VELUX roof window installation on a loft conversion

A roof window is not fitted the way a wall window is. It sits in the pitched plane of the roof, tucked between the rafters, with the tiles or slates cut back around it and a metal flashing kit weaving the covering into the frame so water runs off and away. On a loft conversion there are two common situations. Either the window is going into a brand-new roof structure that the conversion has just created, in which case the opening is framed as the roof is built, or it is going into an existing slope on a house that is already standing, which means opening up a covering that has been weathering for decades.

Both start the same way inside. The room below is finished to a habitable standard, so the plasterboard, the insulation and the internal lining all have to meet the frame cleanly. A roof window brings its own lining kit for exactly this, so the reveal around the glass is square and finished rather than a ragged hole in the ceiling slope. The frame is screwed to the rafters or to timber trimmers spanning between them, the flashing is dressed into the roof covering outside, and the insulation and vapour collars are sealed around the frame so the warm damp air of the room cannot reach the cold outer edge of the unit and condense there.

The order of work matters. Fit the frame, dress the flashing, seal the collars, then line the reveal. Skip the collar stage and the loveliest finished plasterboard reveal in Essex will still grow a black line of mould along its cold edge within two winters.

The single biggest constraint on a loft window is the rafter spacing. Most house roofs are built with rafters at 400mm or 600mm centres, which leaves a clear gap of roughly 360mm or 560mm between them. A roof window narrower than that gap can drop straight into a single bay with no structural alteration to the roof at all, which is the quickest and cheapest way to glaze a loft. Go wider than the bay and a rafter has to be cut, which means a structural trimmer above and below the opening to carry the load the rafter was taking, and usually a note from whoever signed off the conversion structure.

VELUX windows use a two-letter, two-digit size code, and it pays to know how it reads before you choose. The letter pair encodes width and height. Here are the codes you will see most often on a domestic loft, with the sizes as published guidance.

Height matters as much as area, and for a reason people rarely think about. The head of the window wants to be high enough that a standing adult can see the sky through it and not just the tops of the neighbouring roofs, and the sill wants to be low enough to see the garden while sitting down. On a standard pitch the guidance most fitters follow puts the sill somewhere around 900mm to 1,100mm above the floor. A taller window like an MK08 or MK06 lets you keep the sill at a comfortable seated height while carrying the head high enough for the view and the light. Two smaller windows side by side often beat one large one for the spread of light across a room, and they keep you inside the rafter bays.

The opening action is not just a comfort choice on a loft. It interacts with headroom, with cleaning and with fire escape.

A centre-pivot window turns on a spindle halfway up the frame, so the top half swings inwards as the bottom half swings out. It is the default, it is the cheapest, and it is easy to reach because you operate it from the middle. The catch is that the lower sash swings down into the room, so you do not want it directly over a sofa or a bed where someone can crack their head on it. A top-hung window is hinged at the top and pushes out from the bottom like an awning, which keeps the whole sash clear of the room and gives an open, unobstructed view when it is up. Top-hung units are the usual choice where the window doubles as the escape route, because they open wide enough to climb through.

For a low sloping ceiling where the window comes down close to the floor, a top-hung sash is far more comfortable to stand at. For a high slope you cannot reach by hand, an electric or solar-powered opener earns its keep, and pairing that with a rain sensor means the window shuts itself the moment a shower starts. On a loft in a Leigh or Clifftown conservation area, a conservation-style window with a central glazing bar and a low, flush black flashing profile is often what the planning authority will accept where a standard white-framed unit would not.

Here is the part that most loft quotes leave off the page. A roof window faces the same way as the roof plane it sits in, and a loft conversion almost always glazes the slope that gives the best view or the easiest structure, without anyone checking the compass. On a south-facing slope in Southend that is a mistake you will feel every summer.

Southend faces south across the Thames Estuary, which is unusual for an English seaside city and means a very large number of loft slopes here point straight at the summer sun with nothing above them to cast shade. A flat wall window facing south takes the midsummer sun at a steep glancing angle, so a good deal of the energy skims off. A pitched roof window on a south slope takes that same sun much closer to square-on, so it collects far more of it per square metre of glass. The window that gives you the best morning light in a bedroom is often the same window that turns the room into a greenhouse by four in the afternoon.

This is the whole basis of how we specify a loft. Orientation first. We survey which way each slope faces before choosing a single unit, because a north-facing loft window and a south-facing one should not have the same glass in them. North wants all the daylight it can get and never overheats, so it takes clear glazing. South wants the light without the heat, which is a question of the G-value, not the frame. Glare belongs in the same conversation. A south or west roof window over a desk or a bed throws direct sun across the room in a way a wall window never does, because it comes from overhead, and no blind fully tames what the wrong glass lets in.

VELUX and every other maker publish their glazing against two numbers that pull in opposite seasons. The U-value is the winter figure, the rate at which heat leaks out on a cold night, and lower is better. The G-value is the summer figure, the share of the sun’s energy the glass lets through, and on a south loft slope lower is what keeps the room usable. These are typical published figures for common roof-window glazing, offered as the shape of the market rather than a set price list.

