Dome answer

Can a plastic dome rooflight be swapped for glass?

A dome is a familiar sight on an older flat roof, and a clouded or yellowed one is a fair prompt to think about glass. The swap is usually possible, but the kerb, the weight and the finished height all need a look first.

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The answer

Usually yes. A plastic dome sits on a kerb, and a glass rooflight can be set on that kerb or on a rebuilt one, provided the structure carries the extra weight and the opening suits the new size. The finished height matters: on a house, the 150mm projection limit of Class C applies, and a flat needs planning permission for a new rooflight.

What actually changes when a dome becomes glass?

A single or twin-skin dome is moulded plastic over a small kerb. A glass rooflight is a flat or low-pitched sealed unit in a rigid frame, so the look, the light and the weight all change.

The two products do the same job, but they are built in different ways, and the differences are what the survey weighs up.

The look from inside

A dome bulges upward, so the light enters from a curved surface and the ceiling reveal is often wide. A glass rooflight is flat or only slightly pitched, set in a square frame with a crisp edge. Indoors the opening reads as a clear panel of sky rather than a bubble, and the frame sharpens the room's lines.

Clarity and ageing

Plastic scratches, dulls and can yellow in the sun. Glass keeps its clarity and cleans with a cloth. A twin-skin dome may also collect damp between its skins, which gives a similar cloudy look to a failed glass unit, although the cause is different. Glass units can fog too, and the guide on how a sealed pane clouds describes that version of the problem.

Insulation and strength

A glass sealed unit generally insulates better than a dome and is rated against the Part L limit of 2.2 W/m²K for a rooflight in an existing dwelling in England. A glass unit has a laminated inner pane that stays together if struck, which thin moulded plastic cannot promise.

Will the existing kerb and roof take glass?

Often the kerb can be kept if it is sound and tall enough. Glass is heavier than plastic, so the structure round the opening is checked, and a weak or low kerb is rebuilt.

The dome stands on a small upstand of timber, concrete or moulded plastic, and the new rooflight needs somewhere solid to stand.

Height and condition

Manufacturers set minimum upstand heights, commonly around 150mm above the finished roof. A dome may sit on a lower one, which is fine for a loose-fitting plastic skin but not enough for a glass rooflight frame. A kerb that is damp, soft or cracked, or too short, is rebuilt and insulated, and the roof covering is made good round it.

Opening shape and size

Domes are often round or square with a standard footprint. A rooflight sold in a different size means the opening is adjusted, and that touches the joists and the covering. Where the new rooflight matches the old kerb's measurements, the work is simpler.

Weight

Glass is considerably heavier than plastic. For a single rooflight in a sound roof that is seldom a problem, but a larger unit or a weak deck is checked before ordering, and any trimming of joists is detailed for Building Control. Cutting a new opening in a roof is building work and is notified to Building Control, the local authority or a registered building control approver. We send that notice where the job is notifiable.

Do the planning rules treat a glass rooflight differently?

The rule looks at how far the finished rooflight projects, not the material. A taller kerb under glass can use up the 150mm allowance on a house, and a flat is subject to planning permission.

Planning does not care whether you have plastic or glass. What counts is the finished height and the kind of home.

On a house

A rooflight on a house is permitted if it projects no more than 150mm beyond the plane of the roof slope and sits no higher than the highest part of the roof. A dome on a low kerb may sit within that, while a glass rooflight on a rebuilt, taller upstand might reach it. The finished height is therefore worked out at the survey, before any order. An Article 4 direction, or a condition on an earlier planning permission, can remove Class C from a house.

On a flat

Flats and maisonettes have no householder permitted development. Ask the council's planners how a dome-to-glass swap is handled at your address, because a new rooflight on a Tyneside flat cannot rely on householder rights. For roof ownership, the title or lease sets out whose consent applies.

Other cases

A conservation area on its own does not remove Class C for a rooflight on a house. A listed house keeps Class C on the planning side, but almost any rooflight on it needs listed building consent, and that consent decides the job. Historic England has guidance.

Which house is yours?

Upstairs flat

Who decides: The upstairs owner, with the downstairs neighbour's consent where the lease or title requires it.
Which rules apply: No householder permitted development, so a new rooflight needs planning permission from the council. Ask how a dome-to-glass change is treated.

Downstairs flat

Who decides: The owner of the downstairs flat, for a rooflight over their own rear room, again subject to what the lease or title says.
Which rules apply: Also no householder permitted development. Planning permission applies, and the title or lease shows whether the roof is shared.

House

Who decides: The homeowner, unless the house is listed or an earlier permission restricts alterations.
Which rules apply: Class C allows up to 150mm projection and no higher than the highest part of the roof. Article 4 directions and conditions can remove it.

What should you do before asking for a quote?

Photograph the dome inside and out, measure the kerb if you can reach it, and check the lease or title on a flat. The survey then settles kerb, structure and finished height.

A little preparation turns the survey into a decision, not a discovery.

What to gather

Take a photograph of the dome from inside and, if the roof is safely reachable, from outside. Note whether it is single or twin skin and whether it opens. In a flat, check the roof clauses of your lease or title and tell the other owner.

What the roof visit settles

Kerb height and state, deck, joists, opening size and covering all come under review. From those we work out the finished height and say which route the rules point to. You then receive a written quote with a price that holds unless the scope alters, and our workmanship guarantee lasts ten years, set down in writing.

Dome-to-glass doubts

Will a glass rooflight be noisier in rain?

Glass is generally quieter than a thin plastic dome in heavy rain, because the sealed unit and laminated pane absorb sound better. The roof construction around the opening also plays a part, and the survey notes it.

Can I keep the dome's opening vent?

A glass rooflight can open too, by hand or electric motor, if you choose an opening model. Some are fixed and do not open at all. Decide whether ventilation matters to the room before the order is placed.

Is glass a security improvement over a dome?

A laminated glass unit in a rigid frame is harder to force than a thin plastic skin, although the frame fixings matter as much as the glass. Ask at the survey if this is a priority for the room.

Do I need new planning permission just for the material?

No planning rule is tied to the material. The test is the finished height on a house and the type of home. On a flat, ask your council. Our rooflight planning guide explains the routes.