Can you match my existing rooflight?
On anything from about 1990 onwards, almost always, because the size codes those units were built to are still in current ranges and the aperture repeats. Before that it gets harder, and on a Victorian iron light or a one-off joinery light made to whatever a carpenter marked out on the bench, an exact match off a price list does not exist and the answer becomes a made-to-order unit instead. The word “match” also carries three different meanings and it is worth separating them at the start, because a homeowner asking the question usually means one of them while the person answering assumes another. The identification side, how to read the plate and work out what you actually have, is at how do I find out what rooflight I have. This page is about whether a match is achievable and what happens when it is not. The wider job sits on the replacing a failed or misted rooflight page.
Three things people mean by matching, and they have different answers
The first is the aperture: will the new unit fit the hole that is already in the roof, without cutting rafters, moving trimmers or rebuilding the reveal. That one is a dimensional question with a yes or no answer, and it is the one that decides whether a job is a day or three days.
The second is the appearance: will the new unit look like the others on the same slope, from the garden and from inside the room. That is a question about frame colour, sightline, glazing tint and cladding profile, and it only matters where more than one unit is visible together. The third is the behaviour: will it open the same way, take the same blinds, work with the same rod, sit at the same handle height. That one is easy to check and easy to forget. Nearly every disappointment about a replacement comes from answering one of the three and assuming the others followed.
Matching the aperture, which is the one that has to be exact
The structural opening is not the same dimension as the frame, and this is where the arithmetic goes wrong. A nominal size code refers to the outer frame, and the structural opening is larger by roughly 40 to 45mm on most modern ranges to give room for packing and for the frame to sit square. So an old opening and a new frame were never the same number, and comparing the two directly produces a false conclusion in both directions.
What actually gets checked is the clear opening between the trimmers and the rafter cheeks, measured at the head, the middle and the cill, plus the diagonals. An opening that has crept out of square over forty years, which is common where a trimmer has deflected under load or a rafter has been wet, will take a frame at its narrowest dimension rather than its widest. Where the numbers land within about 10mm of a current size, packing and a made-good reveal deal with it. Beyond that the opening is adjusted, and adjusting the length within one rafter bay is far easier than adjusting the width across two.
What you have against how closely it can be matched
| What is in the roof | How close a match is realistic | What decides it |
|---|---|---|
| Unit from 2000 onwards, plate legible | Code for code, same brand, same size | Almost all current codes carry straight across. Frame profile may differ slightly |
| 1990s lacquered pine centre pivot, three-digit code | Direct equivalent in a current code | The three-digit codes map to current ones. The frame material choice reopens |
| 1980s timber roof window | Usually an equivalent, sometimes with the opening adjusted 30 to 40mm | Some early sizes have gone. Length adjustment stays in one rafter bay |
| 1970s acrylic dome on a site-built kerb | Made-to-size unit to the aperture | Imperial-derived kerb sizes with no modern equivalent. The kerb usually gets rebuilt anyway |
| Pressed steel light in an outbuilding or workshop roof | New unit, new or re-trimmed opening | The original was rarely formed to any standard, so there is nothing to match to |
| Victorian or Edwardian cast iron light | Conservation unit to the same sightline, or the original frame kept and reglazed | Imperial aperture and, on a listed building, consent decides the route |
| One-off joinery light in a Cotswold stone slate roof | Made-to-order conservation rooflight to your dimension | Those units are manufactured to the size asked for, which is why they solve old apertures |
| 2000s flush glazed flat roof unit | Made-to-size to the existing kerb | Kerb external size, structural aperture and daylight opening all get confirmed separately |
Matching appearance across a slope where only one unit changes
This is the case people underestimate. Where three units sit in a row on a Cheltenham loft conversion or across a Quedgeley rear slope and only one is being changed, the eye compares them constantly and small differences read clearly.
Four things vary. Frame colour: a clear-lacquered pine sash from 1998 has gone honey-gold with age and a new lacquered pine sash is pale, so they will not agree for several years. Sightline: manufacturers have changed sash sections repeatedly, and a slimmer modern profile shows more glass. Glazing tint: a current low emissivity coating, and more so a solar control one, has a faint cast that an uncoated 1990s unit does not, which shows in reflected light from outside and in the colour of daylight inside. Cladding profile: the external aluminium cover pieces have changed shape and finish across generations. None of that is a defect. It is worth knowing before the order rather than after, and it is the strongest argument for doing a slope together, discussed at should I replace all my rooflights at once.
