Flat Roof Skylight Installation

A flat roof rooflight is the quietest way to put daylight into a room. No ridge, no pitched glazing bars, no shape rising out of the roof: just a plane of glass sitting almost level with the deck, reading from below as a rectangle of sky cut into the ceiling. That simplicity is exactly why the detailing has to be right. On a pitched roof, gravity helps you. On a flat roof, water has nowhere obvious to go, the covering has to be turned up and terminated against something, and the glass is presented to the sky at close to the worst possible angle for summer heat. This page covers how fixed and opening rooflights get installed on flat and low-pitch roofs across Southend-on-Sea and south Essex, from a single unit over a bathroom to a run of four across the back of an extension, and what separates a detail that lasts twenty years from one that gives trouble in its third winter.

01Fixed, opening, walk-on: which type of rooflight suits the roof

Rooflight and skylight mean the same thing in ordinary speech. The trade tends to say rooflight for anything set into a roof plane and skylight more loosely, and nobody will misunderstand you either way. What matters is which of the several quite different products you actually want, because they get installed differently, cost differently and behave differently once they are in.

Fixed flush-glazed units

The most common choice, and usually the right one. A sealed glass unit bonded into a slim aluminium perimeter frame, dropped onto a kerb, with almost no visible frame from inside. The glass itself is built with a slight internal slope, typically 5 degrees, so that even on a level kerb the outer pane sheds water rather than holding it. Sizes run from around 600 by 900 millimetres up to roughly 1200 by 2400 as a standard product, and beyond that you move into structural glass made to order. There is nothing to operate, nothing to go wrong mechanically and nothing to let air in, which is why they are the default over hallways, landings, bathrooms and the middle of open-plan kitchens.

Hinged opening units

The same glass and frame, on a hinge, opening either on a manual pole or on a chain or spindle actuator. On a flat roof these usually open to somewhere between 200 and 400 millimetres of clear gap, which is not much to look at but is a great deal in terms of air movement, because hot air leaves through the highest opening in a room and takes the rest of the stale air with it. A single opening unit at the far end of a kitchen will shift more heat on a still evening than two open doors at floor level. If the unit is going to be motorised, and above about two metres of ceiling height it should be, the wiring needs planning early rather than chased in afterwards. That side of it is covered on our electric skylight page.

Access hatches and terrace hinges

A heavier hinge arrangement with a stay, sized so a person can get through it onto the roof. Worth having on a flat roof that anyone will ever need to stand on, and required thinking if the flat roof is being used as a terrace, because that changes what the Building Regulations expect of guarding around the edges.

Walk-on and floor lights

Structural glass set flush into a roof or terrace deck, strong enough to be walked over, usually with a non-slip ceramic frit or an etched surface on the outer pane. These bring daylight into a basement or a lower storey under a raised terrace. The glass build-up is much heavier, commonly three laminated leaves, and the loading has to be designed rather than assumed. Expect two to three times the cost per square metre of a standard fixed unit.

Box rooflights and sliding units

A raised glass box, or a fixed pane with a second pane that slides back over it on a track to leave an open hole. These are architectural rather than practical, they sit taller in the roof plane, and they need more room around them. Handsome on the right building, overkill on a modest extension.

Domes and polycarbonate

The traditional flat roof rooflight was a moulded acrylic or polycarbonate dome on a splayed kerb, and there are thousands of them still in service across the 1960s and 1970s stock in this area. They are cheap, light and can be replaced quickly. They are also poor thermally, they yellow and craze with ultraviolet exposure, and they scatter light rather than giving you a view of sky. Where cost or a very large area of industrial roof rules, they still make sense, and we cover them on our polycarbonate rooflight page. On a house, glass is almost always worth the difference.

Whichever type you choose, one detail is not negotiable. Overhead glazing takes a laminated inner pane, so that if it is ever broken the pieces stay bonded to the interlayer instead of arriving on the floor below. Part K of the Building Regulations covers safety glazing, and any serious product will already be built this way, with a toughened outer leaf and a laminated inner.

