Replacing a Failed or Misted Rooflight

The signs that a rooflight has come to the end

Four observations settle it in almost every case, and all four can be made from inside without a ladder.

Moisture sitting between the panes, visible as a haze or as tide marks on the inner face of the outer glass. A frame that has gone chalky, brittle and slightly powdery to the touch, which is ultraviolet degradation of unstabilised plastic. Daylight visible around the edge of the internal lining where none should be, which means the perimeter has moved. And a unit that has yellowed or crazed to the point that the light coming through it has a colour of its own.

Any one of those is a unit at the end of its working life. Two together make the case beyond argument. None of them is dramatic, which is why so many rooflights are left in place for a decade after they stopped doing the job properly.

Four ways a rooflight fails, and which are terminal

It is worth separating the failure of the unit from the failure of the detail around it, because the two ask for different work.

The first two are the ordinary case and they are a single day’s work. The third adds half a day and some carpentry. The fourth is the one that changes the shape of the job, because setting new glass into a roof that is itself finished simply moves the problem along by a year or two.

Misting between the panes, and the physics behind it

A sealed unit is two sheets of glass separated by a spacer bar filled with desiccant, with a primary seal of butyl and a secondary seal of polysulphide or silicone around the outside. The cavity is dry when it leaves the factory, and the desiccant holds it dry by absorbing the small amount of moisture that migrates in.

The desiccant has a finite capacity. Once it is saturated, the next cold night puts the inner face of the outer pane below the dew point of the air trapped in the cavity, and water forms where nobody can reach it. On a rooflight this happens sooner than on a vertical window, because the unit spends every summer day taking direct solar load and every winter night radiating to a clear sky. That daily cycle works the seals harder than anything in a wall.

Once misting starts it does not reverse. The unit is finished, and the only honest answer is a new one.

Domed polycarbonate, and the age it reaches

A great deal of Essex flat roofing from the nineteen seventies and eighties carries a domed rooflight: a single or double skin of acrylic or polycarbonate on an aluminium or plastic kerb frame, usually in a bathroom, a stairwell, a garage or a rear extension of the period.

These have a working life of roughly twenty to twenty five years and a great many are now well beyond forty. The failure is ultraviolet, not mechanical. Unstabilised acrylic yellows and goes brittle, polycarbonate crazes into a fine network of surface cracks, and both lose light transmission steadily as they go. A dome that started at eighty per cent transmission can be down near half of that and the room below simply feels grey without anybody being able to say why.

The kerb frames of that era are usually shallower than current practice wants, which matters for what comes next.

Early roof windows, and the frames still in Essex slopes

Centre pivot roof windows have been fitted into pitched slopes here since the nineteen sixties, and there are three broad generations still in service.

The earliest have a painted or bare timber sash, single or early double glazing, and a flashing detail that relied on the tiles being dressed close. The middle generation, from roughly the nineteen eighties and nineties, introduced the modern flashing kit and better factory finishes but used glass that is thermally poor by any current measure. The recent generation carries a properly warm unit, a laminated inner pane and a much improved seal.

Swapping between generations is generally straightforward, because the outer frame dimensions of the common sizes have been fairly stable over decades. What has changed is the sash depth, the flashing kit geometry and the way the unit sits relative to the covering, so the flashing is always new even when the opening is not touched. A like for like VELUX replacement into an existing timber trimmed opening is a one day job in ordinary circumstances.

Like for like, or a change of type at the same time

The moment the old unit comes out is the cheapest moment there will ever be to change what is there, because the opening is formed, the access is set up and the labour is already committed.

Two changes are worth considering. Going from a small dome to a larger flush unit, where the structure allows the opening to be widened while the roof is open. And going from one unit to a different family altogether, such as replacing two tired domes over a kitchen with a single flat rooflight, or replacing a dark stairwell dome with a sun tunnel where the roofline no longer suits an opening light.

The counter argument is real. Widening an opening brings structure, Building Control interest and a longer programme into what was a single day. Where the existing size works and the room is fine, like for like is the sensible answer and there is no shame in it.

Measuring an existing opening, and the three numbers

Three dimensions govern a replacement and they are routinely confused with one another, which is how the wrong unit arrives on a Tuesday morning.

The structural opening is the hole in the roof structure, measured between the trimmers or between the inner faces of the kerb timbers. The kerb outside size is what a flat rooflight sits over, measured across the outside faces of the upstand. The unit size is the frame itself, which for a flat rooflight typically overhangs the kerb by a defined amount on each side.

