What moves a flat roof rooflight cost, line by line

A flat roof rooflight cost is rarely one big number. It is a stack of smaller ones, and two quotes for the same opening can differ because they have counted different things. The general UK market range, as a guide, runs from about £1,200 to £3,200 for a fixed unit supplied and installed on a straightforward job. That matches the table on our costs page, and it is a guide, not a price list.

The range is wide because the five decisions are independent. A small fixed unit on a sound roof with easy access sits near the bottom. A wide unit with laminated acoustic glass, a built-up insulated upstand, a tired covering around it and a scaffold to reach it sits near the top, and sometimes beyond. Nothing on this page is a promise of any price from Clear Skylights. Every figure we give is written after a survey.

The order here follows the order in which a quote is assembled. Size comes first because it fixes the amount of glass, frame and carpentry. Glass comes next because it is the largest material cost. The upstand and the roof covering follow, because they decide the waterproofing work. Access and paperwork come last, because they are the lines that are easiest to forget and the ones that cause the most argument on the day.

Before any of that, one point about the word flat. A flat roof is never quite level. It is laid to a fall so that water leaves it, and that fall affects the upstand and the positioning of the unit. Our page on falls, drainage and why flat rooflights are never perfectly level covers it, and it is a good thing to have read before you compare figures.

Doubling the glass area does more than double the cost. Bigger panes are thicker, heavier and harder to handle, and the structure beneath them has more to carry.

At the small end, a fixed rooflight over a landing or a utility room is a one-pane job that two people can lift. Glass is cut to order, the frame arrives in one piece and the opening in the deck is modest. As the width and length grow, the glass has to thicken to stay stiff, and the weight climbs with it. A pane that two people can carry is one thing. A pane that needs a hoist or a suction lifter is another, and the labour changes accordingly.

Size also decides the carpentry. A flat roof is carried on joists, and each joist cut for an opening has to be replaced by a trimmer and the neighbouring joists doubled. A narrow unit that fits between two joists needs only short trimmers at either end. A wide unit that crosses several joists needs a larger frame of timber and sometimes a steel, and every extra joist adds cutting, fixing and inspection. The page on why flat rooflights need joist trimming explains the work in detail.

Shape matters as well as area. A long, narrow unit has to be stiffer than a square one of the same glass area, and it may need a slimmer or a stronger profile. If you are weighing a slot of light along a wall, read whether a flat rooflight can be long and narrow before fixing a budget, and for extensions see what size suits a single storey extension.

Finally, ask which size the quote uses. A catalogue name may describe the outer frame, the opening or the visible glass, and the three are not the same. Insist on all of them on the page. The daylight follows the glass, so that is the figure worth comparing.

The kind of rooflight you choose sets the starting price before size and glass are added. A dome is the simplest, a frameless flat glass unit is the most refined, and opening versions add hardware on top of either.

The table shows direction only. The ordering can change once size and glass are known. A large double skin dome with a stout upstand can cost more than a small fixed glass unit with a plain timber kerb.

A dome is made of formed polycarbonate or acrylic skins, and it is the least costly way to bring daylight into a flat roof. It is also the type most affected by rain sound, because thin plastic rings under a shower. That is a trade-off, and whether a flat glass rooflight is better than a dome deserves a proper look, as does the plain guide to dome rooflights.

A framed fixed glass unit is the standard choice for extensions and kitchens. The frame is aluminium, or sometimes timber or PVC, the glass is sealed into it, and the joint to the upstand is formed in rubber or sealant. A frameless unit has the glass bonded to a hidden sub-frame so that the top is almost flush. That takes more fabrication and more careful fitting, and it shows in the price. Our guide to frameless flat rooflights shows where the extra money goes.

An opening unit adds a hinge, an actuator and a control, and an electric version adds a cable run and a rain sensor. A fixed unit costs less and has fewer parts that can tire. If you only want light, fix it. If the room overheats, an opening one at height can let the heat out, and electric and opening rooflight installation describes the options.

Glass is where two similar units part company. The build is the number of panes, the thickness of each, the coatings and the gap between them, and every part of it has a cost.

