window where the light falls on the floor you actually use.
It is a good moment to think about two windows rather than one large one. To light a long room evenly, two smaller units placed beside each other keep each opening within the rafter spacing.
Upgrades worth choosing while the roof is open
With the old unit out, choices that were hard to make before become simple to make now. Here are the ones people ask about, set side by side so you can compare them. Each is explained in the sections that follow.
| Upgrade | Suits | What it does |
|---|---|---|
| Better glazing | Cold or noisy rooms | Raises insulation, cuts draughts |
| Blind or shutter | Bedrooms, lofts | Controls glare, heat and darkness |
| Electric opening | Out of reach windows | Opens by remote or wall switch |
| Rain sensor | Kitchens, bathrooms | Closes the window when rain starts |
| Ventilation flap | Rooms kept shut | Lets in fresh air with the sash closed |
Glass, blinds and the way it opens
Glass is the first choice. Modern units are double glazed or better, with coatings and gas fills that slow the loss of heat. Different glass options suit different rooms, and the manufacturer's range includes laminated panes for quiet, tougher panes for exposed roofs and obscured finishes for bathrooms. Choosing the glass at the survey keeps the quote fixed and the order simple.
Blinds come second. A skylight lets in the sky, and in summer that can mean too much of it. Blinds made for the window sit in side channels so they hold their shape, and they come in blackout, light filtering and insulating forms. Fitting the blind while the window is new is easier than adding it later.
Then comes the way the window opens. A manual sash is fine within reach. Higher up, an electric motor with a remote control saves a stool and a stretch. A rain sensor closes the window on its own, which suits a room you leave open on a warm day.
Ventilation deserves a word of its own. Damp air rises, and a roof window is a natural place for it to leave. A small trickle vent keeps air moving even when the sash is shut, which helps a bathroom or a loft bedroom stay fresh.
Flashings: where the weather is kept out
The window is only half of what keeps a room dry. The flashing is the metal collar that joins the frame to the roof covering, and it has to suit the covering. A slate roof, a plain tile roof and a profiled tile roof each use a different kit, shaped to lie flat against the pieces around it.
A flashing kit is chosen at the survey and arrives with the window. It is fitted in a fixed order: the lower apron first, then the sides, then the top, each lapped over the one below so water runs down and out. A neat fitting leaves no gaps for wind-driven rain to reach.
Old flashings should not be reused on a new window. They have taken years of weather, and a new frame deserves new metal. Keeping the roof watertight is the most important thing the job does, and it is why the surround is inspected before the new frame goes in.
If the covering around the old window is tired, we say so in the written quote rather than on the day. The quote you are given then holds, whatever the roof turns out to hold.
The rules that apply to a replacement
Windows in an existing house are covered by Part L of the Building Regulations. The relevant figures are in Approved Document L, Table 4.2, and they set the standard a new unit has to meet. Both figures below are limits on heat loss, so a lower number means a warmer room.
Roof windows and rooflights are separate categories in the table. A VELUX window in a pitched roof is a roof window; a flat glazed unit on an upstand is a rooflight. The survey records which one you have so the right limit is checked.
Heat-loss limits for a replacement
- 1.4 W/m²K
- Part L Table 4.2 allows a roof window this much, or WER band B
- 2.2 W/m²K
- the same table allows a flat rooflight this much
Escape windows, Building Control and planning
In an emergency a roof window can be the way out, though only from a storey whose floor is no higher than 4.5 m above the ground. Above a two-storey house, the loft needs a protected stair as its route out; a roof window cannot stand in for it. If your loft room is the one in question, the survey goes through this before a window is chosen.
Everything is installed to current Building Regulations, and if the job is notifiable, telling Building Control is part of our service. That removes a form from your list and keeps the paperwork in order should you sell the house. Ask us for the details at the survey and we will show you what has been filed.
Planning is a separate matter. A window of similar size and appearance in the same place is a different question from a larger opening, a new position or a street with extra protection. Whether your street carries extra protection through a conservation designation is for the council's planning team to say, and our planning guide shows how to ask.
Inside the room: finish and tidiness
A replacement is judged from underneath. The lining around the opening, the plaster, the paint and the clean edge where the ceiling meets the window frame are what you will look at every day. We finish the inside as part of the job, so you are not left with a bare reveal and a list of trades to call.
Dust sheets go down before work starts. The room is protected, the floor is covered and the debris leaves with us. When the new window is in, the lining is closed, the surface is made good and the sash is checked for a smooth action.
This matters most in rooms people sleep in. A bedroom with a new, blacked-out window and a freshly finished reveal feels like a different space the same evening. We offer a 10-year workmanship guarantee in writing, so the finish and the fitting are both covered.
Before you book a replacement
- Which room does the window serve?
- Is the glass misted, or the frame cracked?
- Do you want a blind fitted at the same time?
- Would an electric opening help?
- Is the opening staying the same size?
Worth asking before you replace
Do you replace VELUX windows around Milton Keynes?
Yes. We install and replace roof windows and rooflights for homes in Milton Keynes and the towns around it. No fee applies to the survey, and the quote you are given in writing holds for the whole job.
Can I keep my existing blind when the window is replaced?
It depends on whether the new window is the same size and model range as the old one. A blind made for one frame may not fit another, so we check at the survey and, where it does not fit, quote a matching blind with the window.
Will replacing the window make the room warmer?
A new unit with modern glazing and a fresh seal around the frame holds heat better than an old one with a failed seal. It is built to meet the current Part L figures, which is part of what the Building Regulations ask of a replacement.
