VELUX Window Installation

Most people call any roof window a VELUX, the way they call any vacuum a Hoover. It is a Danish manufacturer, it has been making pitched-roof windows since 1942, and its windows sit in a very large number of loft conversions, rear extensions and converted garages across south Essex. We fit them, we replace tired ones with current models, and we spend a fair amount of the survey talking customers out of the glass that came up first on the price list. This page covers what a VELUX window installation Southend homeowners actually get: how to find your size code off the data plate, the difference between a centre-pivot and a top-hung sash, why the flashing kit matters as much as the window, what the glazing options do on a slope that faces the estuary, and what the whole thing costs.

01New windows and like-for-like replacements

Two quite different jobs share the same name, and the price gap between them is wide enough that it is worth being clear which one you are asking for.

A new opening

Nothing exists yet. The roof is intact, the room below is either a loft being converted or a room with a sloping ceiling and no daylight. We open the covering from outside, cut and trim the rafters, build a timber frame within the roof structure, set the window into it, weather it with a flashing kit, and then line the inside so the reveal is plastered and finished. On a bare loft the internal lining is often somebody else’s trade, on a finished room it is usually ours. Expect a day for a straightforward single window, two for a pair or for anything awkward to reach.

A like-for-like replacement

There is already a roof window and it has come to the end of its useful life. The seal on the glazing unit has gone and the pane is permanently hazy. The timber sash has softened at the bottom rail. The old lacquered pine has gone grey because it was never recoated. Or it is simply single glazed, which was still being fitted into the 1980s, and the room is unusable in February and unbearable in July.

If the replacement is the same size code as the original, the structural opening stays exactly as it is. The old window comes out, the new one goes into the same trimmed frame, and the difference in the room is entirely a matter of glass and airtightness. That is a half-day job for one window on an accessible slope. What does not carry over is the flashing. The flashing kit is sized to the window and to your roof covering, it work-hardens over twenty-five years of thermal movement, and lifting it off usually deforms it. Anyone quoting a replacement without a new flashing kit is quoting the wrong job.

Two things get confused with a replacement and neither of them is our work. If the roof covering around the window has failed and water is tracking down the rafters, that is a roofer’s job first and a window question second. And if the window is sound but the operating handle or a hinge has worn, the manufacturer sells replacement parts and a competent joiner can deal with it. What we do is take a window out and install a properly specified new one, which is the honest answer when the unit itself has reached the end.

A third case sits between the two. Sometimes a customer wants to go bigger: a 550mm by 780mm window from 1994 in a room that now needs twice the daylight. That is a new opening in structural terms even though there is a window there already, because the rafters have to be cut wider and the trimming redone. Price it as new work.

02Reading the data plate: finding your size code

Before anyone can quote a replacement properly, somebody needs the size code. It is printed on a data plate on the window itself, and you can find it in about ninety seconds without leaving the room.

Where it is

On a centre-pivot window, open the sash and rotate it until it is roughly horizontal, which is the position you use for cleaning. The plate is on the top edge of the sash, in the middle, facing you once the sash has turned. On a top-hung window, open it out and look at the top rail of the sash from underneath. On older windows the plate is a small foil sticker; on newer ones it is a printed panel. Take a photograph of it rather than trying to write it down at arm’s length above your head.

What is on it

The plate carries several pieces of information and only some of them matter to you.

• The type code, three letters, telling you what kind of window it is. GGL is a centre-pivot with a timber sash, GGU is the same window with a white polyurethane finish over the timber core. GPL and GPU are the top-hung equivalents. GDL is the balcony type that opens out into a rail. GIL and GIU are non-opening elements that sit below an opening window to extend the glass down the slope.
• The size code, a letter and two digits, such as MK04 or SK06. This is the one that decides everything about a replacement.
• A pane code, two digits after the type, describing the glazing that was in it when it left the factory. The 60-series covers ordinary double glazing; a 66 means the inner pane is laminated. This tells you what you have, not what you have to buy again.
• A manufacturing date and serial number, which give the age of the window. Useful context: a window made in 1998 is doing well to still be sealed.

