Why steel is used at all, and why it needs protecting
Steel gives a conservation rooflight its slim, flat profile, but bare mild steel left out in the weather will corrode, so the frame depends entirely on what is put over it.
The reason conservation rooflights and skylights are traditionally steel is strength in a thin section. A steel frame and glazing bar can be much narrower than a timber or plastic equivalent and still carry a double glazed unit, which is what lets the rooflight sit down flush in the slates and read like a Victorian original from the street. The background to that profile is set out in what makes a conservation rooflight.
The same material that gives the slim look is the one that rusts if water and oxygen reach it. So the question is never really whether steel can rust. It is whether the particular unit on your roof keeps the steel covered for its whole life, including the edges, corners, drainage holes and fixing points where a coating is easiest to break.
The protection layers you will see on a specification
Most steel conservation units rely on two layers working together: a zinc layer that protects the steel even where it is scratched, and a coloured finish that protects the zinc.
| Layer | What it does | What to check |
|---|---|---|
| Hot-dip galvanising | Zinc bonded to the steel | Done after fabrication, not before |
| Powder coating | Colour and a sealed skin | Surface preparation and thickness |
| Stainless fixings | No rust streaks from screws | Grade of stainless used |
| Frame material | Steel, stainless or aluminium | Which metal the whole frame is |
Galvanising matters most when it happens after the frame is cut and welded, because then the zinc reaches the welds and cut edges as well as the flat faces. Zinc also corrodes in preference to the steel beside it, so a small nick through the paint does not simply open a path for rust in the way it would on painted bare steel. Some makers state a galvanising standard such as BS EN ISO 1461 on their data sheets; others describe their own process. The details vary between ranges, so we read the maker's specification for the exact unit rather than assume every steel rooflight goes through the same factory process.
Powder coating then gives the colour, usually a dark grey or black chosen to sit quietly in the slope, as covered in what colour conservation rooflights are. How well it lasts depends on the surface preparation underneath and the thickness applied, and claims about coating life are the maker's own, tied to their warranty terms.
Where rust shows first when it does appear
When a steel rooflight does show rust, it almost always starts at a point where the protection was broken or where water sits, not in the middle of a coated face.
- Damage during handling or fitting, such as a frame dropped on a scaffold board or scraped against a slate, which cuts through both layers.
- Screws and fixings of the wrong metal, which rust themselves and leave orange streaks down an otherwise sound frame.
- Standing water on a unit set at too low a pitch for its design, or where debris blocks the drainage path.
- Condensation on the inside face, where warm, damp air from a bathroom or bedroom meets a cold frame night after night.
The last point is as much about the room as the roof. A rooflight over a shower in a Mill Road loft will see far more moisture than one over a landing, and a frame with a thermal break, good ventilation and an extractor all reduce how long the inside face stays wet. That side of the subject sits in thermal performance of conservation rooflights.
Old cast iron is a different story
The rusted rooflights people picture are usually original Victorian cast-iron units, which were painted rather than galvanised and have had well over a century of weather.
Many terraces in Romsey and Petersfield, and older cottages in villages around Ely and St Ives, still have small cast-iron lights over lofts and stairs. They were made with a single pane of glass and a painted finish, so once the paint went, the iron was exposed every winter. By now many are rusted through at the upstand, rattling in their openings, and letting in cold air around a glass pane that was never sealed to modern standards.
At that point the sensible route is replacement, not more work on a unit that has reached the end of its life. A modern conservation unit of the same size and proportion keeps the look officers expect while adding double glazing and a protected frame. How that swap is planned, including whether consent is needed, is covered in replacing cast iron rooflights.
Steel, stainless or aluminium if rust worries you
If corrosion is your main concern, the frame material is worth weighing, but each option trades something against the look and cost.
Galvanised and powder coated mild steel is the traditional choice and gives the slimmest sections. Stainless steel frames, offered in some ranges, resist rust without relying on a coating, at a higher price. Aluminium does not rust at all, because it forms its own oxide skin, but it usually needs a slightly deeper section for the same strength, which some officers notice on a sensitive front slope. The balance between them is set out in steel vs aluminium conservation rooflights. Cambridge sits well inland, so the salt-laden air that attacks metal on a coastal roof is not the deciding factor here; fitting quality and moisture inside the room matter more.
For a view of the whole service, from the light plan to the finished opening, see conservation rooflight installation in Cambridge, or read how we plan the light before you choose a slope.
Questions people also ask
How long does the coating on a steel conservation rooflight last?
It depends on the maker's process and the conditions on your roof, and coating guarantees are set by each manufacturer. The wider question of service life is answered in how long conservation rooflights last.
Do conservation skylights rust faster over a bathroom?
A bathroom puts more moisture on the inside face of the frame, so good ventilation and a thermally broken frame matter more there. The outside coating is not affected by what happens in the room.
What should I do if my old rooflight is rusting through?
Plan to replace it. A rusted cast-iron or failed steel unit is usually also cold and draughty, and a new conservation rooflight in the same opening solves all three. Book a light survey and we will look at the size, slope and consent position together.
Can rust stains come from the roof rather than the rooflight?
Yes. Old iron nails, clips or flashings nearby can streak down onto a sound frame. We check what else is on the slope during the survey so the new unit is not blamed for someone else's metal.
