Conservation rooflight design comes down to one idea: the unit should disappear into an old roof instead of announcing itself on it. A standard roof window is a box of white frame that stands up from the slates. A conservation rooflight is a slim, dark, nearly level pane that borrows the look of the cast-iron rooflights once set into mills, warehouses and terraces across Merseyside. If you want a heritage skylight style that suits a slate roof, the details below are what separate the right product from one that only claims the name.
This guide is about the product. It describes the features, how they fit into a slate roof and how to judge proportions. For the planning side, Article 4 directions included, turn to rooflights in a conservation area; how the glass is built up has a page to itself, double-glazed conservation rooflights.
What makes a rooflight a conservation rooflight
The name does not describe a certificate. It describes a family of design choices that all push in the same direction, which is to make the rooflight look as if it grew out of the roof. The features come as a set, and a unit missing most of them is a roof window with a heritage label.
- A frame that sits close to flush with the slate, with no visible kerb
- A slim profile, so the frame reads as a line, not a surround
- A dark finish, usually black or a deep grey, that sits with slate
- A single vertical glazing bar, or a pair of panes divided in the old style
- Glass that is clean and flat, without a bulge
- Fixings and hinges kept out of sight
Most of this is easier to see than to read about. From the pavement, a good one is a thin dark rectangle with a line down the middle that catches the sky. A poor one is a white box with a dome. The first suits a slate terrace in Toxteth or Birkenhead, and the second is the shape that planners and neighbours look at twice.
Flush fitting into slate
The word flush gets used loosely, so it helps to be exact about what it means on a roof. A roof window sits on timber battens above the line of the slates, with a flashing kit that rises from the covering to the frame and raises the whole unit. The result stands noticeably proud. A conservation rooflight is set into the roof so the glass sits just above the slate surface. It is made to be fitted between the rafters, with the slates running up to the frame on every side and the frame lying in the plane of the roof, rather than perched on a kerb above it.
That detail matters for two reasons. The first is appearance: a low unit casts almost no shadow and does not break the line of the roof when seen end-on. The second is weathering. Where the frame is close to the slate, the water has a short path across it, and the flashing is what seals the gap. On a slate roof the flashing is often lead, or a kit made for the unit, with the slates lapping over it at the head and sides and an apron dressed over the slates below, so water runs off the glass and back onto the roof. The lead is formed on site to suit the slate, and its quality decides how the rooflight lasts.
We take care here because this is the job that goes wrong when it is rushed. Old slate can split when it is lifted, and a roofer lifting courses to dress lead around a frame needs to know which slates to reuse and which to replace. We source matching slate where a few break, and we check the batten spacing before the frame goes in so the gauge of the slates is unchanged around the unit. See our conservation rooflight installation page for how a finished roof comes together.
A flush rooflight is judged from the street and kept dry by the lead.
Glazing bars and what they do
The bar down the middle of a conservation rooflight is the most recognisable feature, and the one most often misunderstood. In many of the original cast-iron rooflights the glass was held in smaller panes, and the frame carried bars to support them. The bars were structural. A modern unit has no such need, because the glass is made in large sheets and sealed as a double-glazed unit. The bar is kept for the look, and it carries a lot of that look.
The common patterns are these:
- Single central bar. A vertical bar splits the pane into two long panels. It is the classic and fits the widest range of roofs.
- No bar. A plain pane, which suits a small unit or a very modern extension on an old building. On a prominent slope it can look blank beside the sash windows below.
- Bar plus a cross bar. A horizontal bar adds a grid effect, which suits a larger unit or a grander property.
The bar sits on top of the glass or is built into the sealed unit. Applied bars sit on the surface and give the shadow line of a true bar. Whichever pattern you choose, the lines of the bar should echo the windows below. A house with two-over-two sashes carries a central bar well. Matching the bar to the window pattern is a cheap way to make a new unit look as if it had always been there.
Glass is also a question of look. Ordinary clear glass is the choice for most heritage roofs. Tinted glass changes the colour of the daylight inside and shows as a different shade against the sky, and clear glass is the usual choice on a heritage roof.
Proportions, size and where to put them
Because the unit is low and slim, it gets its character from its shape. Tall, narrow rectangles that follow the proportions of a sash window suit a slate terrace. Wide, squat units look like skylights from a catalogue. On a steep pitch, a portrait unit lines up with the roof; on a shallow pitch, a landscape one can look right, though the choice affects how the bar divides the glass.