Decoding the size

The letter is the width and the digits are the height, both in millimetres of the outer frame. Once you know the pattern you can read any code on sight.

So the widths run C at 550, F at 660, M at 780, P at 940, S at 1140 and U at 1340. The heights run 01 at 700, 02 at 780, 04 at 980, 06 at 1180, 08 at 1400 and 10 at 1600. MK06 therefore means 780mm wide by 1180mm high, and you can work out any other code the same way.

If the plate has weathered off, which happens on windows fitted before the mid-1990s, measure the outer frame yourself from inside: width across the frame at its widest, and height up the slope. Match it to the table and you will almost always land on a standard code. Where you land between two sizes, the window is likely to be from an older generation with slightly different frame dimensions, and the structural opening will need checking on site before anything is ordered. That is a survey job, not a phone job.

03Centre-pivot, top-hung, and how you will actually use the window

The choice between the two main opening actions is usually made on price, and it should be made on how the room works.

Centre-pivot

The sash turns about a hinge roughly halfway up its height. Push the bar at the top and the bottom of the sash swings out while the top swings in. It is the cheaper action, it needs no clearance in front of the window, and it rotates a full 180 degrees so the outer face comes inside for cleaning, which matters a great deal on a second-floor slope that no ladder will ever reach. It works on pitches from about 15 degrees up to nearly vertical.

The catch is the sash itself. When it is open, half of it is inside the room, sitting at head height in the middle of the opening. If the window is positioned low enough to see out of while sitting down, an open centre-pivot sash cuts straight across that view. And you cannot lean out of one, or put a desk directly under it, or stand at it comfortably.

Top-hung

The hinge is at the top and the whole sash swings outwards, usually to around 45 degrees. The opening is clear, you can lean out and look down the garden, and nothing intrudes into the room. Most top-hung windows also have a secondary pivot so the sash can still be rotated for cleaning, which is the arrangement you want on anything above the ground floor. They cost more, they need clear air in front of the sash so they cannot be used where a dormer cheek or a neighbouring roof plane is in the way, and they are generally rated to a lower maximum pitch, around 55 degrees.

Top-hung is also the action that gives you a usable escape opening, which matters if the window is doing duty as a means of escape from a loft room. More on that below.

The other options

• Non-opening elements. No opening at all, no handle, slimmer sightlines and a lower price for the same glass area. Sensible on a slope you will never want to open, or paired below an opening window to bring the glass line down towards the floor.
• Balcony windows. The upper sash swings out and the lower section slides up to form a rail, turning the opening into a shallow balcony in about ten seconds. They need a good deal of rafter height and a room deep enough behind them, and they are one of the few things that genuinely changes how a loft room feels.
• Electric and solar-powered. The same windows with a motor, a rain sensor and a wall control or remote. Worth it on anything you cannot reach, which usually means a window above a stairwell or set high in a vaulted ceiling. We have written that up separately on our electric skylight page.

Choosing between them

The practical test is where the bottom rail of the window sits relative to your eye. If the window is high in the slope and you are only ever going to look at sky through it, centre-pivot is the sensible money. If the bottom of the window comes down to around a metre above the floor and you want to stand at it, lean out, or see the estuary from a chair, pay the difference for top-hung. On a bathroom, centre-pivot every time, because you will be cleaning it and you will not be leaning out of it.

04Orientation: why the south slope needs different glass

Southend-on-Sea sits facing south across the Thames Estuary, which is a peculiar thing for an English seaside city and the reason we survey compass direction before we open a product catalogue. A pitched roof window is not the same problem as a flat rooflight, and the geometry is worth understanding because it decides your glass.

Take a 1930s semi with a tiled roof pitched at 30 degrees, and put a window in the south-facing slope. A surface tilted 30 degrees from horizontal has its face pointing at a spot 60 degrees above the horizon. At this latitude the sun reaches almost exactly 62 degrees at midday in late June. Those two numbers being within a couple of degrees of each other is not a coincidence you want in your bedroom ceiling: for a few hours either side of noon in high summer, that window is pointed straight at the sun. Nothing about the angle is working in your favour.

