Flat Roof Skylight Installation

Flush or kerb-mounted

Rooflights for flat roofs divide into two families, and the difference is not cosmetic. It changes the roof build-up, the drainage, the price and the way the finished roof reads from a bedroom window next door.

A kerb-mounted unit sits on a raised timber perimeter built up off the roof structure. The frame is lifted clear of the roof surface, typically by at least 150mm above the finished covering, and the covering is dressed up the outside of that kerb and terminated under the frame. The glass sits proud of the roof plane and is usually laid to a slight fall. This is the conventional arrangement and the one that most reliably stays dry.

A flush unit finishes at or very close to the level of the surrounding roof, so the roof reads as an unbroken plane with a panel of glass in it. It is specified where the roof is overlooked, where a terrace surface has to continue across the glass, or because the owner wants the roof to read as a roof. It still sits on a kerb; the kerb is lower and the surrounding roof is built up to meet it, usually with a perimeter drainage detail a kerb-mounted unit does not need.

Non-walk-on units are glazed to be looked at and rained on. They are not designed to carry a person.

Walk-on units are engineered for foot traffic, with a thicker laminated build-up, a slip-resistant top surface and a frame designed for the point loads that come with people standing on glass. They cost considerably more, and they are the correct answer wherever the roof doubles as a terrace or an access route to one.

The covering underneath

The rooflight terminates into whatever waterproof covering the roof has, or will have. Three coverings account for most domestic flat roofs around Oxford, and each behaves differently where it meets an upstand.

Single ply membrane

A sheet material, usually PVC or TPO, laid in wide widths and welded at the joints with hot air. It is the most forgiving covering to detail around a rooflight, because the same material welds into corners and up the vertical face of a kerb, so the waterproofing runs continuously from deck to upstand without a change of material. Preformed pieces close the external corners, and an edge trim holds the top of the sheet before the frame closes over it.

Torch-applied felt

High performance bituminous felt in two or three layers, bonded with heat. Well laid it is long-lived, and it dresses up an upstand perfectly well. The weak point is always the corner: felt does not stretch, so external corners need cut and folded gussets that depend entirely on the care taken at the moment they are formed. A felt roof around a rooflight is only as good as its four corners.

Lead

Lead is the traditional covering on Oxford’s older flat roofs, on dormer tops, small rear additions and bay roofs, and it is still specified on listed buildings and in conservation areas where the covering is itself part of what is protected. It is dressed and welted rather than bonded, and it moves. That movement is the point of the material and also its constraint: bays have to be sized correctly, and a sheet restrained at both a kerb and a perimeter upstand will fatigue if the bay is too large.

Whatever the covering, the principle is identical. The covering is the waterproofing and the rooflight frame is a weathering layer above it. Any detail reversing those roles, where a bead of sealant is the only thing holding water out, will let go within a few winters.

Falls and standing water

A flat roof is not flat. It is a very shallow pitched roof laid to a fall so that water leaves it, and the difference between a roof that lasts and one that does not is usually the fall.

Current practice designs to a finished fall of around 1:80, which means building to something nearer 1:40 so that deflection, tolerance and settlement still leave a positive fall everywhere once the roof is loaded. Falls are formed by firring pieces on the joists, by tapered insulation, or by laying the structure itself to a slope.

Introducing a rooflight interrupts that scheme. A kerb stands in the path of water running down the roof, and unless the falls account for it, water arrives at the upslope face, spreads sideways and gathers in the two upslope corners. Those corners are where flat roof rooflights fail.

Why standing water at the perimeter is the enemy

Ponding at a kerb keeps the termination of the covering permanently wet, so any weakness in a weld, a lap or a folded corner is tested continuously rather than for the few hours after rain. It concentrates dirt, moss and grit at the line where the covering turns from horizontal to vertical. In an Oxford winter it freezes, and the expansion works at the joint from the inside.

The failure is slow, which is what makes it dangerous, and it rarely surfaces under the point it entered.

The answer is designed rather than hoped for. Tapered insulation can be cut to divert water around a kerb instead of into it, so the flow splits before it reaches the unit and rejoins below it. On a wider unit that becomes a proper back gutter behind the upslope face. It separates a perimeter that dries out after every shower from one that never dries at all.

