Upgrading Skylights in a Loft Conversion

Chapter 1

What an older loft conversion tends to have in its roof

A loft conversion done ten, twenty or thirty years ago was usually glazed with whatever roof windows suited the budget and the regulations of the day, and a loft conversion skylight upgrade is the chance to bring that glass, size and control up to the standard of the room below it.

Walk into a converted loft in a Romsey terrace or a 1930s semi off Hills Road and the pattern is familiar. There is a rear dormer with a proper window, and on the front or side slope there are one or two small roof windows set between the rafters. They were often chosen because they fitted the rafter spacing without any trimming, not because anyone asked what the light would do in the room. The glass is plain double glazing with an air or argon gap, the frames are pine with a varnish that has gone orange, and the blinds, if any, are a roller that no longer stays down.

None of that means the room was badly built. It means the rooflight technology and the expectations have moved on. A modern roof window has a warmer frame, a low-emissivity coating, a laminated inner pane, better seals and a far wider choice of glazing and blinds. Swapping the old units for new ones is one of the few changes to a loft room that alters how it feels at every hour of the day, and it can usually be done without touching the plaster, the floor or the rest of the roof.

Before deciding anything, it helps to know exactly what is up there. The make, the size code and the approximate age are usually printed on a small plate on the frame, visible when the sash is opened. Our guide on how to find out what skylight you have shows where to look. That label decides whether a straight swap is possible, whether the new rooflight will sit at the same depth in the roof, and how much of the internal lining can stay.

Chapter 2

What renewed glass changes in a loft room

The glass is where an upgrade earns its keep: a loft room sits directly under the roof, so the glazing is the thinnest thermal layer between the bed and the sky, and better glass is felt on the first cold night and the first hot afternoon.

Older double glazing in roof windows loses heat quickly. Warm air rises, pools at the top of the room and meets the coldest surface in the space, which is often the roof window. That is why older loft rooms get condensation running down the glass on winter mornings and a cold draught falling from the window onto the bed or desk beneath it. Current Building Regulations Part L sets a limiting U-value for a new or replacement rooflight or roof window in an existing home, and since the 2021 changes that figure is 2.2 W/m²K, now measured with the window lying in the horizontal plane rather than standing upright. Any new unit we fit meets it. Many current roof windows come in well below it, and triple glazed versions go further again.

Beyond heat loss, three other glass choices matter in a loft:

• A laminated inner pane. Overhead glazing should hold together if it breaks. Modern roof windows usually carry a laminated inner pane as standard, and our guide to safety glass in rooflights explains why that matters over a bed or a stair.
• Solar control coatings. A south or west slope can push a small loft room into uncomfortable heat on a clear June afternoon. A coating that cuts the solar energy passing through the glass takes the edge off without making the room dim.
• Acoustic laminate. Loft rooms in Cambridge sit close to the airport on the eastern edge of the city, the railway through Chesterton and busy routes like Mill Road and Newmarket Road. A laminated acoustic pane reduces both traffic noise and the drum of heavy rain on the glass.

The same principle runs through all of this: the glass you choose should answer the slope it sits on and the way the room is used, not a catalogue default.

Chapter 3

Going bigger, going lower, or adding a second window

Many older loft rooms have roof windows that are simply too small or set too high, and an upgrade can correct both, provided the rafters, the head height and the approvals allow it.

The width of a roof window between rafters is governed by the rafter spacing. In older Cambridge houses that spacing is irregular, and in a loft conversion the rafters may have been doubled or strengthened. A new window wider than the gap needs a rafter trimmed out and the load carried by trimmers either side, which is ordinary work but is structural and needs to be designed and notified. Going taller is often easier: a longer window in the same bay keeps the rafters intact and brings the bottom of the glass down to a height where you can see out.

That view line matters more in a loft than anywhere else. A small window high on the slope lights the ceiling and shows you only sky. Bring the bottom edge down towards desk height and the room gains a real outlook, over the rooftops towards the Catholic Church spire on Hills Road, across the Fens or into the trees along the Cam. It also puts the handle within easy reach, which decides whether a top hung or centre pivot window will be comfortable to use.

Sometimes the better answer is a second window rather than a bigger one. Two roof windows side by side, or one above the other in the same bay, spread light across a wider part of the room and avoid cutting more structure than necessary. Our answer on replacing a skylight with a bigger one covers the structure and approval side in more detail, and the wider set of choices, from like for like to changing position, is laid out in skylight replacement options.

Chapter 4

How the sun reaches a Cambridge loft through the year

Cambridge sits at about 52 degrees north on almost flat land, so the height of the midday sun swings hard through the year, and a roof window in a loft feels very different in December than it does in June.

At noon on the winter solstice the sun is only about 14 degrees above the horizon. At the spring and autumn equinoxes it reaches about 38 degrees, and at the summer solstice it climbs to about 61 degrees. Those three numbers explain most of what happens under a loft rooflight. In winter the sun stays low, so light through a south-facing roof window slants deep into the room and sits on the far wall. In summer it arrives almost overhead, strikes the glass nearly square on and drops a hot, sharp pool of light straight onto the floor or the bed. Because the Cambridge skyline has so few hills or tall buildings to shade it, a roof window here sees nearly the whole of that arc.

The slope the window sits on decides which part of the arc it catches. The table gives a plain guide to what to aim for when you replace a loft skylight on each slope.

The full comparison of each orientation sits in our guide on which way a skylight should face, and the way we turn it into a written plan for a specific room is set out on how we plan the light.

Chapter 5

Escape windows and the fire plan for the loft

If a roof window was fitted as part of the loft's original fire escape arrangement, the replacement must keep that role, so it is worth finding out what the approved plans said before choosing a new size or type.

