Why plastic domes are worth replacing now
A dome rooflight replacement is one of the simplest ways to change how a flat-roofed room feels, because the old dome was usually the cheapest daylight the original builder could buy. Swapping it for a modern dome, or for flat glass, gives the room a clearer, quieter and warmer skylight on a roof that can often stay largely as it is.
Moulded plastic domes went onto a great many flat roofs from the 1960s onwards: over single-storey kitchen extensions, bathrooms, landings in flat-roofed houses, garages converted into rooms, and the back additions of terraces. They were light, easy to handle and needed no glazier. The dome shape shed rain without any fall on the roof, and the plastic skin could be single, twin or even triple layered.
Age is what changes them. Acrylic domes tend to go brittle and develop fine surface crazing. Polycarbonate domes resist impact well but can yellow and haze as ultraviolet light works on the surface. Opal domes that once gave a clean, milky glow start to look grubby from inside however often they are cleaned, because the dirt is now in the material. The fixings and the small plastic or timber kerb underneath age too, and the thermal performance of a thin plastic skin was never close to a modern sealed glass unit.
None of this means the idea of a dome was wrong. It means the old one has done its time and there is now a better skylight available for that opening. The question worth answering first is which kind of replacement the room actually needs.
Three ways to replace a dome skylight
There are three sensible routes, and they differ mainly in how much happens to the upstand, the short raised frame the rooflight sits on above the roof surface. The choice between them decides the cost, the time on the roof and how the room will look afterwards.
- A new dome on the existing upstand. Where the kerb is sound, square and high enough, a modern multi-skin dome made to the same base size can go straight back on. This is the lightest touch and suits a utility room, a garage or a store where the view of the sky matters less than cheap, even light.
- A new dome with a new insulated upstand. Many modern domes are sold as a kit with a factory-made insulated PVC or composite upstand. The old kerb comes off, the new upstand is fixed over the opening, and the roof covering is dressed up its sides. The room gains a warmer frame as well as a better dome.
- A flat glass rooflight on a new or rebuilt upstand. The dome goes altogether and a double or triple glazed flat rooflight takes its place. This is the route that changes the room most: a clear view of the sky, far less noise in rain, and a glazed unit that can be specified for heat and glare.
Each route suits a different kind of room. A dome over a downstairs cloakroom and a dome over the kitchen table where the family eats breakfast are not the same problem, and treating them as the same is how people end up with a replacement that is merely newer rather than better. For the related case of a flat glass unit that has itself reached the end of its life, see our guide to replacing an old flat rooflight.
What the upstand has to do
The upstand is the part of the job that decides whether a replacement is simple or involved. It raises the rooflight clear of standing water, carries the weight of the unit, gives the roof covering something to turn up against, and, in the case of flat glass, sets the fall that lets rain run off.
Older domes often sit on a low timber kerb made up on site from a few lengths of softwood, sometimes no more than a couple of courses above the roof deck once the felt was laid. Current roofing guidance asks for the top of the upstand to finish at least 150mm above the finished roof surface, so that driven rain and a blocked outlet cannot push water over the edge. A kerb that falls short of that, or that has softened at the corners, is a reason to rebuild rather than reuse.
A dome does not need its upstand to slope, because its curved shape sheds water on its own. Flat glass does. Rain must run off the pane rather than sit on it, and most flat rooflight makers ask for a fall of at least 5 degrees across the glass. That fall can come from a kerb built with one side higher than the other, from a factory upstand made with the angle already in it, or from the unit itself where the maker builds the pitch into the frame. If the old kerb is level, moving from a dome to flat glass therefore almost always means new upstand work.
- Joists, with doubled trimmers either side of the opening to carry the loads of the cut joists around it.
- Deck (usually 18mm ply or OSB) with a vapour control layer laid on it.
- Insulation boards above the deck (a warm roof), often tapered to give the roof its falls.
- Waterproof membrane dressed up the upstand and over its top, so the joint is lapped, not sealed.
- Upstand, or kerb: insulated timber, finishing at least 150mm above the finished roof surface to keep standing water and splash off the joint.
- Rooflight unit fixed over the kerb, its frame skirt shedding water outside the membrane turn-up.
- Plastered reveal lining the opening from the ceiling up to the frame, finished white to bounce the light down.
- Roof falls to an outlet: designed at about 1:40 so the finished roof never falls less than 1:80.