Where a different make gives the closer visual result
Staying with the same brand is the usual instinct and it is often right, because the flashing kit geometry, the bracket positions and the frame depth are known quantities. But it is not automatically the closest match, particularly on an older unit whose own manufacturer has since redesigned the range.
VELUX, Fakro, Keylite and Roto all publish sizes at the same nominal widths of 550, 660, 780, 940, 1140 and 1340mm, and where the visual target is a 1990s profile, another make’s current section is sometimes nearer to it than the original manufacturer’s is. Three things then need confirming rather than assuming: the structural opening tolerance, which is not identical between makers and where 10mm is enough to stop a frame going in square; the bracket positions and frame depth into the covering, which move where the battens have to land; and the flashing kit geometry, which sets how far the covering is cut back and therefore how many slates come off. Those get resolved at survey with the covering opened at the head. Brand differences in mechanism and glazing are compared at choosing between the major roof window brands.
Matching how it opens, and the parts that travel with it
Behaviour is the third meaning and it is quick to settle. A centre pivot and a top hung of the same nominal size do not give the same clear opening, do not put the handle in the same place and do not give the same view out from a chair, so a swap between them is a choice rather than a match. Where the unit serves an escape opening from a room in the roof, the clear openable dimensions are a regulatory requirement rather than a preference, set out at roof window escape openings and the rules.
Blinds are the accessory that catches people. Internal blinds are made to the size code and clip into a groove or bracket set in the frame profile of their own generation, so a blind from a 1998 sash will not go into a modern frame at an identical nominal size. External awnings fix by generation too. Telescopic control rods are generic and almost always carry across, which is the one straightforward item on the list. Where blinds are wanted, they are cheaper ordered with the unit and they arrive handed and sized correctly, which is covered at roof window blinds and when to specify them.
When the size has genuinely gone, and what happens instead
Some sizes have disappeared. Units from the seventies and early eighties were built to imperial-derived dimensions and one-off lights were made to whatever suited the roof. There are four honest routes from there and the choice is made at survey rather than from a photograph: adjust the opening a small amount to land on a current size, adjust it properly by trimming another rafter to reach a larger standard size, order a made-to-size unit to the aperture, or specify a conservation rooflight, which is manufactured to the dimension you ask for. The full comparison of the four is at matching a replacement to an obsolete size.
Conservation rooflights deserve a word here because Gloucestershire has more call for them than most counties. A slim cast or pressed steel frame with a single central glazing bar, low profile, designed to sit nearly flush in a covering 20 to 30mm thick. On the Cotswold stone slate roofs through Painswick, Bibury, Northleach and Chipping Campden that is often the only unit that reads correctly, and because they are made to order they answer the awkward-aperture problem and the appearance problem at the same time. Sizes and finishes are at conservation rooflight installation.
Where the original is the thing being protected
On a listed building an original cast or wrought iron rooflight is frequently part of what the listing covers, so matching stops being a shopping question and becomes a consent one. The pattern in this county is fairly predictable. Where the iron frame is sound, the usual outcome is that the frame stays and the glazing is renewed into it, in slim putty-glazed or beaded form, occasionally with a thin double glazed unit where the rebate will take one. Where the frame has corroded past use, a modern conservation unit matched to the original sightline in the same aperture is normally accepted.
The sequence that works is pre-application advice with photographs and measured dimensions, then the order, then the platform. Doing it the other way round produces a made-to-order unit sitting in a garage while an application is decided. The listed and conservation area position is at can I put a rooflight in a listed building.
What to send so the match can be confirmed before anything is ordered
A match is confirmed from four pieces of information and most homeowners can gather them in twenty minutes. A clear photograph of the data plate, rotated into view if it is a centre pivot. A photograph of the unit from inside the room showing the frame, the sightline and the reveal. A photograph from outside far enough back to show the unit against the covering and any neighbouring units on the same slope. And the internal measurement across the plastered reveal at the head and at the cill, which gives a working figure even when no plate survives.
Send those to info@severnskylightsgloucester.co.uk with the address. From that set it is usually possible to name the make, the size code and a current equivalent by return, along with an honest note on where the appearance will differ. Then an indicative range by email, a survey with the covering opened at the head to confirm the opening, and a fixed price. Fifteen years on Gloucestershire roofs is mostly a matter of having seen which codes carry across and which do not. Related questions are collected in the guides, and the whole approach 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.…