02Warm roof, cold roof, and where the insulation actually sits

You cannot detail a rooflight properly without knowing how the roof underneath it is built, because the kerb has to keep the insulation line continuous. Get that wrong and the perimeter of your beautiful new rooflight becomes a cold ring running right around the opening, with condensation forming on the plaster reveal on every cold morning. People then blame the glass, which is almost never the culprit.

Cold roof

The older arrangement, and still what you find on most flat roofs built in this area before about 2000. Insulation sits between the joists, the deck goes over the top, and a ventilated air gap is meant to be maintained above the insulation with cross-ventilation at the eaves. It works when the ventilation path is genuinely open and the ceiling below has an effective vapour barrier. In practice, downlighters, loft hatches and later alterations puncture the barrier, the ventilation gets blocked by insulation pushed too high, and moist air from the room condenses on the underside of a cold deck. A great many soft, spongy flat roof decks in south Essex are the result of that rather than of any rain getting in. Cutting a rooflight opening into a cold roof means keeping the ventilation path around the new opening, not just closing it off.

Warm roof

The modern standard and what any new extension will be built as. Structural deck first, then a vapour control layer, then rigid insulation, typically PIR board of 100 to 150 millimetres depending on the target U-value, then the waterproof covering directly on top. The insulation is above the structure, the deck stays warm, the dew point sits within the insulation rather than on the timber, and there is no ventilation void to maintain. It is simpler, it performs better, and it is more tolerant of the small imperfections that real buildings have.

For a rooflight, the warm roof build-up creates the detail that matters. The kerb has to be insulated on its outer face so that the insulation running across the roof turns up and meets the rooflight frame without a gap. A bare timber kerb sitting in a 140mm blanket of PIR is a thermal short circuit around the whole perimeter. When we set out a kerb, the insulation upstand is part of the drawing, not an afterthought.

Inverted and ballasted roofs

Less common on houses but present on flat roofs that double as terraces or that carry a green roof. The waterproofing goes on the deck and the insulation goes over the top of it, held down by paving, gravel or a growing medium. The kerb has to be tall enough to clear all of that, which usually means 300 millimetres or more rather than the usual 150.

What Part L asks for

New and enlarged rooflights have to meet a limiting thermal standard under Part L of the Building Regulations, and the flat roof around them has its own, considerably tighter, target. One quirk is worth knowing because it trips people up when comparing quotes. Rooflight U-values are conventionally declared in the vertical plane, the way a window is tested, and a unit installed close to horizontal performs slightly worse than that declared figure because of the way air convects inside a horizontal cavity. Two suppliers can quote the same number and mean different things. Ask whether the figure quoted is the vertically declared value or the in-situ horizontal one, and ask whether it is the whole unit including the frame or just the centre of the glass.

03Falls, ponding and where the water is meant to go

Flat roofs are not flat. They are built with a designed slope, and the slope is the entire drainage strategy. British practice is to design a fall of about 1 in 40 in order to reliably achieve 1 in 80 once construction tolerances, deflection and settlement have had their say. Aim for 1 in 80 on the drawing and you will end up with parts of the roof running level or slightly backwards, and that is where water sits.

A rooflight kerb is an obstruction in the middle of that drainage path. Water arriving from upslope has to get around it. If the unit is positioned without thinking about the direction of fall, you create a dead pocket on the upslope side where water collects, sits, evaporates and leaves a permanent tide mark of silt against the kerb. Standing water is not by itself a leak, but it is the place where a leak will eventually start, because it works on every joint and lap continuously rather than for an hour after rain.

Three things solve it, and on most jobs you use more than one.

• Position with the fall in mind. Set the long axis of the rooflight running down the slope rather than across it, so water divides and passes either side instead of banking up against a two metre dam.
• Tapered insulation. On a warm roof, the falls can be formed in the insulation itself, and small tapered pieces can be set upslope of a kerb to divert water around it. This is standard practice on commercial roofs and underused on domestic ones.
• Outlet position. Do not put the rooflight over or immediately next to the rainwater outlet. The outlet needs to stay accessible, and it needs the lowest point of the roof to itself.