Now do the same sum in December, when the midday sun struggles to 15 degrees. The same window meets it at 45 degrees off square, so it is still collecting usefully, and that is the good news. Unlike a near-level rooflight, a south slope earns its keep in winter. The window that cooks you in July is genuinely warming the room in January. This is why the answer is never simply to shade everything.

Three other things make a loft window a harder thermal problem than a window in a wall:

• The room is at the top of the house. Warm air from the whole building collects there. Whatever the window contributes lands on top of that, not instead of it.
• There is very little mass to absorb it. A loft room is plasterboard, insulation and a timber floor. There is nothing heavy to soak up a couple of hours of gain and release it slowly the way a solid ground-floor wall does.
• The insulation is often thinner than it should be. Conversions done before the mid-2000s frequently have 100mm between the rafters and nothing over them, so the roof itself is contributing heat as well.

Put a number on it. Two MK06 windows come to about 1.3 square metres of glass. A surface facing the July sun squarely at this latitude can be taking somewhere approaching 800 watts per square metre on a clear day. With ordinary clear double glazing at a G-value near 0.6, that is roughly 620 watts arriving in the room, continuously, through the middle of the day. A solar-control unit at 0.3 halves it. The room still looks bright, because the eye adjusts to light levels logarithmically and barely registers the difference, but you have taken out the equivalent of a small heater running above the bed.

West-facing slopes deserve their own warning. The sun is lower in the afternoon, so it strikes a west slope closer to face-on than the midday sun ever does, and it does it at four and five o’clock when the day’s heat has already built up. If you have a choice about which slope a window goes into, and the room works either way, north and east are the comfortable options and west is the one that catches people out.

North-facing slopes are the opposite problem, and a mild one. They get no direct sun except a glancing pass at either end of a midsummer day. The light is flat, steady and slightly blue, it is the best light in the house for anything visual, and there is no case at all for solar-control glass on that side. Clear double glazing with a good low-emissivity coating, and spend the saving on a bigger window.

05Glazing options, and the two numbers that decide the room

Every roof window quote mentions a U-value. Almost none mention the other figure, and the other figure is the one that determines whether you can sleep in the room in August.

U-value measures how quickly heat escapes through the unit, in watts per square metre for each degree of temperature difference, written W/m²K. Lower is better. It governs your winter heating bill and whether the inside face of the glass gets cold enough to run with condensation on a frosty morning.

G-value measures how much of the sun’s energy gets through the glass and turns into heat indoors, on a scale from 0 to 1. Lower means less heat. Ordinary clear double glazing sits around 0.6. A good solar-control unit sits around 0.3, and the better ones go lower. Ask for this figure by name, because it is rarely volunteered.

Here is how the common build-ups compare. These are typical industry figures for sealed units of the kind that go into roof windows, not one manufacturer’s range, and the exact values shift with cavity width, gas fill and coating generation.

A few notes on the choices in that table.

Triple glazing is not automatically the upgrade. It brings the U-value down usefully, and it adds around ten kilogrammes per square metre of weight, which the hinges have to carry for the next twenty-five years. More to the point, a plain triple unit does very little about solar gain. On a south slope in this city, a solar-control double will make the room more comfortable across the year than a plain triple will, for less money. Where both matter, solar-control triple exists and is the expensive but correct answer. There is more on the whole balance on our energy-efficient skylight page.

Laminated inner panes are worth having overhead. Toughened glass fails into a shower of small blunt pieces, which is acceptable on a vertical window and less so directly above a bed. A laminated pane holds together on its interlayer. Part K of the Building Regulations governs safety glazing, and laminated inside is the specification we would put on any window above a sleeping or seating position regardless of what the regulations technically demand for that pitch.

Rain noise is a real consideration on a roof window in an exposed spot. A roof window is a drum head compared to a tiled roof. Straightforward double glazing under driving rain from the south-west is audible in a way most people find pleasant for a fortnight and occasionally regret at three in the morning. A laminated pane, or a unit with different pane thicknesses either side of the cavity, takes a noticeable edge off it.

