The answer
The flat roof skylight cost depends on the unit you choose, what the existing roof is like around the opening, and how the work is reached. The same rooflight can cost different amounts on different roofs, so the price follows a free survey of your roof.
What the unit adds to the cost
The skylight itself is the part people picture first, and it moves the cost in three ways.
Size
A larger opening needs more glass, a bigger frame and a bigger kerb. It also crosses more of the roof structure, so there is more to trim. A modest unit that sits between two joists costs less to fit than a wide one that crosses several.
Fixed or opening
A fixed unit is a sealed pane that stays shut. A venting unit tilts open on hinges, worked by hand or by a motor, and many electric models carry a rain sensor. The moving parts, the motor and the wiring all add to the unit. Over a kitchen or a bathroom an opening version is useful for clearing steam, while a landing with a stair window nearby may not need it.
Glazing
Two sealed panes make the standard build. Triple glazing, low-emissivity coatings and solar control glass improve thermal efficiency or cut summer heat, and each step is part of the unit's specification. Building Regulations cap the U-value of a new rooflight at 2.2 W/m²K, so every unit we fit meets that limit.
For more on how those choices compare, read flat rooflight or roof lantern, which sets the low flat unit beside the taller glazed one.
The frame and its finish
Aluminium frames can be slim without losing strength, and a good one is thermally broken, with insulation separating the warm and cold faces. A powder-coated RAL colour matched to your windows and doors is a choice that sits in the specification as well. These are small steps compared with size, but together they separate a basic unit from a well-specified one.
What the roof adds
Two flat roofs with the same rooflight can need very different work, because the cost sits in the roof as much as in the glass.
The kerb is the first item. A new opening needs a new kerb, built to suit the unit, and where there is already a kerb from an old dome it may need raising or rebuilding to take a glass unit. The covering is the second: felt, EPDM and GRP each meet a kerb in their own way, and a covering that is tired may be better renewed round the opening than joined to.
Then there is the structure. A new hole in the roof is structural work, and it is notifiable. We notify Building Control about the new opening. Where the opening crosses a joist, trimmers carry the load, and a wide span can call for a heavier timber.
Finally the fall: a roof that does not drain well may need a small timber wedge against the kerb so water runs round it instead of pooling. The step-by-step version of all this is in the kerb, deck and covering work in an existing flat roof.
Access and the finish inside
How the unit reaches the roof also affects the work. A small unit goes up a ladder, while a large, heavy one on a tight terrace may need a hoist or a crane. We note the route at the survey.
Indoors, the ceiling opening is lined, plastered and painted as agreed, so the room is left finished.
Planning rules add steps more than they add cost. Whether a house rooflight stays within permitted development depends on how far it stands proud of the roof, and flats and maisonettes sit outside that right, which means permission is normally required, and the council can confirm. The conditions are in the skylight planning guide.
Timing and the weather
Flat roof work is planned around the weather, since the roof cannot stay open in rain. A job that has to be done in stages, such as one that also renews the covering round the kerb, takes longer than a straight swap, and the programme is agreed at the survey.
How we arrive at a fixed quote
A figure given over the phone, before anyone has stood on the roof, is a guess. A felt roof can look sound from the garden and still have a soft deck under it, or joists running the wrong way for the opening you hoped for. Only a visit to the roof shows which.
So we survey first, free of charge. We measure the room and the roof, note the covering and the fall, and agree the position. The quote we write afterwards is fixed, and it states which room the work is for, which unit is going in and how large it is. Our 10-year workmanship guarantee then covers the installation.
Our costs page explains what drives the price across every kind of skylight and rooflight we fit. To ask about a flat roof, message us on WhatsApp (07880 284133) and say which room it is, with a photograph of the roof if you have one.
Two quotes for "a flat rooflight" can look far apart for good reasons: one may be for a small fixed unit on a sound kerb and the other for a large opening one with a new kerb and fresh covering. Asking each firm to give the room, the unit and its measurements makes them comparable.