Then there is the glass itself. A sealed unit lying at zero degrees holds a film of water and everything the water carries. Within a season you get drying marks, pollen residue and, on a shaded roof, a green film of algae along one edge. Estuary sites add a fine salt haze that dries to a matt bloom. This is why decent flush-glazed units are built with an internal slope in the glass, and why we would rather set a kerb with a slight designed pitch, 5 to 10 degrees, where the roof geometry allows it. It is invisible from the garden and it makes the difference between glass that stays clear and glass you notice every time you look up.

Self-cleaning coatings help on a sloped unit and do very little on a level one, because the coating needs rain to sheet off and carry the loosened dirt away. On a near-level pane, the water simply pools and dries in place. Anyone selling self-cleaning glass for a zero-pitch installation is selling you an idea rather than a result.

04The kerb, and how each roof covering is dressed to it

The kerb is the raised collar around the opening that lifts the glazing above the water line and gives the roof covering something to terminate against. It carries the unit, it keeps the insulation continuous and it is the single component most likely to be the reason a rooflight gives trouble. It is also completely invisible once the job is done, which is precisely why it gets skimped.

Kerb types

• Site-built timber. Treated softwood, usually 50 by 100 or 50 by 150, built off the trimmers around the opening, sheathed in exterior-grade plywood or OSB and faced with rigid insulation. The most common and perfectly good when built accurately.
• Factory insulated kerbs. Preformed units in insulated composite or GRP, delivered as one piece, mechanically secured over the opening. They arrive square, which removes the main source of error, and they suit installers who want a known dimension to order glass against.
• Integral upstands. Some rooflights are supplied with their own insulated upstand as part of the product, so the whole assembly lands on the deck and the covering dresses straight up it. Simple, and increasingly common on flat roof units.

Height and tolerance

The working figure is 150 millimetres from the finished roof surface to the top of the kerb, measured at the highest point of the local falls rather than the lowest. Membrane manufacturers want that height for a sound termination and rooflight manufacturers assume it. Where a designer wants the unit to sit lower for the look of it, the trade-off should be stated out loud rather than discovered later. On an inverted roof or under a terrace deck, the figure climbs.

Squareness matters as much as height. A flush-glazed unit is made to millimetre dimensions taken off the external face of the kerb, and the gaskets only compress evenly if the frame lands on a flat, true plane. A few millimetres of twist across the diagonals and one corner sits proud. We survey the built kerb before ordering glass, every time, because the drawing tells you what somebody intended and the tape tells you what exists.

Detailing to the covering

The principle is the same for every roof covering: the waterproof layer runs up the outside face of the kerb, over the top edge, and terminates under the rooflight frame, so water can only ever run away from the timber. How that is achieved varies quite a lot.

• Single-ply membrane (PVC or TPO). Sheet dressed up the kerb and hot-air welded at the corners, usually with prefabricated corner pieces so there is no folded bulk at the change of direction. The top edge is terminated with a coated metal trim that the membrane welds directly to, which gives a clean mechanical stop rather than relying on adhesive. The most reliable of the domestic options and what we would generally want to see around a new unit.
• Reinforced bitumen membrane (felt). Two or three layers, torch-applied or self-adhesive, with a timber or insulation angle fillet at the base of the kerb so the sheet turns through the corner on a curve instead of a right angle. Felt cracks where it is forced to fold sharply, and the fillet is the difference between a detail that lasts and one that splits at the corners. The top edge is turned over the kerb head and held with a batten or trim.
• GRP (fibreglass). Laminated in place over a plywood deck and taken continuously up and over the kerb head, with preformed trims bonded into the laminate at the edges. GRP has no joints, which is its advantage, but it is brittle at internal corners and it moves less than the timber underneath it. A properly formed fillet at the base of the kerb and a laminate of adequate thickness at the corners are what keep hairline cracking away. Cure temperature matters too, which is why nobody should be laying GRP on a cold, damp December morning and pretending it will be fine.
• EPDM rubber. A single sheet with no field joints on most domestic roofs, bonded to the deck and dressed up the kerb, with moulded corner pieces at the four changes of direction. Very forgiving of movement, and quick, but the bond at the kerb head is the part to get right because there is no welded seam holding it.