Manufacturers quote to different conventions and some quote the glass daylight opening as well, which is smaller again. A replacement is ordered against a measured kerb, taken at both ends and in the middle, because a kerb built in 1979 is very rarely square and the diagonal will usually differ by more than people expect.

Whether the old kerb stays or goes

Keeping the kerb saves a day and a good deal of disturbance to the covering, so it is always the first question at survey.

A kerb can stay where the timber is sound and dry throughout, the top is level and square enough to seat a new frame without packing, the height above the finished surface is adequate for the position, and the roof covering dressed up its face is itself in good order. That combination is common on flat roofs laid since about the year two thousand.

A kerb goes where the timber has taken water, where the height is under the working figure and the roof has a history of standing water, or where the covering around it is at the end of its life anyway. Rebuilding a kerb is half a day of carpentry and it is the single most valuable half day on the whole job, because the upstand is what stands between the new glass and the water on the roof.

Timber kerbs, moisture and the fixing line

The failure mode of an old timber kerb is specific and worth understanding, because it decides whether a new unit has anything to fix into.

Water that gets past a tired detail runs down the inside face of the upstand and collects at the junction between the kerb and the deck. The top of the kerb can look and feel perfectly sound while the bottom fifty millimetres has gone soft. A new frame is fixed through its flange into the top of the kerb, so a soft base does not always announce itself at the moment of fixing. It announces itself later, when the fixings have nothing left to hold and the frame lifts slightly under wind.

The test is mechanical, not visual. A bradawl or a screwdriver pushed into the timber at the deck junction tells you in two seconds what a photograph never will, and it is done at survey rather than on the day.

Imperial openings, and what fits them now

Openings formed before the mid nineteen seventies were set out in feet and inches, and the legacy is still in the roofs. Two feet by three feet, three by four, four by four. Converted, those are 610 by 914, 914 by 1219 and 1219 by 1219 millimetres.

Current rooflight ranges are built around metric sizes that do not land on those numbers. The usual outcomes are three. The nearest larger standard unit fits if the opening can be eased by twenty or thirty millimetres, which is often just a matter of the lining rather than the structure. The nearest smaller unit fits with a packed and re-lined reveal, losing a little glass. Or a bespoke unit is made to the measured size, which costs more and adds two to four weeks of lead time but leaves the structure alone entirely.

On a trussed roof the third option is frequently the right one, because easing an opening is exactly what a truss will not tolerate.

Working to a finished ceiling

A new installation forms its opening before anything below it exists. A replacement works the other way round, into a room that is decorated, with a plastered reveal and often a light fitting or a cupboard immediately underneath.

That constrains the work in useful ways. The internal lining sets where the new frame has to land, because a frame that sits ten millimetres further out than the old one leaves a shadow gap that has to be made good in plaster. Any change in unit depth changes how the reveal meets the frame. And where the old unit had a wide timber lining and the new one has a slim aluminium flange, the reveal will need making good regardless.

The honest position is that a replacement usually involves some plastering at the perimeter. It is a small area, and it is far better to expect it and price it than to discover it at four o’clock on the day.

The covering around the old unit, and how far it is disturbed

How much roof comes back depends entirely on what the covering is, and it is the single biggest difference between an easy replacement and an awkward one.

• Single ply membrane is cut back around the kerb and a new section welded in. The weld is a hot air process, the joint is tested, and the result is as good as the original.
• Felt, whether pour and roll or torch on, is stripped back a course and rebuilt in layers. Straightforward on a sound roof and revealing on a tired one.
• GRP is cut back and laminated in with new matting and resin, which needs a dry day and a reasonable temperature to cure properly.
• Tiles and slate around a roof window come off in courses, are stacked, and go back around the new flashing kit. Handmade clay tiles are lifted carefully because matching a broken one is a hunt.
• Asphalt is the awkward one, needing hot work and a specialist, and it is identified at survey rather than on the day.

Flashing a replacement into a pitched slope

The flashing kit is always new. This is not an upsell, it is a geometry problem: kits are matched to a specific frame profile and a specific covering depth, and an old apron dressed to a thirty year old sash will not sit correctly against a new one.

Older details were frequently formed in lead, dressed by hand rather than supplied as a kit, and on a well built job that lead can be in fine condition. It still comes off, because it was worked to the shape of the frame that is leaving. Lead is stripped, kept, and goes for recycling rather than into a skip.

The new kit is chosen by covering profile. A plain clay tile, a deep concrete interlocking tile, a natural slate and a low profile pantile each take a different kit, and using the nearest one to hand is how a proud apron and a slow water track arrive three winters later. On a conservation rooflight in a protected area the flashing is what makes the unit read as flush, so it earns even more attention.