Size is the other thing the upstand fixes. Domes are sold by their base or kerb size, while flat glass units are specified either to sit over a builder's upstand or with their own integral upstand, and makers publish sizes in different ways. The internal opening in the ceiling is the dimension that matters to the room, so the survey measures that, the kerb, and the roof build-up before anything is ordered. Our answer on whether a replacement rooflight has to be the same size covers going larger or smaller.
How the light changes when the dome goes
A dome and a flat glass rooflight give a room two different kinds of light, and the difference is bigger than it sounds. Opal and tinted domes diffuse light into a soft, even glow; clear glass shows the sky and lets a defined patch of sunlight move across the room.
That diffusion is not all loss. Under an opal dome there is no sun patch, no hot spot on the worktop and no glare, which is why domes were popular over bathrooms and small halls. The trade-off is that the light is flat and the sky is invisible. Swap to clear glass and the room suddenly has a view upwards, a sense of weather and time of day, and on a sunny afternoon a bright shape on the floor that tracks across it from late morning to evening.
On a flat roof the rooflight faces the whole sky, and the Cambridge landscape has very little to shade it: few hills, and on many streets only neighbouring roofs of similar height. So the sun path matters more than the compass direction of the roof. In June the midday sun stands about 61 degrees above the horizon here and a sun patch lands close to the opening; in December it barely reaches 14 degrees and the patch, when there is sun at all, stretches far across the room and up the opposite wall. Our light planning guide shows how that plays out in a real room.
There are ways to keep some of the softness of a dome if the room needs it. Satin or acid-etched inner panes diffuse the light while still giving a far clearer, cleaner glow than aged plastic. Solar control coatings take the edge off summer heat. A pleated or integral blind can be closed on bright days and opened on grey ones. For a fair view of how much brighter a new unit is likely to be, read will a new rooflight be brighter than the old one.
Heat, noise and condensation
The comfort gain from a dome skylight replacement is usually felt before the light gain. A modern glazed rooflight holds warmth far better than a thin plastic skin, is much quieter in rain, and keeps its inner surface warmer so that condensation is less likely to form on it.
Part L of the Building Regulations sets a limiting U-value of 2.2 W/m²K for a new or replacement rooflight, and since the 2021 changes that figure is measured with the unit lying flat, which is how a flat roof rooflight actually sits. Many older single and twin skin domes fall well short of that. A quality modern multi-skin dome can meet it, and a double glazed flat unit with a warm edge spacer and a thermally broken frame will usually beat it comfortably, with triple glazing going further again.
Noise is the difference people mention afterwards. Rain on a single skin plastic dome drums, and a heavy shower over a bathroom or bedroom can be loud enough to wake someone. Laminated glass, particularly with an acoustic interlayer, turns that into a faint patter.
Condensation follows temperature. A cold dome surface above a steamy kitchen or bathroom collects water that then drips or runs down the kerb, marking the lining and sometimes the paint around the opening. A warmer glazed inner pane and an insulated upstand reduce that sharply, and an opening unit can purge moist air where it gathers at ceiling height. We explain more about moisture around tired units in can a misted rooflight cause damp.
Comparing the three routes
The table sets the three routes side by side. It is a starting point for the conversation at the survey, not a verdict, because the state of the kerb and the roof covering can move any job from one column to another.
| Route | Upstand work | Light in the room | Suits |
|---|---|---|---|
| New dome, old kerb | None if kerb is sound | Soft, diffused, no view | Stores, garages, utility rooms |
| New dome, new upstand | New insulated upstand | Soft or clear, warmer | Bathrooms, halls, landings |
| Flat glass | New upstand with fall | Clear sky view, sun patch | Kitchens, living rooms, studies |
Two things can change the answer quickly. If the flat roof covering is near the end of its life, the most economical moment to replace the rooflight is when the roof is renewed, and our guide to replacing rooflights during a reroof explains how the trades are sequenced. And if the dome is in the wrong place for how the room is now used, replacement is the chance to move it.
Where dome rooflights turn up in Cambridge homes
Domes appear across every kind of Cambridge housing that has a flat roof somewhere, from single-storey rear extensions on interwar semis to the flat-roofed houses of the 1960s and 1970s. The house type tells you a lot about what sits under the dome and what the replacement can be.