Read the U-value column and the spread is modest. Read the G-value column and it more than halves, from 0.60 down to 0.26, on a quantity that is far larger in raw watts on a sunny afternoon than the trickle of conducted heat the U-value governs. That is why, on a south loft slope, the glass specification is the decision and the frame is almost an afterthought. Solar-control glazing lets you keep a bright, clear-looking room while shedding most of the heat before it arrives, and the good coatings do it without turning the glass dark, so you are not trading a hot room for a gloomy one. There is a fuller treatment of this trade-off on our guide to solar-control glazing, and it is worth a read before you pick a unit for a sunny slope.

One safety point the table does not carry. Anything glazed overhead needs a laminated inner pane under the safety glazing rules, so that if the inner face ever breaks it holds together rather than falling into the room. Roof windows come this way as standard, and the laminated interlayer also blocks nearly all the ultraviolet, which is what keeps flooring and furniture in the room below from fading in the sun.

A loft conversion is a change of use as well as a building job, and roof windows sit inside three separate regulatory questions.

The first is thermal. New and enlarged roof windows are notifiable under Building Regulations, and Part L sets a ceiling on the U-value of the glazing. Any unit worth buying clears that ceiling comfortably. We make the Building Control notification to Southend-on-Sea City Council, or to the relevant local authority for your address, as part of the installation.

The second is safety and escape. Part K covers the overhead safety glazing already mentioned. Part B is the one that catches people out on a loft. A habitable loft room usually needs a means of escape, and on many conversions the roof window is it. An escape roof window has to give a clear opening of at least 0.33 square metres, with the opening no less than 450mm high and 450mm wide, and the bottom of that opening sitting between roughly 600mm and 1,100mm above the floor so a person can actually climb out. That constraint often decides both the size of the window and how low it sits in the slope, so it needs settling before anything else about the glazing. This is a whole-house question, and how the escape window ties into the wider loft rules is set out on the VELUX window installation service page.

The third is planning. Roof windows are generally permitted development on a house where they project no more than 150mm beyond the roof plane and sit below the ridge, with obscure glazing and restricted opening required below 1.7m on side elevations that face a neighbour. The exceptions matter locally. Flats and maisonettes, listed buildings and conservation areas fall outside permitted development, 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 loft conversion there needs a planning application where an identical one a mile away would not. Treat all of this as general guidance and check your own address with the local authority before ordering glass.

On an existing loft slope the covering comes off around the opening first. Tiles or slates are lifted, the battens cut back, and the underlay opened to the size of the frame plus its flashing allowance. The frame goes in secured to the rafters or to timber trimmers, set at the correct height in the slope and packed dead square, because a frame fitted out of square will never seal or operate properly no matter how good the flashing is.

Then the layers that keep the weather out. An underfelt collar ties the roof’s breather membrane into the top of the frame so any water getting under the tiles is guided over the window and back onto the covering below it. The metal flashing kit is dressed in next, and the kit is specific to the covering: a low-profile flashing for slate and flat interlocking tiles, a deeper one for big profiled concrete tiles that stand proud of the roof. Inside, an insulation collar wraps the frame to close the cold bridge, and a vapour barrier is sealed to the room side so warm moist air cannot reach the cold frame edge. Only then does the plasterboard reveal go on.

A single roof window into an existing slope is typically a one-day job, two if the covering is awkward or the internal reveal is being plastered and made good the same visit. A loft with several windows runs longer in proportion. Weather governs the outside stage, since the covering is open while the frame goes in, so the roof is never left exposed overnight.

As industry-typical guidance, a mid-size roof window supplied and fitted into an existing loft slope, flashed and with the reveal finished, tends to fall somewhere in the region of £1,200 to £2,500. The spread comes from size, from whether a rafter has to be trimmed, from the glazing you choose and from access to that part of the roof. Solar-control glazing adds to the glass cost and repays it every summer on a south slope. Electric or solar openers, blinds and conservation flashings each move the figure. Present any single quote against those variables rather than against a headline price.

Four questions settle most of it in one conversation. Which way does this slope face, because that decides the glass before anything else. What is the G-value of the glazing proposed, and if the answer is only that it is energy efficient, ask again, because energy ratings in this country lean on winter performance and say nothing about a July afternoon. Does this window need to serve as the fire escape, because that sets its size and height in the slope. And who notifies Building Control, because a new roof window is notifiable and that notification should be part of the job, not left to you.

We have been installing skylights and roof windows across south Essex for over fifteen years, every installation carrying a ten-year workmanship guarantee and notified to the local authority on your behalf. If you are converting a loft on a south-facing slope, the orientation survey is where we start, because the aspect decides the glass and the glass decides whether the room is one you will actually want to sit in. Ask us for a specification with the G-value written on it for each slope you are glazing.