Flat Roof Rooflight Installation

What counts as a flat roof once glass is involved

Anything under about ten degrees behaves as a flat roof for glazing purposes, whatever it is called on the drawing. At that angle water no longer clears the surface on its own. It moves because the roof has been laid to a designed fall, and if that fall is compromised anywhere, the water stops and waits.

Above ten degrees the conversation changes to a roof window trimmed between rafters, because the manufacturers rate their weathering by pitch and the flashing kits assume a covering that laps. Below it, you are in kerb territory. In practice the Essex flat roof stock is rear extensions, side returns, garage conversions, dormer cheeks and the wide low decks on the new town housing, and almost all of it sits between one and five degrees.

The three ways a unit meets a flat deck

There are only three arrangements in common use, and knowing which one you are buying settles most of the later questions.

The first is kerb-mounted. A builder forms an upstand off the roof structure, the covering is dressed up its sides, and the unit lands on top and is fixed down through its frame. The second is a proprietary insulated kerb: a moulded upstand supplied to match the unit, thermally broken, set on the deck and dressed the same way. The third is flush or near-flush glazing, where the visible surface sits close to the level of the roof finish and the drainage is handled inside the frame rather than by height above the water.

Kerb-mounted is the workhorse. Proprietary kerbs suit warm decks and tight programmes. Flush glazing looks the best from a bedroom window above and asks the most of the roof around it.

The kerb is the half of the job nobody sees

The sealed unit is made in a factory to a standard, by one of a small number of manufacturers, and it is reliably good. It arrives the same whoever fits it. What varies from one installation to the next is the two or three hundred millimetres of roof immediately around it.

That is why the kerb matters more than the brand on the frame. Its height decides how much standing water it can tolerate. Its squareness decides whether the unit beds evenly or rocks on a high corner. Its insulation decides whether the internal reveal grows a cold line in January. Its fixing decides whether it stays put in a February gale off the estuary.

Building a timber kerb, dimension by dimension

A site-built kerb is studwork. The usual build-up is 50mm by 100mm or 50mm by 150mm timber, framed as a rectangle, faced externally with 18mm exterior grade plywood and fixed down into the trimmers that form the opening, not just into the deck sheet.

Height is measured from the finished roof surface to the top of the kerb, and the working figure is 150mm. On a roof with a clean fall and a proven outlet, some manufacturers will warrant less. On a roof that is genuinely flat, that has held water in the past, or that sits exposed, 150mm is the floor rather than the target.

Two details get skipped and both cause trouble. The top of the kerb wants to be level and true across its length, because the unit is rigid and will not follow a twist. And the corners want plating or a proper corner post, since a kerb that racks under wind load opens the covering at exactly the point where the covering has the least material.

Proprietary insulated kerbs, and what they change

A proprietary kerb arrives as a moulded upstand, usually in a rigid insulant with a GRP or PVC skin, sized to the unit it will carry. It sets down onto the deck over the opening, is mechanically fixed and bonded, and the covering is dressed up it exactly as it would be up plywood.

The gain is thermal. A timber kerb is a continuous run of structure bridging from the warm side to the cold side, and unless it is insulated on the inside face it will run cooler than the ceiling around it. An insulated kerb carries the insulation line up and over the opening without a break, which shows up as a warmer internal reveal and less condensation in the corners.

The cost is flexibility. Proprietary kerbs come in the manufacturer’s sizes and want the structural opening cut to suit, so they belong in a job where the opening is being formed new rather than one where an existing hole is being reused.

Warm deck, cold deck and where the insulation runs

Two build-ups dominate. In a warm deck, the insulation sits above the structural deck, with a vapour control layer below it and the waterproofing on top. In a cold deck, the insulation sits between the joists and the void above it is ventilated to outside air.

The distinction matters at a rooflight because the kerb has to carry the relevant line up and over. On a warm deck the insulation is taken up the outside of the kerb and the vapour control layer is dressed up the inside, so the two envelopes stay continuous. On a cold deck the ventilation path across the void has to keep working with a large box sitting in the middle of it, which usually means the opening is positioned to leave clear air on both sides.

Warm deck is the modern default and it is what nearly every new Essex extension is built as. Cold deck turns up on flat roofs from the sixties and seventies and it is the build-up most likely to produce interstitial condensation once somebody cuts a hole in it.

Cutting a hole in a deck without weakening it

A flat roof deck spans one way. The joists run between two supports, they carry the deck and the covering and whatever snow lands on it, and cutting through them removes load path that has to be replaced.

