The Victorian original it is copying
A conservation rooflight is a modern, double glazed unit built to look like the cast iron rooflights that went into slate and clay tile roofs through the nineteenth and early twentieth centuries. Every feature that defines it traces back to that plain original.
The Victorian rooflight was a working object, not a design statement. It was a small cast iron frame, often with a single vertical bar down the middle, glazed with thin sheet glass and hinged at the top so it could be propped open with a notched stay. It sat low in the roof because the metal was thin and the slates or tiles lapped around it. From the street it read as a dark, flat rectangle in the slope, barely thicker than the slates themselves. Many survive on the rear slopes of Cambridge terraces, over attic stairs, box rooms and back bedrooms, usually single glazed, often cold and prone to condensation, and sometimes past the point where they can be kept.
The trouble came when these were replaced in the late twentieth century with standard timber or PVC roof windows. A standard roof window is excellent at keeping heat in, but it sits proud of the roof covering, carries a thick frame with a visible aluminium cladding profile, and is usually squarer in proportion. On a plain Victorian slope that change is obvious from the pavement. Conservation officers began asking for something closer to the original, and manufacturers answered with a new category of product: the conservation rooflight, sometimes sold as a heritage skylight.
So the useful way to judge any unit sold under that name is to ask how faithfully it reproduces the old cast iron rooflight from the outside, while giving you modern glazing, seals and insulation on the inside. The features below are the ones that decide it. If you are dealing with an original that needs to go, replacing cast iron rooflights covers matching a new unit to what is there now.
Flush fit: sitting in the plane of the roof
The single feature that most separates a conservation rooflight from an ordinary roof window is how low it sits. A true conservation unit is designed so its outer frame finishes close to the surface of the slates or tiles, rather than standing up on a deep frame above them.
Flush fit is achieved in two ways at once. First, the frame is shallow: the outer steel section is slim enough that it does not need to stand high to shed water. Second, the unit is set down into the roof, with the flashing designed so the slates or tiles overlap onto it at the sides and head in the way they would have met the old iron frame. The result from the street is a flat plane of glass and dark metal at almost the same level as the roof covering, instead of a box sitting on it.
- The roof plane: the line of the tiles or slates. How far a window sits above it is what the eye picks up from the street.
- Standard roof window: frame and sash sit proud of the roof, the usual choice where appearance is not controlled.
- Standard flashing kit: a wide, visible collar around the frame.
- Conservation rooflight: set down so the glass sits flush with the slates, in a slim dark frame.
- Slim central glazing bar, echoing the cast-iron rooflights of Victorian roofs.
- Low-profile flashing tucked under the slates, so almost nothing shows around the frame.
How flush a unit can sit depends on the roof covering as much as the rooflight. Natural slate is thin and gives the lowest line. Plain clay tiles are thicker and lap in more courses, so the unit may need a slightly different flashing kit. Deep profile tiles and pantiles need a flashing that follows their wave, and the frame will sit a little proud of the troughs. Manufacturers generally make a separate flashing for each covering, and choosing the right one matters more than any other part of the order. Our guide to conservation rooflights on slate and tile goes through each covering in turn, and the short answer on whether a conservation rooflight is flush fitted explains what the term does and does not promise.
Flush fit is now treated by many planning officers as the minimum a conservation unit should achieve, not an optional extra. A unit that calls itself conservation style but stands well above the tiles will often be questioned, even if its frame is dark and slim.
A slim steel frame and what it replaces
Steel is the traditional choice because it is strong enough to be thin. A slim external frame is what lets a conservation rooflight look like cast iron rather than a modern roof window with a thick border.
On a standard roof window the visible frame around the glass is broad, because it houses timber or PVC, insulation and the cladding that protects them. A steel conservation rooflight can carry the same glass with a much narrower external section, so the proportion of glass to frame looks right for an old roof. The frame is usually galvanised and then powder coated, which is what protects it on an exposed slope for decades. Behind that outer frame, better units add a thermal break so the steel is not a cold bridge straight through to the room, which matters for condensation on the inside face.