Asbestos, and where it turns up on older roofs

Anything built or altered before the year two thousand can carry asbestos containing material, and rooflights sit in exactly the places where it was used.

The common ones are corrugated cement sheet roofs on garages and outbuildings, where an old translucent panel sits in the run. Bitumen based felts and mastics from the nineteen sixties and seventies. Some early insulation board used as a lining under a rooflight opening or as a soffit inside a light well. None of these is dangerous while it is undisturbed, and all of them become a controlled matter the moment somebody starts cutting.

Where the survey identifies suspect material, it is sampled and, if confirmed, handled by a specialist licensed for that work before the opening is touched. It adds time to a programme and it is not negotiable. Finding it at survey is straightforward. Finding it with a disc cutter is not.

The thermal jump you get at replacement

This is the part of a replacement that consistently surprises people, and the numbers are large.

A single skin acrylic dome from the nineteen seventies performs at roughly 5.0 to 5.5 W/m²K. A twin skin dome of the same era sits nearer 3.0. An early double glazed roof window with a plain air filled cavity is around 3.0 as well. A current double glazed rooflight with a low emissivity coating, an argon fill and a warm edge spacer will be somewhere between 1.0 and 1.4 depending on the unit and how the whole assembly is measured.

Going from 5.0 to 1.2 on a square metre and a half of roof is a change you can feel standing under it in January. The inner pane is warmer, so it condenses far less, so the marks on the reveal stop appearing. The energy performance of the glass is the easy half of that gain. The perimeter insulation detail is the other half.

Part L, and the standard a replacement is held to

Replacing a rooflight in an existing dwelling is controlled work and the new unit has to meet a maximum U-value set by Part L of the Building Regulations. The figure is a ceiling, not a target, and it is measured for the whole unit rather than for the glass alone.

Two things follow from that. First, a unit advertised on its centre pane figure may not meet the standard once the frame is included, so the number that matters on a quote is the whole unit value with the measurement basis stated next to it. Second, a rooflight in a vertical or near vertical plane is assessed differently from one lying in a flat roof, because the standards treat the two orientations separately.

Any current unit from a mainstream manufacturer will clear the requirement comfortably. It is the old stock still being sold off cheaply that does not, which is one more reason the specification belongs in writing.

Notification, and what the certificate does years later

Because it is controlled work, a replacement is notified to Building Control. Sunspire handles the notification as part of the job, and the completion paperwork follows in the client’s name.

The value of that paperwork is not felt on the day. It is felt at sale, when a buyer’s solicitor sends a standard enquiry asking for certificates covering any alteration to the property, and an undocumented rooflight becomes a retention on the completion money or an indemnity policy nobody wanted to buy. Regularising work after the event is possible and it is slower, more expensive and entirely avoidable.

The same applies where the original rooflight was itself installed without notification, which is common on work done informally in the nineteen eighties and nineties. Replacing it properly and notifying that replacement closes the question for good.

Escape openings, and the size an older unit may not meet

Where a rooflight serves as a means of escape from a loft room or an inner room, the opening has a dimensional standard to meet, and older units frequently do not meet it.

The requirement is a clear openable area with minimum clear height and width, achieved with the sash in its escape position, and with the bottom of the opening within a stated height range above the floor. A centre pivot window fitted in 1990 into a loft conversion may have satisfied the guidance of its day and may fall short of current expectations, particularly on the clear opening once the sash is swung.

This is a matter for the building, not for preference. Where a replacement serves an escape route, the unit is selected against the clear opening it achieves rather than against its nominal frame size, and top hung or escape hinged models exist precisely for this reason.

Condensation that returns after a new unit goes in

Occasionally a brand new rooflight runs with water on the inside face, and the owner reasonably concludes that something has gone wrong. Usually nothing has.

Surface condensation on the room side is not a fault in the unit. It is warm moist air meeting the coldest surface in the room, and in a kitchen or bathroom the coldest surface is very often the glass regardless of how good it is. A better unit raises the inner pane temperature and reduces the frequency, but it cannot remove moisture that has nowhere to go.

The fix is air movement, and a replacement is the natural moment to sort it. That might be a trickle vent in the new frame, an extract fan that actually discharges to outside rather than into the void, or simply choosing an opening unit rather than a fixed one so the room can be purged. Where reach is the obstacle, an electric opener makes ventilation something that actually happens rather than something that is intended.

Blinds, and what does not carry across generations

Blinds fitted to rooflights run in side channels within the frame, because a hanging blind on a sloped or horizontal surface sags away from the glass under its own weight.