The standard solution is trimming. The joists either side of the opening are doubled, trimmers are fixed across at each end with joist hangers rather than skew nails, and the cut joists hang off those trimmers. For a typical domestic opening up to about 1.2 metres across the span, doubled joists and hangers are enough. Beyond that, or where the joists are shallow to begin with, the trimmers themselves want sizing by calculation and the doubling may go to three.

Where an opening is genuinely wide, the load has to reach a wall and a steel becomes the honest answer. That changes the shape of the project rather than the price alone, because a calculation and a longer lead time come with it.

Where the opening sits, and why the centre is not automatic

The instinct is to centre the rooflight on the room. It is often wrong, for three reasons.

First, the joist run may not co-operate. An opening that lands neatly between two joists needs no trimming at all if it is narrow enough, and a shift of two hundred millimetres can turn a structural job into a carpentry one. Second, the fall of the roof means one part of the deck is lower than another, and putting a kerb at the low end sits it in the path of everything the roof collects. Third, the room reads from where people stand, and light landing on the far worktop is worth more than light landing symmetrically on the floor.

On a rear extension with a run of bifolds, pushing the unit back towards the original house is usually right. That is the part of the room the glazed wall never reaches, and it is also, conveniently, the high side of most falls.

Diverting water around the kerb, and where it goes next

Once a box is standing in the middle of a flat roof, the water arriving from above it has to get past. It will not do that by accident.

The fall is designed, conventionally at a minimum of 1:80 on the drawing to deliver at least 1:80 once deflection and construction tolerance are accounted for. Around a rooflight the fall is shaped to run water down the two long sides rather than allowing it to arrive square on the upslope face and stall there. On a tapered insulation scheme this is drawn as a cutting pattern before delivery. On a firred timber deck it is set out on site by adjusting the firring depths.

The water then has to leave. That means an outlet, and an outlet that stays clear. Where the only outlet on a small extension sits at the far corner from the new opening, the whole diagonal has to keep working, and a hollow that develops over a decade is what turns a good detail into a slow problem.

How each covering terminates at the kerb

The covering does not use a flashing kit on a flat roof. The membrane itself is the flashing, dressed up the kerb face and terminated under the unit, and each material handles that differently.

Age matters as much as material. Where a survey finds a covering close to the end of its working life, the sensible order of work is to renew the covering and set the glass into a roof that is new, rather than cutting a fresh opening into a surface that will be lifted in three years anyway.

Flush glazing, and the fall hidden inside the frame

A flush unit reads as a sheet of glass lying in the roof. It is the most striking of the three arrangements and it is also the one with the least margin.

What makes it work is that the glass is never truly level. Either the frame carries a built-in fall of around three to five degrees, or the kerb underneath is tapered so the unit sits at that angle while appearing flush from the ground. That small slope is what moves water off the glass and stops the outer pane sitting under a film of standing water and its dirt.

Genuine zero-fall glass exists and it has a place, mostly where a unit forms part of a terrace surface. It relies on the drainage of the surrounding roof being close to perfect, which is a reasonable assumption on a new build and an optimistic one on a thirty year old deck.

The glass build-up on a horizontal unit

Overhead glazing is specified differently from vertical glazing because the failure mode is different. Glass in a wall that breaks falls outwards. Glass over a room falls on the room.

The standard build-up is a toughened outer pane and a laminated inner pane. The outer is toughened because it takes the thermal load and the impact of hail and dropped tools, and toughening lets it take a large temperature difference across its face without fracturing. The inner is laminated so that if it does break, the interlayer holds the fragments in the frame rather than releasing them downwards.

Between them sits a cavity of sixteen to twenty millimetres, usually argon filled, with a low emissivity coating on the cavity side of the inner pane and a warm edge spacer around the perimeter. That combination is what gets a modern flat rooflight comfortably inside the U-value ceiling that Building Regulations set for the work.

Dirt, streaking and the angle of the glass

A near-horizontal pane collects everything the sky puts down and holds most of it. Pollen in May, tree debris in October, and in the flight paths around Stansted a fine film that vertical windows never seem to acquire.

Self-cleaning coatings help, and it is worth understanding what they need. The coating breaks down organic film in daylight and then relies on rain sheeting across the surface to carry it away. That sheeting action wants an angle. Below about five degrees the water sits rather than runs, and the coating does less than the brochure suggests.

Which is the practical argument for building the fall in rather than chasing a dead level line. Two extra degrees costs nothing on the kerb and makes the difference between glass that mostly looks after itself and glass that wants a ladder twice a year.

Summer heat through a horizontal aperture

A flat rooflight faces the whole dome of the sky. That is why it delivers so much more daylight than a window of the same area, and it is also why it is the hardest orientation to manage in July.