Upstand height

Upstand height is not a number taken from a catalogue. It is a function of the fall, the covering, the exposure of the roof and what the roof drains into.

The purpose of the upstand is to lift the termination of the covering and the base of the frame above the highest level water will ever reach. That is not the design water level; it is the level water reaches when the outlet is half blocked with leaves in November. A minimum of 150mm above the finished surface is the usual starting point, and there are reasons to go higher.

The fall raises the upslope side. On a roof falling away from the rooflight, the surface upslope sits higher than the surface downslope. Measure the upstand from the higher side, or a kerb that reads 150mm at the front measures rather less where it matters.

A blocked outlet raises the level everywhere. One outlet serving a small rear extension roof is common, and a single point of failure. Where there is no overflow, the standing level is set by the lowest point of the perimeter, and the kerb has to clear it.

Against that sits the reason people want a low kerb: appearance. A tall kerb is visible from an upper window next door, and on a flush installation it defeats the object. The resolution is not to lower the kerb but to raise the roof around it, so the unit reads as flush while the covering still terminates clear of any water. That costs more in design and in insulation, and it is the right answer where a flush appearance is genuinely required.

Warm roof, cold roof

This one distinction accounts for more misdiagnosed water inside a room than every other flat roof defect combined.

In a warm roof, the insulation sits above the structural deck and below the waterproof covering, with a vapour control layer immediately beneath the insulation. The whole structure, joists and deck included, stays on the warm side. There is no ventilated void, nothing to block, and the dew point sits within or above the insulation. Every new flat roof we build is a warm roof, and every new kerb is detailed as part of that build-up.

In a cold roof, the insulation sits between the joists, below the deck, so the deck and covering are on the cold side. Warm moist air from the room below migrates into the joist void, reaches the underside of a deck at outside temperature and condenses there. Cold roofs need a continuous ventilated air gap above the insulation with cross-ventilation at both ends, and in domestic construction that path is very often absent, blocked by insulation pushed too high, or interrupted by exactly the sort of obstruction a new kerb creates.

The condensation mistaken for ingress

The symptom is a damp mark on the ceiling near the rooflight, or water running down the internal reveal, appearing in cold weather and improving in spring. The reasonable conclusion is that the roof is letting water in. Very often it is not. The moisture comes from inside the house, condenses on cold surfaces at the perimeter and runs back into the plasterboard.

Two tells separate them. Condensation appears in cold still weather rather than during rain, and it shows at the reveals and the frame rather than as a spreading stain in the field of the ceiling. Where we survey a roof and find a cold build-up with an interrupted ventilation path, we say so, and the conversation becomes one about how the roof is constructed rather than which unit to order.

Vapour control continuity

A warm roof works because three layers are continuous: the vapour control layer, the insulation and the covering. A rooflight kerb is a hole cut through all three, and every one of them has to be closed around it.

The vapour control layer sits directly on the deck, below the insulation, and stops moist internal air reaching the cold outer part of the build-up. At a rooflight it is carried up the inner face of the kerb, lapped and sealed, and taken high enough that the frame closes over it. A vapour layer stopping at the edge of the opening leaves a continuous slot around the perimeter through which warm damp air enters the build-up. The consequence is interstitial condensation, invisible for years, forming inside the insulation and progressively soaking it. Insulation that has taken water no longer insulates, and the cold spot widens.

The insulation across the deck has to meet insulation on the outer face of the kerb without a gap.

Thermal bridging at the kerb

A bare timber kerb is 100mm or more of solid material connecting the warm inside of the building to the cold outside, in an unbroken loop around the rooflight. In cold weather its internal face, and with it the lower part of the frame and the plasterboard reveal, runs several degrees below the rest of the ceiling. Warm moist air reaches it and water forms, in a neat line around the unit.

This is the most common reason a new rooflight is blamed for a problem it did not cause. Insulating the outer face of the kerb as a continuation of the deck insulation, so the thermal line is unbroken from deck to frame, removes it. The detail gets omitted when a kerb is treated as carpentry rather than as part of the roof.

The deck below

All of the above assumes the structure underneath can carry it, and on a flat roof that assumption deserves testing before a unit is ordered.

