Should I replace all my rooflights at once?
If they were fitted at the same time and they sit on the same elevation, yes, and the number that settles it is the manufacture date stamped on the data plate rather than how the units look from the room today. Sealed units glazed in the same week, exposed to the same sun and the same weather, reach the end of their service life within a couple of years of each other. Replacing one and leaving three is a decision to bring the platform back twice more inside five years. Where the units are of different ages, on different slopes or in different roofs, the arithmetic changes and doing them one at a time is perfectly sensible. The programme, the sequencing and the scaffold economics are set out in full at replacing several rooflights at once. This page is the short version: how to decide. The replacement job generally is on the replacing a failed or misted rooflight page.
The date on the plate is the answer, not the appearance
Every manufactured roof window carries a plate with a manufacture date on it, usually as a week and year or a plain year. That single number tells you more about the other units on the roof than any amount of looking at them, because it tells you whether they were glazed on the same day.
Appearance is a poor guide by comparison. Misting is a threshold event rather than a gradual one. A sealed unit’s primary seal fails, the desiccant in the spacer saturates over the following months, and from that point the cavity fogs whenever the outer pane is cold. Before that threshold the glass looks perfect. So three units that look clear and one that has gone cloudy are not three healthy units and one failure. They are four units at the same point in the same process, one of which has crossed the line first. How to find and read the plate is at how do I find out what rooflight I have.
Why a whole slope tends to go within a couple of years
The reason is manufacturing and exposure together. Units bought for one house came off the same production run with the same batch of butyl in the primary seal, the same desiccant and the same secondary structural seal. They went into the roof in the same week and they have taken the same daily temperature cycle and the same ultraviolet dose ever since.
Roof glazing works harder at that than vertical glazing does. It lies close to horizontal so rain sits on it and the edge seal spends more of its life wet, it takes direct sun for the whole arc of the day with nothing shading it, and the temperature swing across a summer afternoon on a south facing slope is large. Fifteen to twenty-five years is a normal service life for an older unit in a roof. When the first one on a slope crosses that line, the others behind it are not far away. The mechanism is described at misted sealed units and why they go.
Elevation separates the roof more sharply than age does
Two units of identical age on the same house can be years apart in condition, and the variable is which way they face. In Gloucestershire the weather arrives from the south west, and so does the sun for the hottest part of the day.
A south or west facing unit on a Quedgeley or Bishops Cleeve rear slope takes the full solar cycle and the driving rain, and it will reach the end of its life first. A north facing unit of the same age on the front slope has had a much easier time and can have several years left in it. A unit tucked under the shelter of a taller building in a Stroud valley terrace, or shaded most of the day by a mature beech in Painswick, is easier again. So the honest grouping is not by house but by slope: do a slope together, and treat the other elevation on its own timetable.
What you have against whether to do them together
| Situation | Sensible route | The reason |
|---|---|---|
| Three or four units, same plate date, one slope, one has misted | All of them | The others are on the same clock and one platform covers the lot |
| Same plate date but split across front and rear slopes | Rear slope now, front when it goes | Different exposure, and a scaffold on one elevation does not reach the other anyway |
| Two units on one slope, ten years apart in date | The older one | Nothing links their seals, and the younger one has most of its life left |
| One unit damaged in a gale, the rest sound and recent | The damaged one | A sudden event, not a service life question, and there may be an insurance claim attached |
| Loft conversion with four units in a row, all early 2000s | All of them, planned | Same batch, same slope, and matching glazing across a row is visible from inside |
| Units in different roofs, a rear addition and a main slope | Separately | Different coverings, different platforms and often different work entirely |
| Conservation area with consent outstanding on one position | Do the consented ones, keep the platform available | Consent timing rather than glass condition is driving the programme |
| One flat roof unit and two pitched roof units | Separately unless the access overlaps | Two different trades’ worth of detailing and two different weather windows |
The mismatch nobody notices until the second unit goes in
There is an appearance argument alongside the practical one and it is worth knowing before the decision is made rather than after. Glazing has changed a great deal in twenty-five years, and a current low emissivity coating with a solar control layer has a faint tint that an uncoated 1990s unit does not.
Put one new unit into a row of three old ones on a slope visible from the garden and the difference shows both ways. From outside, the new glass reads slightly different in reflected light. From inside, the new unit lets through light with a marginally different cast and the frame profile and sightline are unlikely to be identical, because manufacturers have changed their frame sections repeatedly. On a single unit on a rear slope that nobody sees, none of it matters. On a row of four across the front of a Cheltenham loft conversion, it does. Matching across a slope is dealt with at can you match my existing rooflight.
Where doing one at a time is the right call
Stating it positively, because the answer is genuinely not always all of them. Budget is a legitimate reason and it needs no defending: replacing the failed unit now and planning the others for a named future year is a real plan, and it is better than a vague intention. Where that is the choice, the useful move is to have all the plates read and recorded during the first visit so the future order can be placed without another survey.
The other clear cases are units of genuinely different ages, units in separate roofs that need separate platforms, a single storm-damaged unit with a claim behind it, and a house being sold in the next few months where clearing the obvious fault is what is needed. There is also the situation where one unit is in the wrong place: over a stud wall added in a later reconfiguration, or hard against a valley. Replacing that one in kind repeats somebody’s 1985 mistake, and it is worth a conversation before it is ordered.
Gloucestershire roofs where the answer is nearly always all of them
Two patterns in this county produce the same recommendation almost every time. The first is the developer-fitted estate house: Quedgeley, Abbeydale, Brockworth, Innsworth, Ashchurch, Bishops Cleeve and the Yate estates went up with narrow roof windows between trusses at 600 centres, all fitted during the build, all from the same delivery. Those are the cleanest replacements available, because the opening was designed with the trusses and there is no structural work, and they are also the most reliably simultaneous failures.
The second is the 1990s and 2000s loft conversion, which typically has three or four units on one rear slope, all fitted in the same fortnight. The units are frequently the room’s only glazing and only ventilation, so a failure is felt immediately, and the row is visible from inside as a row. Cotswold stone slate roofs are the exception in the other direction: on a Painswick or Northleach roof the covering work around each opening is substantial and the units are usually few and individually placed, so they get considered one at a time on their merits.
How to decide in an afternoon
The whole decision needs about half an hour with a stepladder and a phone. Go to each unit in turn, rotate the sash if it is a centre pivot and engage the barrel bolt, photograph the plate so the product code, the size code, the glazing code and the date are legible, and note which slope the unit is on and roughly which way it faces. Then look through each pane in daylight for cloudiness or streaking that does not wipe off from either side.
Send that set to info@severnskylightsgloucester.co.uk with the address and a photograph of the roof from the garden. From plate dates and elevations it is usually possible to say straight away whether the roof is one job or two, and an indicative range by email follows, then a survey, then a fixed price. Grouping the work also means one Building Control notification rather than several, which is set out at do replacement rooflights need building regulations. Everything else about how the company works is on the skylight and rooflight installation page.
Next on this subject.
- Question Do rooflights need planning permission? Usually not. Putting a rooflight into the roof of a house is normally permitted development, which means the national rules…
- Question Do I need building regulations approval for a rooflight? Yes. Forming a new opening in a roof is notifiable work under the Building Regulations, and it is notifiable whether…
- Question Who notifies building control about a rooflight? In law the duty sits with the person having the work carried out, which means the homeowner, and in practice…
- Question How much does a rooflight cost? There is no single figure, and any site that publishes one is quoting for a job it has not seen.…