Anti-dew and easy-clean coatings. Easy-clean coatings are worth the modest premium on any window you cannot reach from a stepladder, because rain does most of the work on a slope of 30 degrees or more. Anti-dew outer coatings reduce the morning misting that appears on the outside of very well-insulated units, which is a sign the glazing is working rather than a defect, but it does block your view until the sun clears it.

06Flashing kits: the part between the window and your roof

The window keeps rain off the room. The flashing kit keeps rain off the window, and it is the component most likely to be got wrong on a cheap installation.

A flashing kit is a set of preformed aluminium and lead-substitute sections that dress the roof covering up and over the frame on all four sides. It comes with an apron at the bottom, side sections that lap over the tiles or slates, and a head section that goes underneath the covering above. It is not generic. It is ordered to the window size code and to the profile and thickness of your roof covering, and installing the wrong one is how water ends up on a rafter.

Two collars go in with the flashing and neither is optional in a properly built job. An insulation collar wraps the outside of the frame in a preformed insulating sleeve, closing the cold bridge between the window and the roof insulation. Skip it and you get a cold line around the reveal that turns into black mould at the head of the window within two winters, which then gets blamed on the glazing. An underfelt collar connects the roofing membrane to the frame so that any water getting past the tiles in driving rain is carried around the window and back onto the felt below it, rather than finding the timber.

That second collar earns its money here. South-facing roofs on this coast take wind-driven rain off the estuary, and the salt in it is unkind to metal fastenings. On an exposed slope at Shoeburyness or along the Leigh cliff top, the assumption that no water ever gets under a tile is optimistic. The membrane collar is what makes it not matter.

The other decision is the recess. A standard installation leaves the window sitting proud of the roof plane by around 40 to 60mm, which is normal and which nobody notices. A recessed installation sets the frame deeper so the glass sits nearly flush with the tiles, and it needs its own flashing kit and a slightly different opening. It looks considerably better on a slate roof, it is often what a conservation officer wants to see in the Clifftown or Leigh Old Town conservation areas, and it costs more in both parts and labour. On a rear slope of a 1930s semi facing the garden, the standard kit is perfectly good.

07Sizing, rafters and the rules that apply

Working with the rafters you have

Rafters in a domestic roof are usually at 400mm or 600mm centres. That governs the window widths that fit easily. A 550mm or 660mm wide window will often drop into a 600mm bay with the two adjacent rafters left completely alone, which is quick, cheap and structurally uneventful. Anything 780mm or wider means cutting at least one rafter and installing trimmers above and below the opening to carry the loads the cut rafter was taking. That is routine carpentry, but it is carpentry, and it is where the price difference between an MK and an FK window comes from once you count labour rather than parts.

Two narrower windows in adjacent bays are frequently a better answer than one wide one. You get more even light across the room, you avoid cutting rafters, and the pair looks deliberate from the garden where a single off-centre window rarely does. On a full-width loft conversion, three windows evenly spaced along a slope will outperform two larger ones for the same total glass area.

Height in the room

The general rule is that the bottom of the window wants to be between 900mm and 1,100mm above the finished floor, and the top wants to be around 2 metres up. That gives a standing adult a view out and a seated one a view of something other than cloud. The steeper the roof pitch, the taller the window you need to cover the same vertical range, which is why a 45 degree Victorian roof takes a 1,400mm sash where a shallow 1960s roof is fine with 980mm.

The internal reveal

How the plasterboard is cut around the opening changes the room more than people expect. The standard detail is a vertical return at the head of the window and a splayed return at the sill, angled down towards the floor. The vertical head throws light deeper into the room rather than straight down the wall. The splayed sill lets warm air from a radiator underneath rise across the inside of the glass, which is the simplest defence against condensation on the pane there is. Square both returns off and you get a room that is darker and a window that mists more. It costs nothing to do it properly, and it is a detail worth asking about.

Planning permission

Roof windows on a house are generally permitted development, so no planning application, provided they project no more than 150mm beyond the plane of the existing roof slope and no part of the window is higher than the highest part of the roof. A standard flashed installation sits comfortably inside that 150mm, which is why most roof windows in most streets went in without anyone applying for anything. Windows in a side-facing roof slope serving a room, where the opening is less than 1.7 metres above the floor, also need obscure glazing and restricted opening.