A deck is typically plywood, oriented strand board or, on older work, softwood boarding, laid over joists spanning between the walls of the extension. Cutting an opening interrupts one or more of those joists, and the load they carried has to be taken round the opening by trimmers, doubled or upsized to suit. In new work the joist layout is set out so the opening falls between joists wherever possible. In an existing roof, joist direction, span and condition all have to be established first.

Deflection

A rooflight frame is a rigid rectangle. A timber roof deflects under load, and it deflects differently mid-span than at the ends. If the opening is trimmed without regard to that, the frame is asked to accommodate movement it cannot: the perimeter seal is loaded unevenly, the glass sits in a frame no longer square, and the gaskets are worked at one corner every time snow sits on the roof.

Deflection also undoes falls. A deck that sags between joists creates a local low point, and a low point beside a kerb is standing water at the perimeter. On larger openings, sizing the trimmers is a calculation rather than a judgement.

Older rear extensions across the city produce the full range: sound softwood boarding, chipboard that has taken moisture and lost its strength, plywood delaminating at the edges. Where a deck has softened, renewing it is part of the work, and far better established at survey than discovered on the morning the covering comes off.

Glass and walk-on units

A rooflight in a flat roof faces directly at the sky. It admits substantially more daylight than a vertical window of the same area, and substantially more heat. It also collects everything that lands on it, because a level pane does not shed debris.

Two figures govern the thermal behaviour. The U-value describes how readily heat passes through the unit, so a lower figure means less heat lost in winter. The G-value, or solar factor, describes how much solar energy passes through, so a lower figure means less heat gained in summer. On a flat roof the G-value deserves more attention than it gets, because a horizontal pane takes the sun at its most direct in the middle of a summer day. Self-cleaning coatings also earn their cost here, because a level pane holds dust and rain does not sluice it off.

The build-up of the glass

The outer leaf is toughened as standard. The inner leaf should be laminated on any unit set over a space people occupy, because laminated glass holds together in a breakage rather than coming down. It also improves acoustic performance under heavy rain, which above a bedroom is easier to design out than to live with.

Walk-on units go further. The upper surface is a thick laminated build-up with a slip-resistant treatment, and the assembly is specified against an imposed load rather than only wind and snow. Where a flat roof is a terrace, or where a rooflight sits in the roof of a lower extension reached by a first-floor door, the walk-on specification is not optional. Adding it later means replacing the unit and usually the kerb with it.

Opening and ventilation

A rooflight at the highest point of a room is the most effective ventilation opening in the house, because warm air leaves upward rather than being pushed sideways at window height. Non-opening units are simplest and suit rooms ventilated adequately elsewhere. Manual units operated by a pole are fine on a low ceiling and rarely used on a high one. Electrically actuated units, wired to a switch and a rain sensor, are the arrangement that actually gets used, and they need a power supply planned into the work rather than added afterwards. We install VELUX, Fakro, Keylite and Roto units alongside bespoke flat glass rooflights.

Oxford’s flat roofs

The common setting for flat roof skylight installation in Oxford is a rear extension, and the city’s rear extensions fall into two groups that behave very differently.

Victorian terraces and villas

The brick terraces of Jericho, Osney, East Oxford and the streets off the Cowley Road, and the larger villas of North Oxford, almost all carry a rear addition. Some are original outriggers with a lead or felt roof over a small back kitchen. Many more are later single-storey additions across the full width of the rear, built at any point in the last hundred years and to any standard. The typical rooflight here goes into a roof that is shallow, low, close to a boundary and heavily overlooked from the upper windows either side and behind.

That overlooking is why the flush option matters here more than it might elsewhere. Seen from a neighbouring first floor, a tall kerb-mounted unit reads as an object on the roof; a flush unit reads as part of the roof surface. Where the property sits in a conservation area, in the North Oxford designated suburb, or where an Article 4 direction applies to the street, the appearance of a rear roof addition becomes a material consideration rather than a private matter. On a listed building, consent is likely to be engaged in its own right, and it covers roof slopes nobody can see from the pavement.

We are installers rather than the local planning authority, and we do not present ourselves as the people who decide. What we can set out is the kind of position a building is likely to be in, what an officer assessing a rear roof addition tends to look for, and which specification usually satisfies it. Where a determination is required it belongs to the council, and we say so plainly.