Once the covering is dressed and terminated, the unit is bedded on a continuous compressible tape or a suitable sealant on the kerb head and secured down through its frame at the manufacturer’s stated centres. External trims then close the joint between frame and covering. Water that gets past the outer seal, and on a horizontal surface some always will, drains through the frame’s own channels and out onto the roof covering, well above the water line.

05South-facing glass over an estuary city, and the number nobody quotes

Southend-on-Sea faces south across the Thames Estuary, which is an unusual orientation for an English seaside settlement and the reason we start every survey with a compass rather than a brochure. A rear extension on the landward side of any street running parallel with the front is pointing its roof directly at the southern sky, with open water beyond it and very little mature tree cover in the way.

Near-horizontal glass behaves differently from a window, and the difference is entirely about the angle the sun strikes it. A vertical window at this latitude takes the high June sun at a glancing angle and gets a good deal of natural shading from its own reveal, from the eaves above it and from anything to either side. A rooflight lying flat in the roof plane has none of that. There is no reveal deep enough to matter, nothing above it, and no time of day when the sun is at a shallow angle to its surface during the months you would most like it to be. It collects. That is not an argument against rooflights: overhead glazing is comfortably the most efficient way to get daylight deep into a plan, worth several times its area in vertical glass, and in a south Essex January that means the lights stay off through the afternoon. It is an argument for choosing the glass deliberately.

Two numbers describe any sealed unit. Everyone quotes the first and almost nobody volunteers the second.

U-value measures heat escaping outwards, in watts per square metre per kelvin. Lower is better and it governs winter comfort, running costs and whether the inner pane runs cold enough to attract condensation.

G-value, sometimes written as solar factor or g, measures the fraction of the sun’s energy that gets through and becomes heat inside. It runs from 0 to 1. Ordinary clear double glazing sits near 0.6. Good solar-control glass sits near 0.3. On a roof plane that is the number that decides whether your kitchen is pleasant in August.

Scale it to a real job. Take a run of three units, each a metre by a metre and a half, so 4.5 square metres of glass across the back of an extension. On a clear July afternoon a horizontal surface here can be receiving in the region of 700 watts per square metre. In clear double glazing that is roughly 1,900 watts of heat arriving in the room. Swap to a solar-control unit at 0.3 and it is around 950. The room looks essentially the same to the eye, because human vision adapts to a very wide range of brightness, but you have removed the equivalent of a large electric heater from the ceiling.

The figures below are general industry values for the sorts of build-up you will be offered. Exact numbers vary by manufacturer, cavity width and gas fill, and they should appear on the quotation rather than being taken from a page like this one.

The honest downside of solar-control coatings is that they cut daylight along with heat, and they carry a very slight tint. Modern neutral coatings read as a barely perceptible grey when you stand directly under them and are gone from your awareness within a week. Older solar glass had a bronze or blue cast that you could not stop seeing. Current neutral products do not, so ask to look through an offcut before you accept one that tints the view.

Two other levers exist and both are worth knowing about. Internal blinds, either pleated cassettes running in side channels or roller blinds, are the cheapest way to control glare, and glare on a screen or a worktop is a separate problem from heat. They should be specified with the rooflight rather than added later, because the channels are neater when they are part of the frame. External shading works far better thermally, because it stops the energy before it reaches the glass, but on a flat roof it is difficult to do without spoiling the look. For most houses, the right answer is good glass first and a blind for the handful of afternoons when the sun is directly overhead. The wider question of thermal performance across a whole house is set out on our energy-efficient skylight page.

06One unit or a run: sizing, spacing and what the roof can carry

The first question on most extensions is not what size, it is how many. A single large rooflight and a run of three smaller ones can deliver similar daylight, and they behave quite differently.