At the height of an Essex summer the sun sits at roughly sixty degrees above the horizon in the middle of the day, which puts it almost square-on to a horizontal pane. There is no wall angle taking the edge off it and no eaves overhang shading it. A generous flat rooflight with clear glass over a south facing kitchen will make that room unusable between two and five in the afternoon.

The answer is the g-value: the fraction of solar energy that gets through. Solar control glass brings it down substantially while keeping most of the visible light, and on a flat unit it is close to a default rather than an upgrade. The detail of coatings and the numbers worth asking for sit on the energy efficient rooflights page.

Condensation on a flat unit, and the channel in the frame

Surface condensation on the room side of the glass is physics rather than a fault. Warm moist air from a kitchen or a bathroom meets a surface that is colder than the dew point, and water forms.

On a vertical window it runs down onto a sill and dries. On a flat rooflight there is nowhere for it to run, so it beads and sits on the underside of the pane and, in the worst case, drops. Good units handle this with a drainage channel formed in the internal frame profile that collects the run-off and takes it to weep holes at the low edge. It is a small feature and it is the difference between a damp mark on a reveal and nothing at all.

Everything else is about the room. Extract ventilation that actually discharges outside, a trickle path for background air, and glass warm enough on the inner face that the dew point is rarely reached. Misting between the panes is a different matter entirely and means the unit has reached the end of its life, which is covered under replacement.

Wind uplift, and how the unit is held down

Wind does not press down on a flat roof. It accelerates over the parapet or the edge and creates suction, and the strongest suction is at the corners and along the perimeter, not in the middle. A rooflight sitting proud of the surface is a small obstruction in that flow and it gets its share.

The unit is therefore mechanically fixed through its frame into the kerb at a specified spacing, typically every three hundred millimetres or so and always within a short distance of each corner, rather than relying on its own weight and a bead of sealant. The kerb in turn is fixed into the trimmers rather than into the deck sheet, because a plywood deck screwed to joists will lift as a panel if the load finds it.

Exposure varies more across Essex than people expect. A sheltered rear extension in central Chelmsford and a bungalow roof on Canvey facing open water across the estuary are not the same design case, and the fixing schedule reflects it.

Kerb size, unit size, and the number a supplier is quoting

Three different measurements get called the size of a rooflight, and confusing them is the most common ordering mistake on a flat roof job.

• Structural opening: the hole cut in the deck, measured between the trimmers.
• Kerb size: usually quoted as the external face of the upstand, which is what the covering wraps.
• Unit size: the outside of the rooflight frame, which sits over the kerb with a defined overhang.

A supplier quoting a 1000 by 2000 rooflight is nearly always quoting the kerb size, meaning the unit itself is slightly larger and the visible glass considerably smaller once the frame is deducted. Ask which of the three you are being given and what the daylight opening comes to, because that last figure is the one you will actually see from the room below.

There is a sequencing trap here as well. On a new extension the unit is often ordered before the kerb exists, from a builder’s dimension on a drawing. The kerb then wants building to the order, not the other way round, and it wants checking with a tape and a diagonal before the delivery arrives.

One large rectangle or a run of smaller units

Beyond about 1.5 metres in either direction the job changes character. The glass becomes a special rather than a stock size, the weight moves past what can be lifted by hand up a ladder, and the structural opening usually needs designing rather than trimming.

A run of two or three smaller units avoids all three. They arrive as standard sizes, they go up without mechanical lifting, and each opening trims between existing joists. The cost per square metre of glass is higher but the cost of the job is often lower, and on a long narrow extension a rhythm of three squares reads better from inside than one letterbox.

The argument for the single large unit is the view and the uninterrupted ceiling. Where the room is wide, the structure is being formed new anyway and a steel is already in the design, the incremental cost of going large is smaller than it first appears.

Opening flat units, and the hinge geometry

An opening flat rooflight hinges along one edge and the whole pane lifts, usually to somewhere around three hundred millimetres of clear opening at the free edge. That geometry matters because the unit becomes a small awning when open, and rain arriving at an angle will get under it.

Which is why almost every opening flat unit is powered rather than manual. There is no reasonable way to reach the free edge of a horizontal pane at ceiling height with a pole, and a powered unit can take a rain sensor that closes it before the weather gets in. The control options and the wiring are covered on the electric rooflight page.

The ventilation gain is real. Warm air collects at the highest point in a room, so an opening at roof level lets it leave while cooler air is drawn in through doors and windows lower down. In a tall extension on a still summer evening that moves a surprising volume of air with no fan running.

Flat roofs across Harlow, Basildon and the Essex new towns

Essex has more flat roof than most counties, and it arrived in two waves. The new town housing at Harlow and Basildon put wide low decks over kitchens, garages and link blocks from the fifties onwards, most of it originally cold deck with felt. Then came the sixties and seventies bungalows and rear additions, on Canvey, around Wickford and across the older commuter suburbs, with the same build-up.