Placement counts as much as the size. A few rules of thumb that keep a roof calm:
- Line the unit up with the windows below or with the rafter spacing, so the roof reads as ordered
- Keep it clear of the ridge, the eaves and the party wall by a few courses of slate
- Use the same size throughout if there are several, with even spacing between them
- Prefer a pair of small units to one large one, which dominates the slope
The structure sets the limits as well. The unit fits between the existing rafters, so the spacing of the timbers decides the width. A Victorian roof with timbers at irregular spacing may need a trimmer fitted by a carpenter, and we check the rafters in the loft before suggesting a size. When the room below is a loft conversion, the position of the head height and the floor line also affects where the unit goes, since a rooflight above the eye line gives sky and one at eye level gives the street.
Fixed or opening, and the frame material
Many conservation rooflights are fixed. A fixed unit keeps the lowest and slimmest profile, because there are no hinges or opening gear to hide. If a loft room needs ventilation, an opening version is available. It is usually top-hung, with the hinges tucked away behind the frame so the shape from the street stays the same. It may be opened by a pole or by a small electric actuator, which keeps the handle out of the way when the unit is in a high slope.
A fixed pane suits a stairwell, a landing or a bathroom with an extractor fan. An opening pane suits a bedroom or a study. Ventilation in a loft conversion is a Building Regulations matter, so the choice is made at survey, with the room in mind, not on the day.
The frame is usually steel or aluminium, finished in a powder coat. Steel gives the thinnest section and the truest look. Aluminium is lighter, with a slightly wider section. Both can be had in black and in a handful of greys. Frames are made to order, so the lead time is longer than for a standard roof window, and the order goes in only after the survey measurements are agreed.
| Choice | Look from the street | Best for |
|---|---|---|
| Steel frame | Thinnest line | Prominent slopes |
| Aluminium frame | Slim, slightly deeper | Slopes seen from further off |
| Single central bar | Classic heritage | Most terraces |
| No bar | Plain, modern | Small units, extensions |
Sizes, colour and getting the choice right
Conservation rooflights are sold in ranges of set sizes, often with the option of a made-to-measure frame. A range suits a standard Victorian rafter spacing and keeps the lead time short. Made-to-measure pays when the opening is odd, for example between two chimney stacks or in a roof that has been altered over the years.
Colour is the simplest decision and the one with most effect. A black or very dark grey frame blends into slate and against sky. A lighter grey stands out on a dark roof. Paint finishes other than the standard dark ones usually mean a special order, and on a roof seen from a conservation street they are rarely worth the trouble.
Before you commit, the questions to put to anyone quoting are practical. Ask how far the frame stands above the slate, what the flashing is made of, whether the bar is built in or applied, and whether the quote names the exact product. We put all of this in the fixed written quote, after a survey of the roof, so you can compare like with like. For what drives the price of these units, see how much a conservation rooflight costs.
Fitting, finish and what you live with afterwards
A conservation rooflight is fitted by the same team that surveyed it. The slates around the opening are lifted and stacked, the opening is trimmed, the frame is set and levelled, and the lead flashing is dressed into the courses. The slates go back and the finish is checked from outside as well as in. Fitting one falls under the Building Regulations as controlled work, so we file the Building Control notification, and a written guarantee on our workmanship then runs for 10 years.
Once it is in, the unit asks for little. Rain washes the glass and an occasional wipe from the inside keeps the bar clean. Black frames show water marks in a hard-water area, which a damp cloth clears. In a house near the coast on the Wirral or at Formby, salt on the frame is worth rinsing off at the change of seasons.
For a house in a protected street, the sequence is the same as for any other, with the planning check at the start, and the planning guide covers that. Once the permission question is answered, the design is the thing you live with every day. Get a free quote
Design points that come up at the kitchen table
Can a conservation rooflight go in a roof that is not in a conservation area?
Yes. The style is available to anyone, and many people choose it for the slim line and the dark frame on a period house or a barn conversion. It is a product choice, not a designation, so no special status is needed.
Will the central bar get in the way of the view?
It is a thin line, and from the room below you look past it. The eye tends to settle on the sky behind it. A plain pane with no bar is available for anyone who values the open sky above how the roof looks from the street.
Do the units leak more because they sit low?
A well-made unit is watertight because of the flashing, not the height of the kerb. The lead is dressed to carry water over and away from the frame, which is why the survey and the roof detail matter more than the product brochure.
How many can I put in one slope?
The rafter spacing and the look of the roof set the number. Two or three small, evenly spaced units usually look better than a single large one, and we draw the layout on a photograph of your roof so you can judge it before the order.