Short answer: The flat roof skylight cost depends on six things: the size of the opening, whether the unit is fixed or opening, the glazing specification, the kerb, the roof covering around it and how easily the roof can be reached. We give no figure until we have surveyed the roof, and the number you then receive is written down and does not move.
Size comes first, and it drives the rest
When people search for a flat rooflight price, they usually want a single number for a single unit. The trouble is that size works on almost every other cost at once. A bigger opening means a bigger piece of glass, a longer kerb, more insulation, more covering to cut and bond, and a heavier lift onto the roof. A small square unit over a landing and a long rooflight over a kitchen island are different jobs, not different sizes of the same job.
Size also decides what the structure has to do. A modest opening can often sit between existing joists. A wide one means cutting a joist and framing the hole with trimmers, so the loads carry round it. That carpentry is part of the job and it grows with the opening. Joists in the flat-roofed back addition of a Victorian terrace are often old timber at uneven spacing, which is why we measure them instead of assuming.
Fixed or opening, and what moves the sash
A fixed rooflight is a sealed unit that lets in light and nothing else. An opening one adds hinges, a seal that has to work under a hinge, a stay or a motor, and a place to put the controls. Many kitchens and bathrooms get a powered one with a rain sensor that closes the sash when it drizzles. Powered units need a cable run to a spur, and the electrical work is a line on the quote of its own.
It is worth asking what the opening is for. If the aim is daylight, a fixed unit does it for less. If the aim is clearing steam from a shower room or letting heat out of a south-facing kitchen, a vent or opener earns its place. How those units work is covered on our electric and opening rooflights page, so for cost purposes the only point is that the choice drives the price.
Glazing and thermal performance
The glass specification is the next lever. Double glazing is the baseline. Triple glazing, solar-control and self-cleaning coatings, tinted or laminated panes and acoustic glass all add to what the unit costs.
Thermal performance matters for more than comfort. A new rooflight in an existing house is controlled work under the Building Regulations 2010, and the 2021 edition of Approved Document L caps rooflights in existing dwellings at 2.2 W/m²K (volume 1, Table 4.2). A unit has to meet it, so very cheap glazing that falls short is not an option we can fit. We check the maker's stated figure before ordering.
The kerb, the covering and the access
Beneath every flat rooflight sits a kerb: a raised upstand lifting the glass off the roof surface and giving the waterproofing something to turn up against. How the kerb is built changes the price. A factory-made insulated kerb supplied with the unit is one route. A timber kerb built on site is another, and a kerb made to a special height or shape takes more time. Heights and build methods for kerbs are covered in the guide to kerbs and upstands.
The roof covering is the part many people forget to ask about. Felt, EPDM and GRP each meet a kerb in their own way, and the covering has to be dressed up the kerb and sealed. If the covering is new, it can be laid with the rooflight in mind. A sound old covering can be tied into, but a worn-out one is better replaced around the opening, since tying new work to a failing surface is poor value. Which covering you have is covered in what suits a felt or rubber roof.
Then there is access. A ground-floor extension roof can be reached from a ladder and the unit lifted by hand. A flat roof above a first-floor landing, or at the back of a terrace with no side passage, may need the unit carried through the house or lifted with equipment. Scaffold, a hoist or a long carry through a lived-in home adds time, and so does a tight street in the older parts of Liverpool.
| Factor | Pushes cost up | Keeps it down |
|---|---|---|
| Size | Wide opening, cut joists | Opening between joists |
| Sash | Powered, rain sensor | Fixed unit |
| Glass | Triple, coated, laminated | Standard double |
| Roof | Failing covering, hard access | Sound covering, easy reach |
How we turn all this into one number
We survey first. That means looking at the covering from the roof, checking the joists from the room below, measuring the opening and noting how the unit will get up there. We also tell you what the Building Control side involves. Should the work be notifiable, we deal with Building Control directly and the paperwork never lands on you.
The survey is followed by a written quote with a fixed price. It names the unit, the kerb, the covering work, any electrics and the making good, so you can see where each part of the price comes from. A written 10-year guarantee on our workmanship stands behind the work. For the general picture of how our pricing is built across all ten services, see our costs page, and for the parent service, the flat roof rooflight installation page sets out the options in full. Ready to go ahead? Get a free quote and the survey gets booked.
Worth knowing first
Can you quote over the phone?
Not reliably, because the answer depends on the covering, the joists and the access, none of which we can see from a call. We can talk through the options on the phone or WhatsApp, then survey before we name a figure.
Does a bigger rooflight always cost more?
Generally yes, but not in a straight line. Once an opening is wide enough to need a joist cut and framed, the structural work steps up, so two sizes close together can differ more than the glass alone suggests.
Is the quote still fixed if you find something on the day?
The quote is fixed because the survey is thorough. Checking the covering and the structure beforehand means the figure we give is the figure on the invoice, and anything that would change it is raised before work starts.