A standard double glazed unit has two panes and a sealed gap. A triple glazed unit adds a pane and a second gap, which costs more and weighs more, and on a flat roof it also means a deeper frame. A laminated pane is two sheets bonded with a clear interlayer. On a flat roof the inner pane is usually toughened and laminated, so that a broken pane holds together above a room. That is a safety reason as much as a cost one.

The quiet reason is separate. A flat rooflight faces the sky, so rain lands directly on the outer pane, and a thin outer pane drums. A thicker outer pane, a laminated acoustic inner pane and well chosen seals make the room noticeably calmer under a shower. Over a bedroom under the Gatwick routes, or beside the M23 or the A23, the same build can soften traffic and aircraft as well. That is a choice made for the room, and it is written down in the quiet spec that comes with every quote. We quote no decibel figures, because the choice is one of specification: pane thickness, interlayer, cavity and seals.

A quieter build costs more than the thinnest standard unit, because there is more glass. Beside the labour, upstand and access, which do not change, the difference is usually modest. If you are comparing, our page on what glass a flat rooflight should have covers the choices, and quieter glazing over a bedroom goes deeper on sound.

Coatings add to the price too. A low emissivity coating and a warm edge spacer improve the thermal figure. A solar control coating costs more again and helps a room that catches strong afternoon sun. Privacy glass is another variant, and obscure glass for privacy explains how it is made and what it changes.

The upstand, often called a kerb, is the raised frame that lifts the glass clear of the water on the roof. Its height, material and insulation all cost money, and many quotes hide it in a single line.

There are two broad routes. A timber upstand is built on site from treated timber, then clad, insulated and covered with the waterproof membrane. A ready-made upstand arrives as a GRP or insulated unit that is fixed to the deck and finished the same way. Timber gives flexibility where the roof has a fall or an awkward shape. A factory unit is quicker to fit and offers a consistent insulation value. Either way, the height matters, because the upstand has to stand clear of the finished roof surface by a safe margin. How high a flat rooflight upstand should be sets out the usual thinking, and height, insulation and finish goes through the build.

Insulation is the quiet cost. An uninsulated upstand is a thermal bridge that feels cold and can collect condensation on the lining inside. Insulating it properly takes material and time, and the lining and reveal below have to be finished to match. The work under the unit is covered in lining the reveal under a flat rooflight.

The seals where glass meets upstand are another line that looks small and is not. Good rubber gaskets and a properly formed joint keep water out and keep rain noise from travelling into the room. A cheap seal might last a few winters, and a good one lasts a great deal longer.

A new rooflight has to be joined into the roof that surrounds it, so the covering you already have shapes the work. Felt, EPDM and GRP each behave differently when they are cut and dressed up an upstand.

A sound felt roof can take a new upstand and a new layer around it, but old felt that has cracked or blistered will not bond, and the join has to land on material that will last. Adding a flat rooflight to a felt roof describes the usual approach. An EPDM rubber roof is a single sheet, which is forgiving, but the sheet has to be cut and bonded with the correct adhesives and the corner details must be made with the maker's own parts, as explained in fitting a flat rooflight into an EPDM roof. A GRP roof is a cured fibreglass shell, so the laminating is done on site, with a limited window of dry and mild weather, and rooflights in GRP fibreglass roofs covers what that involves.

The price changes when the covering is past its best. If the membrane around the opening is near the end of its life, it makes little sense to set a new unit into it. The honest quote then separates two things: the rooflight work and the work to bring the surrounding covering up to a condition that can carry it. Sometimes the answer is to renew a small section of the roof at the same visit. That costs more, and it avoids a second disturbance later.

Insulation and build-up count here too. A warm roof, with insulation above the deck, leaves the joists dry and the detail simple. A cold roof, with insulation between the joists, has a ventilated void that has to be kept clear around the new opening. Whether the existing build-up suits a new unit is covered in whether the roof needs insulating for a new rooflight.

A fall that is too shallow, or ponding near the opening, adds a further line. Correcting a fall means tapered insulation or a re-laid section, and that can be the largest single extra on an otherwise small job.

