How much does a flat roof skylight cost?

A flat roof skylight price is really five costs stacked together, and quotes vary because installers split them differently. Knowing the parts lets you read a quote properly rather than judging it on the headline number.

• The unit. The glazed rooflight itself. A small fixed double-glazed flat unit might be a few hundred pounds; a large opening or triple-glazed one is well into four figures before anyone touches the roof.
• The kerb, or upstand. Flat glazing cannot sit flush with the roof surface or it will pond water. It needs a raised kerb, either a timber one built on site or a prefabricated insulated one supplied with the unit. The insulated version costs more and performs better at the edge.
• Forming the opening. Cutting through the deck, trimming the joists and sometimes adding a structural member so nothing above the hole is left unsupported. A modest opening between existing joists is quick. A wide one that interrupts the structure is not.
• Weathering it in. The flashing, the dressing of the covering up the kerb and the detail where the glass meets the frame. This is where a flat roof either stays dry for decades or does not, so it is not the place to shave money.
• Labour, access and making good. Time on the roof, scaffold or a tower if the roof is high, and the plasterwork to the reveal inside afterwards.

Add Building Control on top. A new or enlarged rooflight is notifiable, because Part L sets a ceiling on how much heat the unit may lose and Part K governs the safety glazing. That notification is part of a proper installation rather than an optional line, and on our flat roof skylight installation work we make it to the local authority on your behalf.

The figures below are industry-general ranges for a supplied-and-fitted job in a standard flat roof, before glass upgrades and before any awkward access. Treat them as a way to sense-check a quote, not as a fixed tariff.

Two units cost more than one of twice the size, because each opening needs its own kerb, its own flashing and its own reveal. If a deep room needs a lot of daylight, a single larger rooflight is usually better value than a row of small ones, and it leaks in fewer places by design. A roof lantern is the exception: it is one opening but a more involved frame, which is why its range runs so wide.

A flat rooflight points at the sky rather than at a compass direction, so it collects the high summer sun almost square on no matter which way the house is turned. That makes the glass specification the decision that matters most on a flat roof, and it is where a cheap quote and a considered one part company.

Clear double glazing has a G-value of around 0.6, meaning roughly 60 per cent of the sun’s energy comes straight through as heat. Solar-control double glazing sits nearer 0.3. On a flat roof over a south-facing rear extension, the sort of 1930s kitchen extension you find across Westcliff, Thorpe Bay and Hadleigh, that difference is the gap between a room you can use in July and a greenhouse. Southend-on-Sea faces south across the estuary with an open horizon and light reflecting off the water, so the sun is on the glass longer and harder than it would be inland.

Solar-control glazing adds to the unit cost, modestly on a small rooflight and more noticeably across a large lantern. Triple glazing adds more again and buys you a lower U-value for winter warmth. Neither is always the right answer: on a north-facing roof the free winter gain from clear glass is worth having, and paying to block it is money spent to make the room colder. Getting orientation, G-value and glare right is the whole point of our energy-efficient skylight installation work, and it is why we price the glass around the room rather than selling one specification to everyone.

Beyond size and glass, a handful of things swing the final figure, and most of them are about the roof rather than the rooflight.

• Opening versus fixed. An electric opening unit brings a motor, a rain sensor and wiring, so it costs more to buy and more to fit. It also lets hot air escape at the highest point of the room, which does real work on a south-facing flat roof that a fixed pane cannot.
• Walk-on glass. Structural glazing you can stand on, used for balconies and light wells, is a different product at a different price, and it is heavier to handle.
• Kerb type. A prefabricated insulated kerb costs more than a timber one built on site but performs better at the cold edge and speeds up the fit.
• The structural span. An opening between existing joists is straightforward. A wide opening that cuts through them needs a trimmer or a beam, which is the single biggest structural variable in the price.
• Access. A single-storey rear extension is reachable from a tower. A rooflight over a two-storey flat roof, or one you can only reach across a neighbour’s ground, brings scaffold and time.
• Making good. A plastered reveal and a painted finish inside add labour that a bare opening does not.

If your existing flat rooflight has misted, cracked or simply lost its seal, taking it out and fitting a new one is a replacement, and it is work we do. The opening and the kerb are already there, so the price sits at the lower end and it is the moment to improve the glass for nothing more than the unit difference. If the roof covering around it has failed rather than the rooflight, that is a job for a roofer instead.

Any price you read online, this page included, is a guide until someone has seen the roof. Three things turn a range into a real number: the finished opening size, whether the unit is fixed or opening, and the glass specification. Measure the opening you want, note which way the roof faces and whether the room already runs hot, and you can price it against the table above yourself.

For a firm quote, our skylight installation costs page sets out where each element lands, and we will look at the structure, the kerb detail and the orientation together rather than pricing the glass in isolation. Ring 01702 898232 or request a quote with your rough opening size and the direction the roof faces, and we will come back with a specification and a price for the whole job, not just the hole in the roof.