Replacing several rooflights at once

A roof full of units has one age, not several

The starting point is that these are not independent items that happen to be similar. They are one batch.

Every unit on a roof was almost certainly glazed within days of every other one, from the same production run, with the same seal chemistry, the same desiccant charge and the same fill purity. They have then had the same weather for the same number of years. The only real variables left are which way each one faces and whether anything shades it, and on a single slope those are close to identical.

So the unit that misted first was not unlucky. It was the one with the smallest manufacturing margin in a batch that is collectively at the end of its design life. On a south or west-facing slope the others are usually within two or three years of the same condition, and often much less. Why sealed units go, and the service life to expect by generation, is set out at misted sealed units and why they go.

The same logic runs beyond the glass. The gaskets are the same age. The lead was fixed by the same hands on the same morning, so if one length has cracked along a nail line, they all have. The underlay was cut the same way at every opening. The insulation was stopped short of every frame. A finding at one opening is a very strong predictor of the others.

Scaffold is the cost that does not divide

Access is the item that makes the arithmetic work, and it behaves differently from everything else on a quotation.

A scaffold to a rear elevation is priced on the erection, the hire period and the dismantle, and those figures barely move whether one unit or four are being changed behind it. Extend the run to reach a second opening on the same slope and the marginal cost is a few standards and a couple of lifts. Take the whole thing down and bring it back in eighteen months and the cost is incurred again in full.

Everything else on the job scales roughly with the number of units: the units themselves, the flashing kits, the lead, the labour on each opening, the waste. Access does not. That is the whole economic case, and it is why the sensible comparison is never the cost of four units against the cost of one, but the cost of one access visit against the cost of two or three.

Three things magnify it. Storey height, because a three-storey rear elevation on a Cheltenham terrace is a different structure from a two-storey semi. Ground conditions, since a scaffold over a conservatory, a flat roof or a sloping garden needs designed support. And access for the scaffolder’s lorry, which in the Stroud valleys around Chalford, Rodborough and Nailsworth genuinely means hand-carrying material up a flight of steps, sometimes a long way. Where the access itself is the expensive part, doing everything behind it in one visit stops being a preference and becomes the obvious answer. Whether a job needs a scaffold at all is covered at do you need scaffolding to fit a rooflight.

One notification, one set of inspections

The regulatory side compresses in the same way.

Replacing a rooflight is notifiable work in most circumstances, because a rooflight is a controlled fitting and a replacement has to meet the current limiting standard rather than the standard the old one met. That is one application whether it covers one unit or five, with one fee, one set of inspection appointments and one completion certificate listing all of the work.

Doing them separately means separate applications, separate fees, separate inspection visits and a paperwork trail with gaps in it. At the point of sale a solicitor asks for certificates covering the alterations, and a single certificate listing four units in one job is a cleaner document than three certificates and one unit nobody notified. Where any of the openings changes size, the structural work goes into the same submission with the engineer’s calculation attached. The general position is at do replacement rooflights need building regulations.

Batch matching, and why glass bought three years apart does not match

This is the argument that persuades people who were unmoved by the scaffold arithmetic, because it is the one they will look at every day.

Modern glass is not neutral. A low emissivity coating is a stack of extremely thin metal oxide layers, typically with a silver layer at the heart of it, deposited under vacuum. Those layers are what give the glass its performance and they also give it a colour cast in reflection: usually a faint neutral grey, sometimes with a green, blue or bronze bias depending on the coating. A solar control coating is heavier again and its cast is more obvious.

Coating recipes are revised. Manufacturers introduce new generations of low emissivity product every few years, and even within one product name the coating stack and the resulting reflectance shift slightly between production campaigns. Add to that the fact that float glass itself has a batch-to-batch variation in iron content, which is what gives ordinary glass its green edge, and two panes of nominally identical specification made three years apart can be visibly different in reflection.

Standing in the garden looking at a roof, that shows up as one unit reading slightly warmer or cooler than the ones beside it, most obviously on an overcast day when the glass is reflecting flat sky rather than showing what is behind it. It is not a defect and no manufacturer would accept it as one. It is the reason glazing for a single visual field is ordered as one batch wherever possible.

Weathered frames beside new ones

The frames do the same thing, more slowly and more visibly.

White polyurethane and white painted timber both drift over fifteen years, usually towards a warm off-white, and the drift is even enough across a whole elevation that nobody notices it until a new frame goes in beside an old one. At that point the difference is obvious from the garden, and it is more obvious than most people expect because the eye compares adjacent surfaces very accurately and absolute colour very badly.

Lacquered pine is worse, because it moves further. A pine sash exposed to a south-facing slope loses its lacquer from the bottom rail inwards, silvers where the timber is exposed, and darkens where water has stood. A new pine sash beside a thirty-year-old one does not look like the same product.

