Dome rooflights on a flat roof: where they still earn their place

A dome is a one-piece moulded plastic cover on a kerb, sold as a finished unit. It is light, it is made in stock sizes, and it is the simplest rooflight to fit into a flat roof that already exists.

Most domes are polycarbonate, a tough clear plastic that resists impact well. You will also meet acrylic domes, which are clearer to look through but more brittle. The plastic is heated and formed into a shallow curve, so the top is never flat. That curve does real work. Rain and leaves run off it, and the shape adds stiffness to a thin sheet.

Underneath the plastic sits the kerb. On a dome this is usually a moulded upstand in GRP or PVC, with a flange that is fixed to the deck and then covered by the roof membrane. Insulated versions fill the kerb with a rigid foam core, so the cold does not travel through the upstand into the room. Our page on flat roof rooflight upstands explains how height and insulation work for every type of unit, dome or glass.

A dome is not glass, and that matters in three ways:

• Plastic scratches and yellows slowly under ultraviolet light, however good the coating is.
• Plastic expands with heat more than glass does, so the fixings and the seal are designed to allow for movement.
• A thin plastic skin carries sound differently from a laminated glass pane. We come back to that below.

A dome earns its place where the room is utilitarian, the budget is tight and the main job is simply to get light in. Garages, stores, outbuildings and plain corridors are the natural homes.

Picture a single garage with a felt roof and no window on the long wall. One dome over the workbench changes the whole space. You can see what you are doing without switching a light on, and the room feels like part of the house rather than a box at the side of it. The same applies to a boiler room, a utility lobby, a flat-roofed porch or a shed that has become a home office for the summer months.

Domes suit these places for practical reasons:

• The opening is small and square, so the joist work stays simple. Our page on joist trimming for flat rooflights shows how little is involved at this scale.
• The unit arrives complete, so there is no heavy glass to lift onto the roof.
• The curved top sheds water better than a flat pane sitting at a shallow angle.
• Nobody sits directly under it for hours, so the noise question carries less weight.

If you are weighing a garage conversion, the page on fitting a rooflight on a garage roof goes into the roof build-up to expect. Whichever unit you pick, the same rule applies: the rooflight is sized and placed for the room underneath, not for the catalogue.

The word skin means a layer of moulded plastic. A dome can have one, two or three of them, and each extra layer traps a pocket of air that slows the loss of heat and changes how the unit sounds in weather.

Single skin

One layer of plastic on the kerb. It is the lightest and lowest cost option, and it passes the most light. It also loses heat freely, so in winter the underside of the dome is cold, and warm damp air from the room condenses on it. In an unheated garage that hardly matters. In a heated room it leads to drips and a stained ceiling. A single skin dome is an honest choice only for an unheated space.

Double skin

Two layers with a sealed air gap, which is the usual domestic specification. The inner skin stays much closer to room temperature, so condensation is far less likely, and the unit feels less cold when you stand beneath it. Light transmission drops a little because the light passes through two layers, but the room still looks bright. For any heated space with a flat roof, a double skin is the minimum we would consider.

Triple skin

Three layers and two air gaps. It holds heat best of the three, which makes it the product to ask about where you want a dome over a heated room and the double is not quite enough. The cost of that comfort is slightly less light again and a thicker, heavier dome. Our separate page on whether a double or triple skin dome is warmer compares the two more closely.

A side by side helps, because the trade-offs line up neatly.

A thin plastic dome is the quickest way to hear a downpour indoors. The skin is light, there is nothing to absorb the impact, and the curve acts like the top of a drum.

That is a property of the material, not a fault in the fitting, and it is where our quiet light approach begins. In a garage you will barely notice. Over a home office or a bedroom, the noise can be the first thing you remember about the room. A second skin helps because the two layers and the air gap between them break up the vibration before it reaches you. A third skin helps again. Fixings, seals and kerb stiffness all play a part, and so does the ceiling build inside the room.

We do not quote decibel figures, because a number on a brochure tells you little about the room you actually sit in. Instead, the quiet spec that comes with every quote writes down what we listened for at the survey and the glazing build we suggest for it. Read more on what goes into the quiet spec.

A dome is a building element like any other, so it has to meet the regulations on energy, safety and fire. The rules are published, and they change what you can choose.

For energy, Approved Document L sets the performance for new and replacement rooflights in a heated space, and it treats a rooflight in an extension differently from one in an unheated outbuilding. A single skin dome is not designed to meet the standard for a heated room, which is another reason it belongs on the garage or the store and not on the kitchen.

For safety, the working at height rules treat a plastic dome as fragile. A dome is a window, not a floor, and nobody should stand on it or lean on it during maintenance. If you need a rooflight that a person can cross, read about walk-on rooflight installation instead. Roof access for cleaning needs planning from the start.

For fire, Approved Document B limits how close plastic rooflights can be to a boundary, and it classifies the materials they are made from. A dome fitted close to a neighbour's wall can need a different product from one in the middle of a large roof. The position of the boundary is checked at the survey, and our page on a flat rooflight near a party wall goes through how it works.

Where the work is notifiable we handle the Building Control notification. We install to current Building Regulations, and the written quote says which product we are using and why.

