What sets the cost of an energy efficient rooflight

A quote for an efficient rooflight has two halves. One is the unit, the other is the work of getting it into your roof and making good around it. Both halves move, and they move for different reasons.

The unit is priced by what it is made of. Two rooflights with the same opening can sit a long way apart on price because one has a basic sealed double unit in a plain frame, and the other has a low-emissivity coating, a warm edge spacer, argon fill and a thermally broken frame. Each of those layers adds material and manufacturing steps. None of them is visible from the room, which is why a cheap quote and a dear one can look identical on a brochure photograph.

The labour half depends on the roof. A flat roof with a sound deck and an existing upstand is a simpler job than a pitched roof that needs a new opening cut through rafters, a trimmer fitted and the tiles or slates lapped back in. Access matters too. A loft room reached by a narrow stair takes longer to work around than a ground floor kitchen with a clear route to the garden.

We keep the two halves separate in the written quote, so you can see what you are paying for in the glass and what you are paying for in the work. Our rooflight and skylight costs page sets out the general market picture, and this page explains the reasons behind the movement.

Most of the price of the glass sits in a handful of choices. Some earn their keep on almost every roof, and some only on certain rooms.

The cheapest sealed unit is two panes of clear glass with air between them. Upgrading it is a matter of adding layers that each cut heat loss a little more.

• A low-emissivity coating on one inside face of the cavity. It reflects heat back into the room and is the single most worthwhile upgrade on a double unit. See low-E coatings on rooflight glass.
• Argon in the cavity instead of air, with a warm edge spacer in place of a metal one. Together they cost little against the unit and stop the cold edge you can feel with your hand on a winter morning.
• A third pane. This adds the most cost and the most weight. It suits a bedroom in an exposed spot, less so a kitchen that stays warm anyway.

The third pane is the upgrade people ask about most, and it is the one to think about hardest. It does lower heat loss, but it also makes the unit heavier and the frame deeper, and on a lantern that can mean stronger bars. Our page on when the third pane earns its place goes through the cases, and there is a separate answer on the weight of triple glazed rooflights.

Laminated glass is a different kind of upgrade. It is chosen for safety overhead and for sound, and it costs more than toughened glass of the same thickness. On a rooflight over a loft bedroom near the Gatwick approach, or a kitchen lantern that drums in rain, it is an upgrade we would usually propose in the quiet spec. The warmth and the quiet can come from one build, which is covered in one glazing build for warmth and quiet.

The frame is the second big line on a quote, and it is where a good deal of heat can escape if it is chosen badly.

Size works on the price in two ways. A bigger unit has more glass, and large panes are dearer per square metre because they are harder to make and to handle. They also need more people and sometimes a crane or a hoist to lift them into place. Doubling the opening more than doubles the labour on a difficult roof.

It also works against your heating. A larger rooflight loses more heat in winter, simply because more of your roof is glass. Part L limits how much glazing a new extension can have, and the sizing is part of the conversation at survey. The Part L guide explains what the rules ask for.

The same rooflight can carry two very different installation costs, depending on what is underneath it.

A flat roof with an existing kerb needs the old unit lifted out and the new one seated, with the upstand insulated and the membrane dressed up to it. If the kerb is tired, it is renewed at the same visit, and that adds materials and time. A new opening in a flat roof needs the deck cut, a new upstand built and the roof covering made good, which is more again.

On a pitched roof, a new opening means structural work. Rafters are trimmed, a header and a trimmer are fitted, and the roof covering is re-laid around the flashing. A roof window in a loft conversion also brings the inside finish into the price: the lining, the plaster reveals and the decoration that make the job tidy. Our page on insulating around a rooflight explains why the upstand and the reveals are part of the efficiency, not an extra.

Two more items sit on the list and are easy to forget. Scaffold or access equipment is priced where the roof cannot be reached safely from a ladder. And the Building Control notification, which we handle where the work is notifiable, is part of doing the job to current Building Regulations and is included in how we plan it.

Comparing two quotes only works if they describe the same thing. A line that says "energy efficient rooflight" tells you nothing.

Ask for the glazing build written out: the pane thicknesses, the coating, the gas fill, the spacer and the frame. Ask whether the thermal figure quoted is for the glass centre or the whole unit, because the whole-unit number is the honest one and is always a little higher. Our answer on whole-unit and centre-pane U-values shows how to tell them apart.

Then check what is not in the glass. Is the upstand renewed or reused? Is the inside made good? Is access included? A quote that leaves those out can look cheap and then grow. Ours names each of them, along with the glazing build, in the quiet spec that comes with the quote.

Finally, weigh the running side. A well-specified unit can trim heating demand in the room, although how much depends on the room, the orientation and the rest of the house. Our page on whether efficient rooflights lower heating bills is honest about the limits.

Is triple glazing always the most efficient choice?

It has the lowest heat loss of the common builds, but the gain over a well-made coated and gas-filled double unit is smaller than the gain from the first coating. It suits some rooms and is wasted on others.

Does a more efficient rooflight cost more to install?

The labour is usually the same. The difference is in the weight of the unit and the frame, which can mean more hands on the day and, on a lantern, stronger support.

Can I upgrade only the glass and keep my old frame?

Sometimes, if the frame is sound and can carry a new unit. Often a worn frame is the weak point, and a whole new rooflight is the better spend. The page on upgrading an old rooflight explains how we decide.

How do I get a price for my own roof?

Tell us the room and the roof, and we survey it, listen for the noise the room hears and write the build down. Get a quote, or read about the full service on the energy efficient rooflights hub.