Powder-coated aluminium is the most stable of the three but not immune: a coating exposed for twenty years chalks slightly and loses gloss, so a new frame in the identical RAL reference reads as slightly brighter. On flat roof units, where the frame is a slim band around a large area of glass, this matters less than it does on a timber roof window where the sash is a substantial visual element.

Sightlines, and mixing profiles on one slope

Beyond colour there is shape, and shape is harder to ignore.

Manufacturers have reduced frame sections steadily as insulation moved into the frame and as slim sightlines became the thing people ask for. The visible face of a current roof window sash is narrower than its 1990s equivalent at the same nominal size, which means the daylight opening is larger and the proportions of the visible rectangle are different. Two units of the same size code, one from each era, side by side on one slope, do not look like a pair.

The same applies across brands. VELUX, Fakro, Keylite and Roto all publish sizes at the same nominal widths, and all four have different external frame profiles, different flashing kit geometry and different sash proportions. Mixing them on one visible elevation is a decision worth making deliberately rather than by default, and the differences are compared at choosing between the major roof window brands.

Where a roof has to be done in stages, the practical answer is to keep each visual group whole. Everything on the road-facing slope in one visit; everything on the rear in another. A slope nobody sees from the ground can differ from one that is visible from the street without anybody caring.

Sequencing down the slope, and the rule that shapes the week

The roof is never left open overnight. That single rule governs the order of work more than anything else.

An opening is not started unless it can be closed and made watertight the same day. So the sequence runs from the top of the slope downwards, one unit at a time, and each one is completed to a weathertight condition before the next is touched. Working downwards matters because every flashing laps under the covering above it: a unit set high on the slope is finished with tiles above it that have already been re-laid, and any water crossing the covering above passes over completed work rather than over an open hole.

The corollary is that a four-unit elevation is not four days of parallel work. It is four sequential operations with a hard stop each afternoon, and the day’s cut-off is set by how long the last opening will take to close rather than by the clock. If the forecast turns during the morning, the outside work stops at a point where the roof is whole and resumes when it clears.

Where two units sit side by side in adjacent bays on the same level, they are sometimes opened together because a single strip of covering serves both and the flashings interleave between them. That is a judgement made on the day with the weather in front of you, and it only happens where both can certainly be closed together.

Two elevations, and the scaffold decision

Where units sit on more than one slope, the arithmetic changes and it is worth taking seriously.

Scaffolding two elevations of a house is not twice the cost of one, but it is a long way above it, and on a detached house with front, rear and side slopes it can be the largest single line on the quotation. Three options come up. Scaffold everything and do it all in one visit, which is the least total cost where the number of units justifies it. Scaffold one elevation, complete it, then strike and re-erect on the other, which costs two erections but only one hire overlap and keeps the disturbance to one side of the house at a time. Or scaffold one elevation now and the other in a later year, which is a genuine staging decision rather than a compromise.

What tips it is usually the condition of the second elevation. A north-facing front slope whose units are visibly still clear may have five or more years left, because north-facing units routinely outlive identical south-facing ones on the same house. A west-facing slope where one has already gone will not.

Mixed products on one roof

Plenty of Gloucestershire houses have more than one type: two roof windows on the pitched rear slope and a flat rooflight over the rear addition, or a lantern over a kitchen and a small unit over the stairs.

Those can share a visit and usually should, but they do not share a programme. Manufactured roof windows in stock sizes are available within days. Made-to-size flat roof units are manufactured to your dimensions and are always a lead time. Lanterns are longer again and may need structural work under them. Conservation rooflights are made to order.

So the ordering runs backwards from the longest lead item, and the scaffold is booked to arrive when everything is on site. It is also worth being clear that a flat roof unit and a pitched roof unit are different trades within the same job: one involves waterproofing chemistry with temperature and dew point constraints, the other involves covering and lead work that can proceed in cold weather. That difference sometimes decides the order within the week.

Ordering as one batch, and checking it once

A multi-unit order is checked at the tailgate rather than on the scaffold, and with several units the check is worth doing methodically.

Each unit is unwrapped and confirmed against the order for size code, glazing code, frame finish, handing and transit damage. Glass is inspected for edge chips, which is where transit damage shows on a sealed unit and which will propagate under thermal stress. Flashing kits are confirmed as the right profile for the covering and the right size for the unit, and where two units sit in adjacent bays, the combination kit that flashes them as a pair is a different item from two single kits. That pairing detail is at can two roof windows share one flashing.

Nothing is stripped until all of it is on site and checked, because a roof opened around a unit that turns out to be the wrong glazing code is a roof that gets sheeted and left, and sheeting is not a roof.