A single unit gives you one strong shaft of light and a simple ceiling. It needs a wider structural opening, which usually means the joists on either side are trimmed more heavily, and the glass gets expensive quickly beyond about two and a half metres in either direction. A run of three or four gives you an even wash of light across the whole depth of the room, keeps every unit within standard product sizes, and preserves the structural continuity of the roof between them. It also gives you the option of making one of them an opening unit and leaving the rest fixed, which is the cheapest ventilation you will ever buy.

For daylight, a useful starting point on a rear extension is overhead glazing amounting to roughly eight to twelve per cent of the floor area of the room below, on top of whatever vertical glazing you have. Under about six per cent and the rooflights read as decorative rather than functional. Over about fifteen per cent, on a south-facing roof, you are into territory where the glass specification has to work hard and blinds stop being optional.

Spacing and margins

Leave at least 300 millimetres of roof between adjacent kerbs, and 400 to 500 is better. That gap has to accommodate two kerb faces, two membrane terminations, the insulation between them and a drainage path. Squeeze it to 150 and there is no room to dress the covering properly, which is where a run of rooflights starts to leak. Around the perimeter, keep a similar margin from any parapet or upstand wall, and think about where anyone cleaning the glass is going to put their feet.

Alignment matters visually more than people expect. From inside, a run of rooflights that is 20 millimetres out of line across the ceiling is obvious, because the reveals draw a straight line and the eye follows it. Setting out is done from a single datum across the whole run rather than measuring each opening from the last one.

Weight and structure

A square metre of sealed double glazing with a toughened outer and a laminated inner weighs somewhere around 27 kilograms, plus the frame. Triple glazing takes that towards 40, and a walk-on build-up can reach 90 or more. So a 1200 by 2400 fixed unit is close to 90 kilograms of glass alone, which is a two-person lift with proper suction handles and a three-person one if the access is awkward.

Cutting the opening interrupts the joists. Those cut joists are carried on trimmers spanning between the neighbours, and the trimmers then carry both the roof loads they have inherited and the rooflight on top. Doubled timber trimmers cope with most domestic openings. Once the opening exceeds about 2.4 metres across the joist direction, or where the roof also carries a parapet, a terrace or a green roof build-up, it stops being a rule of thumb and becomes a calculation for a structural engineer. On a new extension the engineer has usually already sized the whole roof. Cutting into an existing flat roof deserves a look from someone who can do the sums, particularly on the 1960s and 1970s extensions in this area where joist sizes were often marginal to begin with.

Where the shape of the roof or the geometry of the extension pushes past standard products, whether that is a rooflight meeting a wall on one side, a non-rectangular opening or a continuous run of structural glass with no visible frames between the panes, that becomes a made-to-order piece with its own survey and lead time. If what you actually want is a raised, pitched glazed structure rather than a flush pane, that is a different product and it lives on the roof lantern page.

07Replacing a failed rooflight

A good proportion of the flat roof work we do is not a new opening at all. It is taking out a unit that has come to the end of its life and installing a properly specified replacement in its place. That is straightforward work when the kerb underneath is sound, and it transforms a room out of proportion to what it costs.

What failure looks like

• Misting between the panes. The perimeter seal of a sealed unit has a service life, typically fifteen to twenty-five years, longer on a well-drained unit and shorter on one that sits in standing water. Once it goes, moist air enters the cavity, condenses on the inside of the glass overnight and dries out in the day, leaving a permanent mineral haze. A misted unit cannot be recovered. It is replaced.
• Yellowed or crazed plastic. Acrylic and polycarbonate domes lose clarity and go brittle under ultraviolet exposure. A dome from the early 1990s is likely to be opaque, discoloured and no longer safe to lean any weight against.
• Single glazing. Older domestic rooflights and many older domes are single-skin. In a heated room that is a cold surface running with condensation all winter and a serious hole in the thermal performance of the roof.
• Perished gaskets and trims. Rubber goes hard and shrinks. Once the gasket has lost its compression the unit is relying on whatever sealant was applied over the top of it, which is not a long-term arrangement.
• The wrong size for the room. Not a defect, but a common reason for replacement. A 600 by 600 dome in a kitchen that has since been opened up into a dining space is doing about a quarter of the job it could be.