Those channels are specific to the frame generation. A blind that ran happily in a nineteen nineties sash will not transfer into a current one, and the fixing points, the channel width and the pane dimensions have all moved. Anyone hoping to keep an existing blackout blind through a replacement should assume it will not go back on.

The practical response is to order blinds with the unit. It costs less than sourcing them afterwards, the fit is exact, and in a bedroom under an east facing slope the blackout is doing serious work at half past four on a June morning. Where the room is only used in daylight, the channels can be left empty and a blind added later, provided the frame has the provision.

Adding an opener to a unit that was always manual

A replacement is the one straightforward opportunity to motorise a rooflight that has been reached with a pole for twenty years.

There are three routes and they differ mainly in the wiring. A mains powered actuator gives the strongest and quietest operation and needs a supply run to the frame, which means a cable in the roof void and an electrician. A low voltage system runs from a transformer sited elsewhere, which is easier to route and lighter work above the ceiling. A solar powered unit carries its own panel and battery on the frame and needs no cable at all, which on an occupied house with a finished ceiling is often the deciding factor.

Cable routing is the whole question. If the void is accessible from a loft hatch, running a supply is a straightforward morning. If the ceiling is sealed below a flat roof, solar becomes considerably more attractive.

What replacement day looks like from inside

For a single unit into a sound existing kerb, one day. For a unit that needs a kerb rebuilt, one to two. For a roof window in a tiled slope with the covering stripped back and reset, one day in ordinary weather.

The room below is protected first, floor and furniture, because dust from an old lining and fragments of brittle acrylic go a long way. The old unit is lifted out from above, not pushed through, so nothing lands in the room. The kerb or opening is prepared, the new frame is set and mechanically fixed, the covering or the flashing kit is dressed and made good, and then the internal reveal is finished.

The reveal is the part people remember, so it is worth saying that a splayed lining spreads light across the ceiling and reads far better than a square one. If the old reveal was square and the plaster is coming back anyway, splaying it is close to free.

The order of work, and the weather

Opening a roof is a commitment, and the sequence exists to keep that commitment short.

Nothing is stripped until the new unit is on site, unwrapped, checked against the measured opening and found correct. Materials for closing up go onto the roof before the old unit comes off, not after. The forecast is read for the working window rather than the day, because a February shower arriving at two o’clock is the entire risk in a January replacement.

The roof is closed and watertight at the end of every working day without exception. Where a two day job has to pause, the opening is decked and weathered rather than sheeted, because a temporary cover that relies on tape and hope is exactly what fails at three in the morning in a gale off the Blackwater.

Doing several units on one visit

Rooflights fitted at the same time tend to fail at the same time, which is why a bungalow with four domes rarely has a problem with only one of them.

Doing them together is materially cheaper per unit than doing them one at a time, and the reasons are practical rather than promotional. The access is set up once. The materials for closing the covering are bought in useful quantities rather than as offcuts. The Building Control notification covers the work as a whole. And the disruption to the household happens once instead of four times across four winters.

The judgement call is whether a unit that is not yet misting should come out alongside those that are. Where it is the same age and the same generation, the honest answer is usually yes. Where it is ten years newer, it stays, and the survey will say so.

What moves the price on a replacement

Sunspire quotes in two stages: a range by email once the basics are known, then a fixed written figure after a survey. On a replacement, the range is usually tight, because the opening already exists and the biggest unknown in a new installation has been removed.

What moves the figure is the state of what is already there. A sound kerb keeps the price at the bottom of the range. A kerb rebuild, a section of soft deck or a covering that needs more than a local reinstatement moves it up. Access moves it in the usual way, since a rear extension roof off a garden is not a third floor slope needing scaffold. And a bespoke unit to an imperial opening costs more than a standard size, both in the unit and in the waiting. The costs page sets out how the ranges are built.

What to send to get a range

Four things are enough for a sensible range by email, and all four can be gathered in ten minutes.

A photograph of the unit from inside, showing the reveal and the frame. A photograph from outside if the roof can be seen safely from a window, a garden or an upstairs room of a neighbouring house. The rough size of the opening, measured internally at the ceiling with a tape. And what the roof covering is, in whatever words come naturally: flat and black, grey tiles, slate.

If the unit carries a data plate, usually on the sash edge or the frame face, a photograph of that is worth more than every other measurement combined, because it gives the manufacturer, the model and the size code directly. From there the survey confirms the kerb and the structure, and the range becomes a fixed price. The areas covered page shows how far that travels from Chelmsford, and the guides go into the detail behind each decision.