On the 1930s semis of Cherry Hinton, Arbury and the roads off Histon Road and Coleridge Road, a later flat-roofed kitchen or garage extension often carries one or two domes. The original room behind is usually deep and dark, and the dome was the only light the extension got. Replacing it with flat glass, sited over the worktop or table rather than wherever the joists allowed, can change how the whole ground floor works.
In the Victorian and Edwardian terraces of Romsey, Petersfield and around Mill Road, flat-roofed rear additions and bathroom outriggers sometimes have a small dome over a bath or a narrow galley kitchen. Space is tight and neighbours are close, so an obscured or satin inner pane can keep privacy while giving far more light than the old plastic.
Flat-roofed modern houses, including some of the architect-designed homes of the 1960s and 1970s in the city and villages, can have domes over stairwells, landings and internal bathrooms. Here the dome is doing the job of a window in a space with no outside wall, and the choice of glass and blind decides whether the stair is a pleasant, well-lit route through the house or a hot shaft in summer.
Where the building is listed or in one of the many conservation areas, visible changes to a roof can need consent. A flat rooflight often sits lower than the dome it replaces, which tends to help, but the position of the roof and what can be seen from the street matter. Our answer on planning permission to replace a rooflight sets out when to check first.
Building Control and the rules for the new unit
Replacing a dome is usually straightforward in regulatory terms, but the new unit has to meet current thermal standards and some changes make the work notifiable. It is better to know which side of that line a job falls on before the old dome comes off.
A replacement rooflight in an existing home generally has to meet the Part L limiting U-value described above. Where the size of the opening changes, the roof structure is altered, or the job involves more than simply exchanging one unit for another, the work may need to be notified to Building Control, and in some cases a structural check of the trimming around the opening is sensible. We handle the Building Control notification where the work is notifiable, so the paperwork follows the job rather than falling to the homeowner. The detail is in our guide to Building Control for replacement rooflights.
Safety glass is the other point. A rooflight over a place people stand, walk or sit should have a laminated inner pane so that if the glass is ever broken it stays in the frame rather than falling into the room. Specifying that from the start costs little and matters a great deal over a kitchen island or a bath.
What happens on the day
A like-for-like dome swap on a sound kerb can be done in a few hours. Moving to flat glass with a new upstand is typically a day, sometimes a little more if the roof covering needs more work around the opening or the internal lining has to be extended.
The sequence is simple. The room below is protected with dust sheets and the opening is guarded from inside. The old dome and its fixings come off, and the kerb is inspected; this is the point at which a soft corner or a low upstand shows itself. If a new upstand is going on, it is fixed over the opening and set to the fall the new unit needs. The roof covering, whether felt, a single ply membrane such as EPDM, or GRP, is then dressed up the sides of the upstand and sealed so that the roof and the rooflight form one continuous weatherproof line. The new dome or glazed unit is fixed down to the maker's instructions.
Inside, the new upstand may be a little deeper or differently sized than the old kerb, so the reveal around the opening is lined, insulated where it can be, and plastered or boarded to a clean finish ready for decoration. Our answer on whether replacing a rooflight will damage your ceiling explains what making good involves. Most single-storey flat roofs can be reached safely with a proper ladder and roof edge protection; higher roofs may need a tower or scaffold, which is agreed in writing before the day. The old dome is taken away with the rest of the waste.
Questions people ask
Can a dome be replaced with flat glass without redoing the whole roof?
Usually, yes. The new upstand is set into the existing roof covering and the membrane or felt is dressed up it locally. The whole roof only needs renewing if the covering is already failing, in which case it makes sense to do both together.
Can I replace a dome that doubles as a roof access hatch?
Yes. Hinged access domes and glazed access rooflights are both available, the glazed ones with gas struts or electric openers. The size of the opening needs to suit a person climbing through, which is checked at the survey.
Will a flat glass skylight look out of place where a dome used to be?
From the ground, a flat rooflight usually looks lower and neater than a dome because it sits closer to the roof line. From inside, the frame of a modern unit is slim and the view is clear, so the opening reads as a window in the ceiling rather than a plastic bubble.
Is a new dome still a sensible choice?
For a store, garage, utility room or anywhere the view of the sky is not the point, a good multi-skin dome on an insulated upstand is a sensible, economical upgrade. Where people spend time under it, flat glass is usually the better long-term choice. The costs guide explains what moves the price, and the replacement service page shows how we approach every kind of tired rooflight. Book a light survey and we will show you, in a written light plan, what each option would do to the room.