That stock is now on its second or third covering. What a survey finds is a deck of chipboard or softwood boarding rather than modern ply, joists at generous centres, and a ventilated void that somebody has since filled with insulation without thinking about the airflow. None of that stops a rooflight going in. It does mean the deck gets opened up and looked at properly before a fixed figure is given.

The other constant is the Victorian side return, in the terraces off Moulsham Street and Broomfield Road and throughout Colchester and Brentwood. Narrow, deep, one external wall, and a flat roof over the infill. It is the single most common flat rooflight job in the county and the one where the daylight gain is most dramatic, because the space had almost none to start with.

Notifiable work, and what Building Control looks at on a flat roof

Forming a new opening in a roof and glazing it is notifiable work under Building Regulations in England. Sunspire handles the notification as part of the job, which matters more than it sounds, because unnotified alterations surface years later during a sale when a solicitor asks for a completion certificate.

Three parts are engaged. Part L sets the thermal performance, and for a new opening in an existing dwelling the U-value of the whole unit including the frame is what is assessed, not the glass alone. Part A covers the structural alteration where joists have been trimmed or a beam introduced. Part B applies where the rooflight forms part of an escape route, and Part K applies where there is a risk of falling, which is why guarding comes into the conversation on any accessible roof.

An inspection typically happens twice: once with the structure open and the trimming visible, and once at completion. Both are straightforward when the detailing was right, and both are far cheaper to satisfy than to correct.

What a flat roof survey turns up

The survey is where a range becomes a fixed price, and on a flat roof it is doing more work than on a pitched one, because nothing is visible from either side.

It establishes the direction and depth of the joists and their spacing, the deck material and its condition, whether the build-up is warm or cold, where the vapour control layer sits, the covering type and its remaining life, the designed fall and any evidence of standing water, the position of outlets, and the ceiling line and services in the void below.

Awkward truths surface at this point and it is the entire purpose of the exercise. A bathroom extract discharging into a cold void. A ceiling that is not where the drawing says. Boarding that has softened at one corner under a long standing hollow. Finding any of those before a price is agreed is a good outcome, not a bad one.

How the installation runs across the days

A single flat rooflight into an existing flat roof is generally one to two days. Two or three units, or a job with a new beam, runs to three or four.

The sequence is consistent. The room below is protected and the ceiling opened where needed. The covering is stripped back well beyond the opening so there is clean material to bond to. The deck is cut, the joists trimmed and the trimmers hung. The kerb is built or set, insulated and squared. The covering is made good up the kerb face and terminated. The unit is set, bedded, fixed and weathered. Then the internal reveal is formed and finished.

Whatever the forecast has decided to do, the roof is closed and watertight at the end of every working day. On a flat roof that is not a courtesy, it is the whole basis of working on one at all.

What moves the figure on a flat roof job

Sunspire quotes in two stages: a range by email once the basics are known, then a fixed written figure after a survey. Between those two points, five things move the number.

Size moves it, but not evenly, because of the step at around 1.5 metres where the glass becomes a special and the structure needs designing. The kerb type moves it, since a proprietary insulated kerb costs more in materials and less in labour than a built one. The covering moves it, because welding single ply around a new upstand and laminating GRP are different trades with different day rates. Access moves it, and a rear extension roof reached off a garden is a different job from a second floor deck over a narrow side return. And structure moves it most of all once a beam enters the drawing.

Indicative ranges for each of those variables sit on the costs page.

The autumn check around the kerb

There is very little to do, which is rather the point of a well detailed flat rooflight. The glass wants washing once or twice a year. The perimeter sealant wants a look every few years.

The genuinely useful habit takes five minutes in October. Clear leaves, moss and grit from around the base of the kerb and from the outlet, and check that water has a clear route from the upslope face of the unit all the way off the roof. Most water problems that appear around flat rooflights start as a drainage path that quietly closed itself, and they are always cheaper to prevent in autumn than to investigate in February.

On a warm deck with a good fall, that is the entire maintenance schedule for the working life of the unit.

The photographs that get you a range

Two photographs and three numbers are usually enough for a sensible range by email. A photograph of the roof from above if there is an upstairs window overlooking it, a photograph of the ceiling from inside, the rough dimensions of the room, and whether you know what the covering is.

If you are earlier than that, start with the roof rather than the product. Which way the joists run, what the covering is and how old, where the water leaves, and whether the build-up is warm or cold. Those four answers narrow the field faster than any showroom visit, and they are the same four the survey will confirm. Guides on each of them sit under guides, and the towns covered are listed under areas.