What the work involves

The survey is the important part, and it happens before anything is ordered. We measure the kerb externally, because modern flush-glazed units are sized against external kerb dimensions while older domes were often sized against the structural opening or against an internal dimension, and the two conventions do not agree. We check the kerb timber for softness, particularly at the corners and on the upslope face where water has been sitting. We look at how the existing covering is terminated, because lifting the old unit will disturb that termination and it has to be reformed.

On the day, the old unit comes off, the kerb head is cleaned back to sound material and any decayed timber is cut out and rebuilt. Where the kerb is too low or the wrong size for a modern unit, an adaptor kerb is built over the existing one, which is usually quicker and less disruptive than taking the original out. The covering is then dressed and terminated correctly at the new kerb head, the new unit is bedded and secured, and the trims go on. Most single replacements are a half-day to a day. A run of three is usually two days.

One boundary worth stating plainly. If the glass and frame are sound and the problem is the flat roof covering around them, that is roofing work and a roofer is the right person to call. We install and replace rooflights. Where the unit itself has failed, or where the covering is being renewed anyway and it makes sense to put a better rooflight in while the roof is open, that is squarely our work.

08Permission, Building Control and the paperwork you will need later

Two separate approval systems apply to rooflights, and clearing one does nothing about the other. People conflate them regularly and it causes trouble years later, usually at the point of sale.

Planning permission

Rooflights on a house are generally permitted development, meaning no planning application is needed, provided they project no more than 150 millimetres beyond the plane of the existing roof and do not sit above the highest part of the roof. On side elevations below 1.7 metres from floor level there are additional requirements for obscure glazing and restricted opening.

On a flat roof this needs reading carefully. A unit on a 150 millimetre kerb, plus the frame above it, plainly stands more than 150 millimetres proud of the roof surface. In practice a rooflight forming part of a new single-storey rear extension is normally assessed as part of that extension rather than as a separate rooflight, and the extension’s own permitted development limits govern. Retrofitting into an existing flat roof is where the question is worth asking directly. Treat everything here as general guidance and confirm the position with the local authority for your address before glass is ordered. A written answer costs nothing and is worth having on file.

The exceptions are firm:

• Flats and maisonettes hold no permitted development rights, so any rooflight needs an application.
• Listed buildings need listed building consent whatever the size or position.
• Conservation areas attract closer scrutiny, particularly on roof planes visible from a highway.
• Article 4 Directions remove permitted development rights for the things they name. The Leigh Cliff and Leigh conservation areas carry such a Direction covering roofing materials and windows, so work that would need nothing elsewhere requires a planning application there. Both appraisals were updated in March 2022. Clifftown and Leigh Old Town are conservation areas too.

Southend-on-Sea has been a city since 2022 and the planning authority is Southend-on-Sea City Council. Jobs beyond the boundary fall to Rochford, Castle Point, Basildon or Thurrock. Whichever applies, ask before you order.

Building Regulations

A new or enlarged rooflight is notifiable under the Building Regulations without exception, whether or not planning permission is involved. Several Approved Documents bear on the work:

• Part L sets the thermal standard for the rooflight and for the roof it sits in. Any decent glazed unit clears the rooflight figure comfortably; the roof build-up is usually the harder target.
• Part K covers safety glazing and protection from falling, which is why overhead glass takes a laminated inner pane and why an accessible flat roof or terrace needs guarding considered.
• Part F becomes relevant if the rooflight is replacing or affecting the ventilation provision of the room, which happens more often than you would think on bathrooms and kitchens.
• Part B matters where a rooflight is close to a boundary, because there are limits on the use of plastic glazing materials in that situation.

We make the Building Control notification to Southend-on-Sea City Council, or to whichever authority covers your address, on your behalf. The completion paperwork comes to you at the end of the job, and it is the document a conveyancing solicitor will ask for when the house is eventually sold. Skipped notifications cause a disproportionate amount of grief later, and it is not something we would ever hand back to a customer to deal with.