The exceptions matter more than the rule, and this area has several.

• Flats and maisonettes have no permitted development rights at all. Every roof window needs an application, including a like-for-like replacement in some cases.
• Listed buildings need listed building consent whatever the size or position.
• Conservation areas restrict work on roof slopes facing a highway, and local policy differs between them. Clifftown, the Victorian estate behind the cliffs, and Leigh Old Town, with its fishing cottages and a core running from the thirteenth century to the eighteenth, are both handled carefully.
• Article 4 Directions remove permitted development rights outright for the things they name. The Leigh Cliff and Leigh conservation areas carry an Article 4 Direction covering changes to roofing materials and windows, so a roof window that would need nothing anywhere else needs a planning application there. Both conservation area appraisals were updated in March 2022.

Treat all of that as the general position and confirm it for your own address. The authority for the city is Southend-on-Sea City Council, and jobs just outside fall to Rochford, Castle Point, Basildon or Thurrock. A written answer from the council costs nothing and is worth having in the file when the house is sold.

Building Regulations

This part is not a judgement call. A new or enlarged roof window is notifiable under the Building Regulations every time. Three parts of the Approved Documents come into it:

• Part L sets a limiting U-value for new windows in an existing dwelling. Any current roof window with a decent sealed unit clears it, but the figure has to be stated and evidenced rather than assumed.
• Part K covers safety glazing and protection from falling, which is where the laminated inner pane and the position of the opening come in.
• Part B covers fire escape, and it is the one that catches loft conversions. Where a roof window serves as a means of escape from a habitable loft room, it needs a clear opening of at least 0.33 square metres with a minimum of 450mm in either direction, and the bottom of that opening between 800mm and 1,100mm above the floor. In practice that means a top-hung window, or a centre-pivot with an escape hinge, and it means the size and position are decided by the regulation rather than by taste. A loft conversion that creates a third storey has stricter requirements again, involving a protected stairway rather than a window, and that is a conversation for whoever is designing the conversion.

We make the Building Control notification to Southend-on-Sea City Council, or to whichever authority covers your address, on your behalf. The completion paperwork comes to you at the end and goes in the drawer with the deeds, because a solicitor will ask for it eventually. Leaving that with a customer to sort out is how a straightforward job turns into a problem five years later.

08Installation day, and the blinds that follow

A single window into an accessible slope is a day. A pair, or a window on a high or awkward elevation, runs to two. Here is the shape of it.

Access

Most roof window installations are worked from inside the roof, with the covering removed from within the opening as the work goes. That surprises people who assume a scaffold is automatic. For a ground-floor rear extension slope or a low bungalow roof, a tower or a properly footed ladder is often enough. For a two-storey slope, particularly a front elevation over a pavement, an edge-protection scaffold is the right call and it should be on the quote as a named line rather than hidden in the total.

Forming the opening

Tiles or slates come off over an area larger than the window. The battens are cut back, the roofing membrane is cut and folded clear, and the rafters are marked. Where a rafter has to go, trimmers are installed above and below the opening first, then the rafter is cut, so the roof is never unsupported. The timber frame that carries the window is then built inside the opening, checked for square across the diagonals and for wind, which is the twist you get when the four corners are not in one plane. A roof window that is fitted into a twisted frame never quite closes evenly against its seals, and no amount of adjustment afterwards fully cures it.

Setting the window

The sash comes out of the frame, which halves the weight and stops anyone putting a knee through the glass. The frame goes into the opening on its brackets, is levelled and packed, and is secured at the manufacturer’s stated points. Then the insulation collar goes on, the membrane collar is connected to the roofing felt, and the flashing kit is built up from the bottom. Apron first, sides lapping over it, head section last and tucked under the covering above, so that every joint sheds downhill. Tiles or slates then go back around the flashing, cut where they need to be, and any that broke on the way off get replaced.