Post-war stock in Headington, Cowley and Botley

Headington, Cowley, Botley, Marston and the estates around them hold a great deal of housing from the decades after the war, and flat roofs are common in it: over garages, over the single-storey rear kitchen extension, and occasionally across a whole first floor. This is where the cold roof problem lives. Insulation between joists, a deck directly above it, and a ventilation gap that was marginal when built and has since been closed by somebody topping up from below.

These roofs also tend to have reached the point where the covering is at the end of its service life, which is an opportunity rather than an obstacle. Where a covering is being renewed anyway, converting the roof to a warm build-up and installing a new rooflight in the same operation removes the condensation problem at the same time as adding the daylight.

When a lantern suits

There are roofs where a flat rooflight is the wrong object, and it is worth saying so before the specification is settled rather than after.

A roof lantern is a raised glazed structure with the glass carried in slopes running up to a ridge. It throws light sideways as well as down, so a room gains brightness at its walls rather than a single pool in the middle, and because the glass is pitched it sheds water and debris instead of holding them.

Where a rear extension is generous in plan, where the ceiling below is high enough to carry a raised structure without the ridge dominating the garden view, and where the roof is neither closely overlooked nor in a setting that would resist a raised addition, a lantern is the better answer. Our roof lantern installation page covers the sizing, the bar arrangement and the planning position for that route.

The flat rooflight wins in the opposite conditions: a shallow roof, a low ceiling, a boundary close by, upper windows looking down on it, a designated setting where the roofscape has to stay legible, or a surface that has to be walked on. On a small rear addition to a listed cottage, a flush unit is the correct answer rather than the consolation prize.

Working at roof level

A flat roof is easier to work on than a pitched one and considerably easier to be complacent about. It is a working platform with unprotected edges, and for part of the day it has an open hole in it.

The work is planned around edge protection, a safe means of getting people and materials up, and a controlled sequence for the opening itself. Once the deck is cut, the hole is covered or guarded at all times. Glass units for flat roofs are heavy and awkward: a large walk-on pane is several times the weight of an equivalent domestic window, and where it cannot be lifted by hand, mechanical lifting is arranged and priced in advance.

Access from the property matters too. A rear extension roof in a terrace with no side access means everything travels through the house or over a garden wall, and that is a legitimate factor in the price.

Building Control

Planning and Building Regulations are separate systems, and satisfying one says nothing about the other. Planning is concerned with appearance and impact. Building Regulations are concerned with whether the work is structurally adequate, insulated, properly weathered and safe. A flat roof rooflight engages several parts at once: the trimmed opening is a structural alteration, thermal performance and insulation continuity are assessed against current standards, and safety glazing requirements apply to glass overhead. We handle the Building Control notification as part of the job. It is not an extra line on the quotation and it is not left with the customer. Notified work leaves a record, which matters because the next time anybody looks closely at a flat roof is usually during a sale.

Cost and programme

A price given without seeing the roof is close to meaningless, because the unit is often the smaller half of the cost. What is useful is knowing what moves the number, so two quotations can be read against each other honestly.

The sequence we work in

The building first. What it is, what designations apply, and what the rear roof is seen from. Much of that is established before anybody visits.

The roof second. Structure, deck, covering, insulation position, fall direction, where the roof drains and whether it stands wet anywhere now. A survey, not a look from a bedroom window.

The specification third. Flush or kerb-mounted, walk-on or not, upstand height against the fall, glass against the orientation, ventilation against how the room will be used.

The consent position fourth. Whether the work sits within permitted development or needs an application, and whether listed building consent is engaged.

The price last. A number given before the first four steps is a guess wearing a suit, and usually a low one that gets revised once the covering is lifted.

Our workmanship carries a ten-year guarantee, with its coverage, exclusions and how to claim set out on its own page rather than buried in small print. We have been installing rooflights for fifteen years, we cover Oxford and up to twenty five miles by road, and we install new units and replacements only. Where an existing unit has reached the end of its life it is replaced along with whatever the survey shows the perimeter needs, which is the only way the new one lasts as long as it should.