Domes come in stock sizes, usually square or rectangular, in steps that match standard roof openings. That is a strength, because the unit is ready to fit and the choice is easy. It is also a limit, because you cannot ask for a size that falls between two steps.

Fitting a dome into an existing roof follows a clear order. The deck is cut back to the size of the kerb. Any joists in the way are trimmed and framed so the load travels round the opening. The kerb is fixed and sealed to the deck. The roof covering, whether felt, EPDM or GRP, is then dressed up the upstand and finished at the top edge. The dome itself goes on last, so that it is not scratched by the work. The steps of a flat roof rooflight installation cover the sequence in more detail.

Kerb height deserves as much attention as size. The waterproof turn-up against the kerb needs enough height above the finished roof surface to stay dry, even when snow or leaf litter builds up. The height depends on the roof covering and the insulation on top of it, and it is fixed at the survey. See how high a flat rooflight upstand should be for the figures we work from.

The roof fall comes in too. A flat roof is never perfectly level, and a kerb is a small dam across the fall. On a very small dome the effect is minor. On a larger unit it decides where the dome should sit, and we read the fall before we mark the deck. The page on falls, drainage and flat rooflights explains why.

Plastic domes age faster than glass, and that ageing is gradual. A good dome lasts for many years, but the surface picks up fine scratches and the clarity softens.

Condensation is the day to day difference. A sealed glass unit keeps its inner face close to room temperature. A single skin dome cannot, and a double skin holds the line only while the air gap stays sealed. If the seal between skins fails you will see moisture or a milky haze in the gap. That cannot be cleared from outside, and the answer is to renew the unit. Our service for misted and failed rooflight replacement covers that route.

Cleaning is simple but it needs care. Use lukewarm water, a soft cloth and a mild detergent. Keep away from abrasive pads, solvents and pressure washers, which damage the surface coating and leave cloudy marks. Because the dome is fragile, clean it from the side of the roof you can reach safely, never by stepping onto it. There is a fuller routine on cleaning a flat rooflight.

Two other details are worth knowing. A dome yellows more quickly in bright, unshaded positions, so a shaded roof keeps its clarity longer. And leaf mould collecting at the base of the kerb is easy to brush away, but left alone it holds moisture against the upstand.

Once people are going to sit under the rooflight, the picture changes. A flat glass unit is a different product with different strengths, and in a lived-in room it deserves a serious look.

Flat glass sits low on the roof and looks tidy from the garden and from the room. It can be toughened on the outside and laminated on the inside, which gives a stiffer pane and a way to specify quieter glazing. It resists scratching and keeps its clarity for longer. It also takes modern coatings that reduce glare and solar heat. The trade-offs are cost and weight. Glass is heavier, it usually needs a bigger structural opening, and the unit may need a lifting plan on site.

The page on whether a flat glass rooflight is better than a dome weighs them in more detail. For a bedroom, study quieter glazing for a flat roof rooflight over a bedroom before you settle on anything.

A simple way to decide:

1. Is the room heated and lived in? Choose a double or triple skin dome at the very least, and consider glass.
2. Will anyone sit, sleep or work directly under it? Lean towards laminated glass, or a triple skin dome if the budget is firm.
3. Is it a garage, store or outbuilding? A double skin dome is usually right, and a single skin where the space is never heated.
4. Is the roof close to a boundary? Check the fire limits before you fall for a product.

Domes are not only fixed. Some have a hinged lid, which is handy in a utility room or a bathroom where steam needs a way out, and some can be operated by a motor and a rain sensor.

An opening dome gives a little fresh air and a little extra light, but it adds moving parts and a frame. The seal between lid and kerb has to be good enough for rain and cold. If the room needs ventilation, Approved Document F describes the rates, and a dome that opens can help meet them. Our page on electric and opening rooflight installation explains the choices when you want more control. For a room with no outside wall at all, a sun tunnel can be the lower-profile answer. Some dome and glass units also take a blind underneath, and fitting a blind to a flat rooflight covers what works.

These are the points that come up when the subject is a dome on a flat roof. Each answer is short, and the pages linked above go further.

Will a dome let water in?

A dome fitted on a proper kerb with a correctly dressed membrane is watertight. Water finds its way in through poor detailing at the kerb, not through the dome itself. That is why we spend the time on the upstand, the fall and the turn-up.

Can I replace an old dome with one the same size?

Sometimes the existing kerb and opening can stay, and the new unit is a straight swap. The kerb is checked first. If it is cracked, too low or poorly insulated, it is renewed as part of the job. Our page on putting a rooflight in an existing flat roof gives the wider picture.

Is a dome safe to walk on?

No. A dome is fragile and must never be stepped on, even for a moment. Plan access to the roof so that nobody needs to cross it.

Which dome should go over a garage I might convert later?

Choose a double skin now. It holds up when the room is heated, and the extra cost is modest. If you expect to live in the room, say so on the quote form, so that the quiet spec reflects the future use.

If you have a garage, store or flat-roofed room that needs light, tell us the size of the opening you have in mind and how the room is used. We will visit, read the roof and the fall, and write down the dome or glass unit that suits the space, along with the quiet spec. You can get a quote or message us on WhatsApp. The rest of the range sits in our flat roof rooflight installation hub.