Aluminium versions exist and some are well made. They can be slim, they do not rust, and they are lighter to handle. The debate between the two is mostly about authenticity, strength at the thinnest sections and how the frame is detailed. We set that out in full in steel vs aluminium conservation rooflights, so here the point is simply that the frame has to be slim, dark and plainly detailed. A conservation officer looking at a product sheet will check the external frame dimension and the material, and a heavy extrusion with a stepped cladding profile is unlikely to pass as a heritage unit whatever its label says.
Look also at what you see from inside. A slim external frame often goes with a slim internal frame, which gives a crisp edge to the lining and more of the opening given over to glass. That affects the light, which we return to below.
The glazing bar: integral or applied
The vertical bar down the centre is the detail people recognise most readily, and the detail most often faked. A conservation rooflight should have a bar that is part of the structure, not a strip stuck on the glass.
Original cast iron rooflights had a bar because thin glass came in small panes and needed support. On a modern unit, an integral glazing bar divides the glazing into two sealed units, each held in the frame, so the bar is genuinely structural and casts the same fine line as the original. An applied bar is a strip bonded to the outside of a single sealed unit. From a distance they can look similar, but up close the applied bar sits on the surface, shows a different depth of shadow, and can look out of place as it weathers. This is why officers increasingly ask directly which type is proposed.
Not every conservation rooflight needs a bar. Smaller units on some roofs looked plain originally, and a bar on a very narrow unit can look fussy. Larger units, or ones replacing an original with a bar, are usually expected to carry one. The number and position of bars should follow the proportions of the opening, not a catalogue default. There is a full explanation in conservation rooflight glazing bars, including the trade-off between one bar and two on wider units.
Proportion, opening and the small details
Beyond flush fit, frame and bar, a conservation rooflight is defined by a handful of details that together make it read as an old unit: its shape, the way it opens, its colour and what you can and cannot see of its fixings and flashings.
Shape. The traditional rooflight is taller than it is wide, running up the slope. Conservation ranges are drawn from that vertical proportion, and one well known maker offers around sixteen standard sizes. A squat or square unit, even in dark steel, tends to read as modern. Conservation rooflight sizes and proportions covers how size is judged against the slope and the other openings on it.
Opening. Old rooflights were top hung and opened outward from the bottom edge, held on a stay. Conservation units follow the same pattern, usually with a winding handle or a stay inside. Many can be motorised, with the actuator hidden in the frame, and some are made in escape sizes, though you should check each against the room it serves.
Colour. The standard finish is a dark colour, typically black or a very dark grey, because that is what painted cast iron looked like against slate and weathered tile. Other colours are sometimes available where a roof calls for them. See what colour conservation rooflights are for when a different shade suits.
Hidden workings. On a good unit you should not see bulky flashing collars, exposed screw heads or bright metal trims from the street. The flashing sits under the roof covering where possible, and the glass is held by the frame rather than by a visible cover strip. These are small points individually, but they are exactly what makes one rooflight look right on a Victorian slope and another look like a replacement.
Checking a product against the features
Product names are not a reliable guide, because "conservation style" and "heritage" are marketing terms with no fixed definition. The practical test is to check each feature on the specification sheet and the drawings.
The table below is a starting checklist for comparing units. It is not a planning test in itself, since the local authority judges the whole proposal, but a unit that falls short on several of these lines is unlikely to be accepted as a conservation rooflight on a sensitive roof.
| Feature | What to look for | Why it matters |
|---|---|---|
| Fit | Frame close to the roof surface | Keeps the slope flat from the street |
| Frame | Slim steel section, thermally broken | Looks like cast iron, less condensation |
| Bar | Integral, splitting the sealed units | Casts a true line, weathers well |
| Shape | Taller than wide | Matches the old vertical openings |
| Finish | Dark powder coat over galvanising | Right colour, protects the steel |
| Flashing | Made for your covering | Lets tiles or slates lap as before |
Pitch is worth checking too. Conservation units are rated for a range of roof pitches, and one leading range is quoted at roughly 17.5 to 65 degrees. Most Victorian slate slopes in Cambridge fall comfortably inside that, but shallow rear outriggers and some later extensions do not, and a unit fitted below its rated pitch will not drain as designed. We measure the pitch at the survey rather than taking it from drawings.