How the crew and the glass reach the roof is a cost in its own right. It is also the line most often missing from a low quote.

A single storey flat roof at the back of a house can be reached with a ladder and a tower, and a modest pane can be carried up. A two storey house with a flat roof above an upper floor, or a terrace with no side passage, may need a full scaffold. A large unit may need a hoist or a small crane on the day, and that means a booking, a road position and sometimes a parking suspension from the council. In Crawley, the closely spaced 1950s to 1970s semis and terraces often have a narrow route to the back, and in the terraced streets of Brighton and Reigate the same is true. The survey looks at the route so that the quote can name it.

Inside the house, the route matters too. Moving plasterboard and glass through a finished extension means protecting floors and stairs. Working over a finished kitchen means covering the units and worktops. That takes time, and it is time that a careful quote includes.

Weather adds a planning cost. A flat roof must not be left open, so the work is scheduled for a dry spell, and a large opening is closed in the same day. That sometimes means a longer working day, and it means a flexible start date.

The work that the eye never sees is frequently the work that a low quote omits. Structure, paperwork and finishing each take time, and each belongs on the quote.

On structure, a survey checks the deck and joists for rot, for spans and for anything hidden such as a soil pipe or a cable. Anything the joists cannot carry is strengthened before the glass goes on. A small opening in a sound roof needs little. A wide opening in an old roof needs more.

A rooflight is building work. It is installed to current Building Regulations, and where the work is notifiable we handle the Building Control notification. The regulations that bear on it include Part L for thermal performance, Part K for safe glazing, Part B for fire and boundary distance, and Part F for ventilation. For a unit near a neighbour, see how close to a party wall a flat rooflight can go. The official guidance on building regulations approval is on gov.uk, and planning is checked for the address and never assumed.

Making good is the last piece. New plasterboard lining, skim, paint and a neat reveal are what turn a hole into a finished light well. Some quotes stop at the roof and leave the room to you. Others finish the ceiling as well. Both can be honest, but they are not the same job and should not be compared as if they were.

A fair comparison lines the items up rather than the totals. Two quotes can be honest and still differ by a large amount, simply because one counts more.

1. Glass: panes, thicknesses, laminated inner pane, coating and spacer named.
2. Upstand: material, height, insulation and who builds it.
3. Roof covering: what is cut, what is bonded and what is renewed.
4. Structure: trimming, doubling and any steel, priced or left as an extra.
5. Access: ladder, scaffold, hoist or crane, included or not.
6. Finishing: lining, plaster, paint and clearing up.
7. Regulations and guarantee: notification handled, and the length and terms of the cover.

A short quote that names none of these may still be fair, but it is harder to hold anyone to. Ours is written after a survey, it comes with a written quiet spec, and the work carries a 10-year workmanship guarantee, described on the guarantee page. To see the overall shape of the job from start to finish, read how a flat roof rooflight is installed.

When you are ready for a figure on your own roof, send a few photographs and the approximate size through the quote form, or message us on WhatsApp. The parent page on flat roof rooflight installation in Crawley shows the wider service.

Why does a flat skylight price change after the survey?

The survey shows what a photograph cannot: the state of the deck, the depth of the joists, the condition of the covering and the route to the roof. Each of those moves a line. A price given before a look is an estimate, and the written quote afterwards is the figure that counts.

Is a dome always cheaper than a flat glass rooflight?

The unit usually is, but the job is often not as far apart as it seems. The upstand, the carpentry, the access and the finishing cost the same for either. The real difference lies in the glass, the look and the sound in rain.

Does an existing flat roof add to the cost?

It can. A sound, well drained roof adds little. A roof with ponding or a tired covering needs work around the opening, and that belongs on the quote. See whether you can put a rooflight in an existing flat roof.

Where can a quote save money without losing quality?

Choose a fixed unit if you do not need ventilation, keep the opening between existing joists where the layout allows, and accept a standard size in place of a made to measure one. Do not trim the upstand insulation or the seals, because those are the parts that last.