What a flat roof skylight costs

Start with what sits in the opening. A fixed rooflight is a sealed double-glazed pane in a frame, with nothing to open, and it is the simplest unit to make and to fit. Adding a hinged vent adds hardware, a second frame section and a more complicated seal. A motorised vent adds a motor, a control and often a rain sensor, and it needs a power feed unless it is a solar model. A walk-on unit is a different build altogether, with thick laminated glass specified for the load it must carry.

Size comes next. A larger opening takes more glass and a stiffer frame, and above a certain span the glass has to thicken or the frame has to be braced. Shape follows: a long, narrow unit can behave differently from a square one of the same area because of the span the frame must cover. Some openings suit a standard size from a manufacturer's range, and some are best made to measure, which is how we handle awkward roofs.

Matching the unit to the room

A ground-floor shower room under a flat roof wants a vent that opens, since steam collects at the ceiling. A boot room with no window at all in its inner half may only need fixed glass. The cheapest unit that does the job for the room is usually the one to start from, and our guide to the best skylight for a flat roof goes through how to choose.

No flat rooflight sits straight on the deck. It rests on a kerb, a raised upstand around the opening that keeps the frame above any standing water and gives the roof covering a face to turn up against. Kerbs come as factory-made insulated sections or as timber built on site and insulated to match. Either one is a line on the quote, and so is the cap that finishes it.

The height is not a fixed number. A roof with insulation laid above the deck needs the kerb to carry that depth of insulation plus the covering, while a roof with insulation between the joists needs less. An older felt roof with years of earlier layers presents a third build-up. We measure the real depth at the survey and set the kerb to suit. For the detail, see our guide to rooflight upstands and kerbs.

The cap matters for the look from the garden. A slim aluminium cap over the kerb gives a tidy edge, and the colour can be matched to the window frames of the room below, so the kerb finish is a choice with its own line. On a bungalow roof that is seen from the upstairs windows of neighbours, we show the finish at the survey.

Kerb height also feeds into planning. A low fixed unit on a shallow kerb can sit inside the 150 mm that permitted development allows, and a taller kerb can push it out, so the price of the kerb and the planning answer are linked. The detail is in our answer on planning for a flat roof skylight.

Roof coverings on flat roofs fall into three families that a rooflight has to join: single-ply membrane, bitumen felt and GRP. Each has its own method for turning up a kerb and sealing at the cap. If the existing covering is sound, the new kerb is joined into it. If it has little life left, a new kerb in tired material is a poor base, and the quote shows how much of the covering is renewed around the opening, or whether the whole area is better recovered at the same visit.

Water has to leave the roof. Flat roofs are laid to a slight fall towards a gutter or an outlet, and a kerb that sits across that line holds water behind it. Where the fall is shallow, a small tapered section behind the kerb steers water to either side, and where the outlet is close to the opening it needs protecting. Each of these is a line, or a small group of lines, that a survey finds and a photograph does not.

Fitting alongside a new covering

When a flat roof is being recovered anyway, the rooflight goes in while the deck is open. The kerb is built into the new build-up from the start, the trimming is easy to frame and no finished covering has to be cut. That saves labour on the rooflight itself, and it is the cheapest moment to ask for one.

Cutting an opening through a flat roof cuts at least one joist, unless the unit happens to fit between two. The joists either side then need trimmers to carry what the cut one used to. An opening that runs with the joists can need very little. A wide one that runs across them, or one in a roof that was only just strong enough when built, can need a steel section or a calculation from a structural engineer. This is often the largest difference between two units of the same size on different houses.

Indoors, a lined shaft links the hole in the roof to the hole in the ceiling. A straight, square shaft throws a hard shadow at its edges, so we splay the sides where the structure allows, which spreads the light across more of the floor. The lining is boarded and finished ready for plaster. If a pendant or a run of downlights sits where the shaft will be, they may need moving by an electrician, and the quote should say who does it.

A flat-roofed extension reached by a clear path at the side is quick to work on from a ladder or a small tower. A roof over a mid-terrace kitchen, with no side passage, means every piece goes through the house, and a flat roof behind a neighbour's fence can only be reached over their land. Larger units may need lifting. Where any of this applies it is written in as a line, since it changes the hours on the job.

Paperwork is a separate group of lines. A new opening in a roof is building work, so the Building Control notification and its fee appear on the quote, and we handle the notification where the work is notifiable. For a rooflight on a kerb, Part L sets a limit of 2.2 W/m²K, assessed for the whole unit with its frame; a roof window in a pitched roof takes the tighter 1.4 W/m²K, or Window Energy Rating band B. Our guide to rooflight U-values explains the figures.

A planning application is a further line on any job that needs one. Lanterns usually do, and low flat rooflights often do not; the lantern planning guide shows where the line falls.

A useful quote describes the job. It names the unit, its size and its glass, then goes down through the kerb, the covering, the structure and the inside finish, and it says where our work stops and the decorator's begins. Put two quotes side by side and mark every line one of them lacks. The gap between them is usually a line missing, not a difference in the glass.

Our survey is free, the written quote is fixed for the scope it describes, and the job carries our 10-year workmanship guarantee in writing. If the rooflight is replacing a failing one, our replacement cost guide covers the like-for-like case, and if you are still deciding whether the roof can take a skylight at all, can you put skylights on a flat roof answers that.

To get a quote, message 07880 284133 on WhatsApp with a photo of the room and one of the roof, or book a survey.