When the rooflight is part of a planning or listed building application, these same features are what the officer reads on the drawings. Our answer on what conservation officers look for in a rooflight adds the planning side: number, position and visibility, which no product can solve on its own.
What the features mean for the light in the room
Every heritage feature has a small effect on the daylight you get. None of them is a reason to avoid a conservation rooflight, but it helps to know where the light goes before the roof is opened.
The glazing bar is the most visible. On a bright day it throws a thin shadow line across the sun patch on the floor or wall, which moves through the day as the sun crosses the slope. On a south or west facing roof in summer that line is crisp; on a north slope, where the light is diffuse sky light rather than direct sun, it barely registers. The bar and the extra frame between two sealed units do take a little glass area out of the opening, and our answer on whether a conservation rooflight lets in less light puts that in proportion.
The slim frame works the other way. Because the external steel section is narrow, a conservation unit can offer more glass for a given opening in the rafters than some standard roof windows of the same outer size. How much light actually reaches the room then depends on the lining: the reveals around the opening, which can be splayed so the light spreads down into the room rather than lighting a deep, straight sided shaft.
Glass choice sits inside all of this. Slim frames can carry low emissivity double glazing, and on a south facing heritage slope a solar control coating can take the edge off summer heat without changing the look from the street much, though some coatings add a slight tint worth checking against the officer's expectations. Laminated inner panes are sensible overhead. The light planning page shows how we read a slope's orientation and sun path, and that reading is what decides the glass, the lining and the exact position of a conservation unit before we quote.
Where conservation rooflights are expected in and around Cambridge
Cambridge City has eighteen conservation areas and South Cambridgeshire many more, so a large share of older homes around the city sit somewhere a conservation rooflight may be asked for. Whether one is required depends on the building, the slope and any Article 4 direction, not on the house type alone.
Listed buildings always need listed building consent for a new rooflight or a change to an existing one, and a conservation unit is normally the starting point for any proposal on a listed roof. In conservation areas, adding a rooflight to a house can still fall within permitted development unless an Article 4 direction removes it, but the planning authority can and does comment on the design when an application is needed, and a conservation unit is often the simplest way to make a proposal acceptable. The Victorian and Edwardian terraces of Romsey, Petersfield and the Mill Road area are a familiar case: rear dormers with conservation rooflights on the front slope are a pattern local architects and builders describe as commonly accepted, but each street and each application is judged on its merits. Rooflights in Cambridge conservation areas explains how Greater Cambridge Shared Planning approaches this, and the council's constraints map tells you whether your address is covered.
Plenty of people choose a conservation rooflight when no rule requires it, for the slim lines and the look on an older cottage or a barn conversion. The features that make it acceptable to an officer are the same ones that make it look right on a plain roof. Our main page on conservation rooflight installation in Cambridge covers the whole service, from the light survey to fitting, and every quote comes with a written light plan showing the position, size, glass and bar layout we recommend.
Questions people ask
Is any black roof window a conservation rooflight?
No. Colour is only one feature. A black standard roof window that stands proud of the tiles, has a thick cladding profile and no integral bar will still look like a modern window from the street. The defining features are flush fit, a slim frame, a vertical proportion and, on larger sizes, an integral glazing bar.
Can a conservation rooflight be as warm as a standard roof window?
Better units use thermally broken frames and modern sealed units, and are designed to meet the current Building Regulations limit for rooflights, which Part L sets at 2.2 W/m2K. The frame design differs from a standard window, so it is worth comparing the declared whole unit figure rather than the glass alone.
Do I need a conservation rooflight if my house is not listed?
Not automatically. It depends on whether the roof is in a conservation area, whether an Article 4 direction applies, whether the slope faces a street and whether planning permission is needed at all. We check this at the survey and, if you would like us to look at your roof, you can book a light survey.
Does the flashing change with the roof covering?
Yes. Slate, plain clay tile and profiled tile each need a flashing matched to the covering so the unit sits as low as that roof allows. Ordering the wrong flashing is one of the easiest ways to lose the flush look a conservation unit is bought for.