09Installation day, what it costs, and what to ask before you sign

How the work runs

Timings depend almost entirely on whether the kerb already exists. A rooflight going onto a prepared, weathered, square kerb is typically a half-day to a day. Forming a new opening in an existing flat roof, trimming the structure, building and insulating the kerb, and reforming the covering around it usually runs to two or three days per unit, less per unit on a run because the messy work is shared.

The sequence on a retrofit goes roughly like this. The room below is protected and the ceiling opening is cut and covered from inside. The deck is opened from above and the joists trimmed, with the structure supported while that happens. The kerb is built, checked for square and level, and insulated. The covering is then dressed up the kerb and terminated at the head. Only then does the glass go on, bedded and secured, with the external trims following. Internally the reveal is squared and prepared, though plastering and decorating are usually a separate trade and it saves argument to agree who is doing that beforehand.

Two practical constraints shape the day. The roof cannot be left open overnight, so on a multi-day job the opening is made watertight at the end of every day, and that dictates how much gets started in an afternoon. And wind decides whether large units get lifted, far more than rain does. A big pane behaves like a sail, so on an exposed estuary roof a gusty morning is reason enough to hold the glazing over until the next day. That call gets made in the customer’s favour every time, because a dropped unit is a worse outcome than a delayed one.

At handover we run water over the finished unit and watch the inside while it is running, which tells you more in ten minutes than waiting a fortnight for weather. Then the paperwork: the manufacturer’s product warranty on the glass and frame, our 10-year workmanship guarantee, and the Building Control notification already lodged. Over fifteen years of installing rooflights along this stretch of the estuary, most of the surprises a flat roof can produce have shown up at least once, and the survey is where we would much rather meet them.

What it costs

The figures below are general market guidance for supplied and installed work in south Essex, not a quotation. They assume aluminium flush-glazed units with solar-control double glazing, which is what we would specify on most south-facing roofs here. A survey is what turns a range into a price.

What moves a price within those ranges is fairly predictable. Glass specification, which is the one place the extra money is always well spent. Frame colour, where anthracite and white are usually stock and anything else adds lead time. Whether the covering has to be reformed or is being renewed anyway. Access, because a rear extension roof reachable from a garden with a tower is a different proposition from one that can only be reached through the house or needs scaffolding. And how much of the substrate work sits inside the quotation, which is the single largest reason two quotes for the same job can be thousands apart. There is a broader breakdown of our pricing on the costs page.

Questions worth asking anyone who quotes

1. What is the G-value of the glass you have priced? If only a U-value comes back, the summer side of the problem has not been considered at all.
2. Is that U-value the whole unit or the centre pane, and is it the declared vertical figure or the in-situ horizontal one? All four numbers exist and they are not the same.
3. Does the price include building the kerb, or does it assume one exists, square, insulated and weathered?
4. How will the covering be terminated at the kerb head? A clear answer naming the trim or the welded detail tells you the person has done this before.
5. Which way does the fall run, and where does water go past the kerb? If nobody has been on the roof with a level, the answer is a guess.
6. Is the inner pane laminated? For overhead glazing it should be.
7. Who notifies Building Control, and when do I get the completion certificate?
8. Will the finished kerb be measured before the glass is ordered? The right answer is yes, without hesitation.
9. How is the roof kept watertight overnight on a multi-day job?
10. What covers the workmanship, separately from the manufacturer’s product warranty? Two different things, and both should be written down.

Be wary of a quotation produced without a site visit, of one that is silent on the kerb, and of anyone who tells you solar control is unnecessary on a south-facing roof in this city. Be wary too of pressure to sign for a discount that expires on the day, which is a habit borrowed from doorstep double glazing and has no business on a job that involves cutting a hole in your roof.

We cover flat roof skylight installation in Southend and across south Essex, from the 1930s semis behind the seafront to the newer estates at Shoeburyness and the villages north of the city. If you are planning an extension, send the drawings and we will tell you what the roof needs before the builder pours anything. If you have a tired dome or a misted unit over a room you have stopped using in summer, we will come and measure the kerb. Phone 01702 898232, or use the quote form and tell us which way the roof faces. It is the first thing we will ask.

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