How much does a flat roof rooflight cost?

The short answer

No figure is published on this site, and the reason is that the same glass unit can sit inside two jobs that have almost nothing in common. A made-to-size rooflight dropped onto a kerb formed by somebody else on a brand new extension, with a driveway to park on, is a fraction of the work involved in cutting an opening into a 1970s cold deck at the back of a Stroud valley cottage where the scaffold is hand-carried up steps and the falls have to be corrected before anything else happens. A published price would be wrong for both. What you get instead is an indicative range by email within a day or two, then a fixed itemised price after a survey. This page sets out honestly what moves that figure, and the flat roof rooflight installation page covers the work itself.

Why one number cannot cover this job

On a pitched roof a manufactured roof window has a size code, a flashing kit and a fairly predictable day’s work, so ballpark figures at least mean something. A flat roof rooflight has none of that. The unit is made to a kerb size that does not exist yet, the kerb depth depends on a build-up nobody has seen, and the amount of carpentry depends on which way the joists run.

Two houses on the same street can differ by a large multiple. Quoting an average would mislead the person at the cheap end into thinking they are being overcharged, and set up the person at the expensive end for a nasty surprise.

Two figures, in this order, and what each is for

First an indicative range by email, based on photographs and a rough size. It is free, it takes a day or two, and it exists so nobody spends an afternoon on a survey for a project that was never going to work financially. Second, after a survey, a fixed price, itemised so it is clear what is unit, what is labour, what is scaffold or access, and what is Building Control. That figure holds unless the scope changes at your request, or unless something structural is uncovered that was written up as a risk in the survey notes beforehand.

The glass unit itself, and the four specification decisions

Area is the obvious one, and it does not scale linearly: past about two and a half metres in one direction the glass gets thicker, heavier and harder to lift, and the frame section changes.

Then the build-up. Double or triple, argon or krypton, the low-emissivity coating, and whether there is a solar control coating on the outer pane, which matters on any unit that faces straight up at a June sun. Laminated inner panes are required over habitable space and they also cut rain drumming noticeably. Then whether it is fixed, hinged, or electrically operated with a rain sensor, which is a different product with an actuator, a power supply and a certificate behind it, as set out under electric rooflight installation. Then the frame finish and whether it is a standard powder coat colour.

The opening: carpentry, trimmers and anything structural

An opening that fits between two joists with a head and cill trimmer is ordinary carpentry. An opening that cuts three joists and carries them on a trimmer spanning over a metre is a designed member, sometimes a flitch plate, occasionally a small steel, and that brings an engineer’s calculation into the Building Control application.

The kerb is built on site: treated timber, insulated on its outside face, square to a few millimetres, with its top edge raked to give the glass its pitch. That is a day of careful work on its own, and it is the part of the job that decides the outcome, so it is not where anyone should be looking to save.

What the existing roof adds before the rooflight starts

Falls. Where the deck already holds water at the position you want, firrings or tapered insulation go in first. That is a separate piece of work with its own materials.
Diverters behind the kerb. A kerb dams the water arriving from upslope, so the tapered set-out has to route around it.
Covering condition. Tying new work into a twenty year old membrane is possible; tying into one that is at the end of its life is a false economy, and it gets said plainly at survey.
Cold deck ventilation. On a pre-1990 rear addition, keeping the void cross-ventilated around a new opening takes design and sometimes extra vents.

Access, and the addresses in this county where it genuinely costs more

Access is the factor homeowners least expect and it moves the figure most. A single storey extension with a driveway alongside is a tower and two people. A flat roof at second floor level behind a Cheltenham parapet needs a proper scaffold with a loading bay for the glass.

The Stroud valleys are the honest case. Terraces at Chalford, Rodborough and Nailsworth step up the hillside, access is by shared lane, footpath or a flight of steps, and there is nowhere flat to stand a tower. Scaffold gets hand-carried, and a large pane may need extra hands or a mechanical lift where a lorry cannot get within fifty metres. That takes longer and it costs more, and it is priced from a survey that has looked at the lane rather than from a photograph.

How far the internal finish is taken

The kerb on a warm deck stands 280 to 320mm above the deck, and every millimetre becomes a plastered reveal. The options run from a plasterboard box taped and left ready for the customer’s decorator, through a fully skimmed and painted reveal, to a splayed reveal that widens the beam of light into the room. Splaying costs another half day of carpentry and plastering, and on a north-facing kitchen it changes the room noticeably. Fitted units, extract hoods and pipework in the ceiling void all add time.

New extension against cutting into an existing roof

	New extension, kerb formed with the deck	Existing roof, opening cut in
Structure	Joists laid out around the opening from the start	Joists cut and trimmed, sometimes designed
Falls	Designed in with the tapered scheme	Checked, and corrected where needed
Waterproofing	One continuous new covering	New work tied into existing laps
Internal finish	Part of the extension plastering	Ceiling opened, reveal formed, decoration follows
Coordination	Kerb size agreed in writing between trades	All one trade, so the interface risk disappears

What to send, and what a useful first reply needs

A photograph of the flat roof from an upstairs window, a photograph of the room below, the covering if you know it, a rough size, and the address, because the postcode often tells you the era of the deck and whether a conservation area is involved. Email info@severnskylightsgloucester.co.uk. The process is set out further on the costs page, and the workmanship carries a ten-year guarantee whichever end of the range the job lands at.