When staging over two seasons is the right answer

The case for doing everything at once is strong and it is not universal. Four situations genuinely favour staging.

Where the units are on different elevations with different exposure, and one group has years of life left. Where a scaffold to one elevation is straightforward and to another is a designed structure over a conservatory or a garden drop. Where the household simply cannot have the whole house sheeted in one fortnight, which is a real constraint with small children or someone working from home. And where the roof itself is due other work: if a covering is coming off in three years anyway, the rooflights on that slope belong to that project.

Where staging is chosen, two things make it work. Keep each visual group whole, so no elevation ends up half old and half new. And record the exact specification of the first batch, including glass make-up, coating name, spacer type and frame finish, so the second batch can be ordered to match as closely as the market still allows. That record goes in the handover pack.

Staying put through a multi-unit programme

Worth setting out plainly, because it is the part that decides whether people go ahead.

A scaffold to a rear elevation means loss of privacy at the back of the house for the hire period, and it means the ground floor windows behind it are darker. Access to the garden through the scaffold is maintained but it is not pleasant. The scaffold is alarmed or the ladder is removed at night as a matter of routine, and it is worth telling a house insurer that a scaffold is up.

Inside, each room with an opening in it is out of use for the day that unit is done, and cutting produces dust even with the room sheeted. The loft hatch is closed and sealed while any cutting happens, which matters more than people expect on a house with an open loft over a landing. Noise is intermittent rather than constant: the loud parts are stripping covering and any cutting of timber.

The practical advice is to clear the rooms properly the evening before rather than on the morning, and to accept that four consecutive days of this is easier to live through than the same four days spread across three years.

Waste, and what leaves the site

Several units generate more than people picture, and it is planned rather than discovered.

Old sashes and frames, which are bulky and awkward and go out through the scaffold rather than through the house. Sealed units, which are glass bonded to a spacer and go as mixed waste rather than as recyclable glass. Old lead, which is worth something and is recycled. Broken tiles or slates, and on a stone slate roof the broken material is worth keeping because sound pieces can be re-dressed. Old bitumen felt underlay, old insulation, and packaging, which on four units is a surprising volume of timber crating and film.

All of it is removed as part of the job, with the site left clear and the gutters cleared of debris, and the waste transfer documentation is part of the handover pack. Whether the old unit is taken away is answered at do you remove the old rooflight and dispose of it.

Where this comes up across the county

The multi-unit job has a distinct geography in Gloucestershire, and it maps onto how the housing was built.

The clearest case is the trussed estates at Quedgeley, Abbeydale, Brockworth, Innsworth, Churchdown, Bishops Cleeve, Ashchurch and Yate. Units went in with the build, between trusses at 600 centres, with trimmers already there and a standard flashing kit for concrete interlocking tile. Three or four on one rear slope, all glazed in the same week, all reaching the end of their lives together. That is the job this page is really about, and it is close to the ideal version of it: no structural work, plaster untouched, one scaffold, four straightforward days.

Loft conversions across Longlevens, Hucclecote, Tuffley, Warden Hill and Charlton Kings are the second group, typically two or three units put in together when the loft was done in the eighties or nineties. The complication there is that the conversion is a finished room with plaster, decoration and often fitted furniture below, so the work is kept outside wherever possible, which is covered at replacing a rooflight in a finished loft room.

Terraces in Gloucester, Tewkesbury and the Forest towns raise a different question, which is party wall and shared access. A scaffold to a mid-terrace rear elevation often needs to cross a neighbour’s ground, and that is a conversation and sometimes a written agreement rather than an assumption. In central Cheltenham the shallow flats behind the Lansdown, Montpellier and Pittville parapets frequently carry two or three old domes at once, all on low uninsulated kerbs, and doing them together is the only sensible way to handle a parapet gutter whose falls and outlets serve all of them.

And on Cotswold stone slate through Painswick, Bibury, Northleach and Minchinhampton, the multi-unit job is driven by material rather than by scaffold. Reclaimed stone to match a two-hundred-year-old roof is sourced when it becomes available rather than when it is wanted, and sourcing enough for three openings in one go is a single, longer wait instead of three separate ones. Conservation units are made to order, so ordering them as a set also keeps the frames and finishes consistent. Those roofs are covered at conservation rooflight installation.

Send photographs of each unit from inside and outside, a close-up of every data plate, and a wide shot of the roof and the access to info@severnskylightsgloucester.co.uk. That gets an indicative range by email, then a survey, then an itemised fixed price separating units, labour, scaffold and Building Control so the effect of doing them together is visible on the page. The workmanship carries a ten-year guarantee, the glass and frames carry the manufacturer’s own warranty as a separate document with its own terms, and the pricing process is on the costs page.