Inside

The sash is rehung and the window is opened and closed repeatedly while the packing is adjusted, until the sash seats evenly on all four seals. Then the reveal: insulation into the gaps around the frame, vapour control layer taped to the frame, and plasterboard cut to the vertical head and splayed sill described earlier. Skim and decoration are usually a follow-on unless it has been agreed otherwise.

Handover

We run water over the finished window and watch it from inside while it is running, which tells you more in five minutes than a fortnight of hoping. Then the demonstration: the handle, the ventilation flap in the top bar that lets you leave a window closed and locked but still breathing, the filter behind it that needs a rinse once a year, and the rotation catch for cleaning. You get the manufacturer’s product warranty, our 10-year workmanship guarantee, and confirmation of the Building Control notification we have already lodged. Over fifteen years of installing skylights and rooflights around this estuary has taught us that the survey is where the surprises should turn up, not the second morning.

Blinds, and which ones actually do anything

Blinds get treated as an afterthought and they are half of the comfort answer on a south slope. The distinction that matters is inside versus outside the glass.

• Internal blackout blinds run in side channels and seal against light at the edges. They are what you want in a bedroom or a nursery in June, when it is light at half past four in the morning. What they do not do is stop heat, because by the time the sun’s energy has passed through the glass it is already in the room and a fabric blind simply absorbs it and radiates it back.
• Internal pleated and roller blinds soften light and give privacy. Useful, undemanding, and the same limitation on heat.
• External awning blinds are a mesh that sits outside the glass and intercepts the sun before it reaches the unit. They stop a large majority of the solar energy, typically in the region of three quarters, and you can still see out through the mesh. On a south or west slope they do more for summer comfort than any internal blind, and they pair well with solar-control glass rather than replacing it. Most are solar powered with their own small panel on the top of the window, so no wiring is needed.
• External shutters block light and heat entirely and add useful sound reduction, at a higher price and with more visual impact from outside.

If a window is going into a south-facing slope of a bedroom and the budget will not stretch to everything, our order of spending is: solar-control glass first, because you cannot change it later without changing the window; external awning blind second; internal blackout blind third, since that one can be added any winter without touching the roof.

09What it costs, and how to compare quotes

Prices move with size, glazing, access, roof covering and how much internal finishing is included. The ranges below are general market guidance for south Essex, supplied and installed, and a survey is what turns a range into a figure.

Broader figures across everything we install are on the costs page, and if you are still deciding between a roof window and something else in the same roof, the pitched roof skylight page covers the wider options.

Questions worth putting to anyone quoting

1. What size code have you quoted, and does it match what is in the roof now? On a replacement this is the whole basis of the price.
2. What is the G-value of the glass? If only a U-value comes back, the summer half of the problem has not been thought about.
3. Is a new flashing kit included, and which type? The answer should name your roof covering, not just say yes.
4. Are the insulation collar and the underfelt collar in the price? These are the two items quietly left out of cheap quotes.
5. Does the price include lining and boarding the reveal, or does it stop at the frame? A large source of quotes that look different but are not.
6. Is a rafter being cut, and who has confirmed the trimming is adequate?
7. Does the window need to serve as a means of escape, and does the one you have quoted meet Part B?
8. Who notifies Building Control, and when do I get the completion certificate?
9. Is scaffold included or is it a variation on the day?
10. Which way does this slope face, and has anyone been out to look at it? A quote given over the phone without that answer is a guess with a number attached.

What a quote you can rely on looks like

It names the window type and size code, states the glazing build-up with both performance figures, names the flashing kit type against your actual roof covering, lists the collars, says explicitly where the work stops on the inside, sets out access arrangements, states who notifies Building Control, and gives a lead time. Standard sizes are usually available within a week or two; anything with solar-control triple glazing or an unusual size takes longer. A single line and a price is asking you to trust rather than compare.

We install and replace roof windows and rooflights across Southend-on-Sea, Leigh, Rochford, Shoeburyness and the wider south Essex area, on everything from Victorian slate to 1930s interlocking tile to new-build estates. Send a photograph of the data plate if you have one, or just tell us which way the slope faces, and we will come and measure it. Phone 01702 898232, or use the quote form.

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