Fire safety guidance for loft conversions has changed over the years. Older conversions were sometimes approved with an openable roof window sized as a means of escape alongside fire doors, while current guidance for a new storey in a typical house generally relies on a protected stair running down to a final exit. What your loft has depends on when it was approved and how. The completion certificate and approved drawings, if you have them, will usually show which windows were counted as escape windows.

Where a roof window does act as an escape window, the guidance in Approved Document B generally looks for an unobstructed openable area of at least 0.33 m², with the opening at least 450 mm high and 450 mm wide, and with the bottom of the opening at a height a person can climb out from. Roof windows sold as escape windows are usually top hung or side hung so that the sash swings fully clear, whereas a centre pivot sash leaves the pivot bar across the middle of the opening. These are guidance figures rather than a guarantee of acceptance: Building Control decides what is required for a particular house, and the answer can depend on the stair, the doors and the layout of the floors below.

This is one of the points we check at the survey, and where the work is notifiable we handle the Building Control notification so the new rooflight is recorded properly for when the house is sold.

Chapter 6

Planning permission and Building Regulations for the upgrade

A like for like swap of a roof window rarely needs planning permission, but it usually counts as controlled work under the Building Regulations, and anything that changes size, position or projection brings planning back into view.

For houses, new or enlarged roof windows are normally covered by permitted development under Class C, as long as they project no more than 150 mm from the roof plane, sit no higher than the highest part of the roof, and any side-facing window is obscure glazed and non-opening unless it is more than 1.7 m above the floor. Flats and maisonettes do not have these rights. Across Cambridge and its villages there are many conservation areas, and where an Article 4 direction removes permitted development for roof changes, a front roof window in a terrace may need a flush conservation rooflight and an application where a like for like swap would not. Our guide to planning permission for rooflights in Cambridge covers how Greater Cambridge Shared Planning and the neighbouring councils handle it.

On the Building Regulations side, a replacement roof window must meet the Part L thermal limit, carry safety glazing where Part K expects it, keep any required ventilation under Part F and, as above, respect the fire strategy under Part B. Enlarging an opening adds Part A structure. The detail is in our guide to Building Regulations for rooflights. We install to current regulations and handle the Building Control notification where the work is notifiable, so the paperwork is in order when you come to sell.

Chapter 7

Blinds, opening styles and control

An upgrade is the easiest time to get the blinds and the opening method right, because a new roof window comes with its own blind channels and an old blind rarely fits the new frame.

In a loft bedroom the first question is darkness. A blackout blind that runs in side channels stops the light leaking round the edges on a June morning, when Cambridge daylight arrives well before 5am. In a loft study the question is glare on a screen, which a pleated or light-filtering blind handles better than blackout. On a south or west slope, an external awning or heat blind stops the sun before it reaches the glass, which keeps far more heat out than any internal blind can. Our answer on whether a skylight needs a blind goes through each room.

The opening style decides how the room is used. A centre pivot window lets furniture sit close underneath and can be cleaned from inside by turning the sash. A top hung window opens outward from the top, gives a clear view with nothing across the glass, and suits windows set low on the slope. For a roof window that is out of reach above a stair or a high bed, electric operation with a rain sensor means it can be opened for air on a warm evening and will close itself when the first drops fall. Solar powered versions avoid running new cables through a finished loft ceiling.

1. Decide what the room most needs: sleep, work, air or a view.
2. Choose the opening style that fits the height of the window and the furniture beneath it.
3. Match the blind to the slope and the use, with external shading on hot slopes.
4. Add electric or solar control where the window is out of reach.

Chapter 8

How a loft roof window is swapped in a finished room

Replacing a roof window in a loft that is already lived in is a tidy, contained job: the old unit comes out, the new rooflight goes in at the right depth, and the lining inside is made good so the room is back in use quickly.

Most of the work happens from inside the loft room. The sash is lifted out, the old flashing and frame are removed, and the roof covering around the opening is opened only as far as needed. The new frame is fixed to the rafters, set at the depth that suits the roof and the manufacturer's flashing system, and the new flashing is dressed to the slates or tiles. Where the original window predates a change in the manufacturer's standard installation depth, as older roof windows from around 2001 and earlier often do, replacement flashings are available that let the new window sit where the old one did so the plaster lining can stay.

Inside, the gap between the new frame and the old lining is insulated, sealed with a vapour control collar and finished. The insulation around the reveal is easy to skimp on and it is what stops a cold line and condensation forming around the frame later. For a like for like swap the room is usually usable again the same day. Enlarging an opening or trimming a rafter adds time, plastering and decorating. The step by step order is in the rooflight replacement process, and if the glass in an old unit has already misted between the panes, our page on misted and failed rooflight replacement explains how we renew it.

For new windows added to a loft as part of the upgrade, see our roof window installation service, and for the wider picture of how we approach skylights and rooflights across the city, start at skylight and rooflight installation in Cambridge. When you are ready, book a light survey and we will look at the loft, the slope and the way you use the room before recommending anything.

Chapter 9

Questions people ask

Can I upgrade just one roof window in the loft and leave the others?

Yes. Each roof window is a separate unit, so one can be replaced now and the others later. It is worth matching the frame finish and blind style if the windows sit side by side, and if one of them is an escape window it should be the first to be checked.

Will a new loft rooflight make the room hotter in summer?

Not if the glass and shading are chosen for the slope. A modern unit with solar control glass and an external heat blind on a south or west slope usually keeps a loft room cooler than the older plain double glazing it replaces, while also keeping more heat in over the winter.

Do I need to redecorate after a loft skylight upgrade?

For a like for like replacement, usually only the reveal around the new frame needs finishing. Enlarging the opening or moving the window means new plaster around the reveal and some decorating, which we include in the written quote so the scope is clear before the roof is opened.