Short version
Both, depending on the age. Original Victorian roof lights were cast iron, set in the roof with putty and small panes. Today's steel conservation rooflight is a modern copy in slim steel, and aluminium versions exist too. Most are made in a dark finish with a thermal break and double glazing, and councils generally look at the appearance and profile more than the metal.
Original cast iron roof lights
Cast iron was the standard material for small roof lights in Victorian houses, workshops and railway buildings. The frames were cast in one piece, with narrow glazing bars and small panes of single glass. Many survive on stone cottages and old industrial buildings, and some can still be seen on roofs around Swindon.
They date from before insulation standards and have no thermal break. They are also heavy, and an original frame is a historic feature that is usually better kept in place than removed. Where one is being kept, the question is a different one from the one this page answers.
What most owners want is a new opening that echoes the original: a slim frame, a bar down the middle and a flush fit. That is the job a modern replica does.
Modern steel and aluminium replicas
A modern steel conservation rooflight copies the old profile with current manufacturing. The frame is slim, the glazing bar sits in the middle, and the unit lies flush with the roof covering. Aluminium versions are lighter and use extruded sections, sometimes finished to look like painted metal.
Steel is the closest match to the original material. A cast iron style skylight in aluminium is a reasonable alternative where weight matters, such as on older rafters. Either way the glass is double glazed and sealed, so the unit performs far better than the original it resembles.
| Material | Character | Weight |
|---|---|---|
| Original cast iron | Historic, single glazed | Heavy |
| Modern steel replica | Closest to the original look | Heavier |
| Aluminium replica | Slim, similar profile | Lighter |
Sightlines, finish and the thermal break
The sightline is the width of frame you see from inside and outside. Conservation units keep it narrow so the glass dominates, with the bar as the only strong line. A flush fit means the frame barely rises above the slates or tiles.
Finish colour is usually a dark grey or black, chosen to sit with slate. Some makers offer other shades, and a council may have a view on colour. A thermal break is a barrier of low-conductivity material splitting the frame into an inner and outer part, and it is what lets a metal unit meet modern energy standards. Part L 2021, Table 4.2, sets the limit for a rooflight at 2.2 W/m²K, and the job plan names the figure for your unit.
On a slate roof, the line of the bar matters as much as the frame. A single central bar sits well on a narrow opening, while a wider one may take two, and the spacing is set so the bars echo the glazing pattern of the original lights found in similar buildings. Photographs of the existing roof help us suggest a layout at the survey.
- Narrow frame, with a bar as the main line
- Dark finish chosen to sit with the roof covering
- Thermal break between inner and outer frame
- Sealed double glazed units, with a laminated inner pane recommended
Weight and fixing
Steel is heavier than aluminium, and a large unit can weigh a good deal once the glass is in. The rafters either side of the opening carry it, so the survey checks their size, spacing and condition. Where a rafter has to be trimmed, that is structural work and is planned in the job plan.
Fixing follows the manufacturer's method, with the unit bedded and flashed to the roof covering. Slate, clay tile and stone slate each need their own detail, and the choice is made before the unit is ordered.
Weight also affects handling on the day. A heavy steel frame goes up in stages, and the order of work is written down in advance so the roof is closed before the team leaves.
Choosing between steel and aluminium
Start with the roof. If the rafters are old and slender, the lighter aluminium frame puts less load on them, and the survey will tell you which way the numbers fall. If the roof is sound and the council prefers a material close to the original, steel is the natural choice.
Then think about the finish. Steel is normally supplied painted or galvanised and then coated, and the coating protects it over time. Aluminium is usually powder coated in a factory, which gives an even colour and little upkeep. Ask for the finish to be written into the job plan, including the colour reference, so what arrives is what was agreed.
Finally, look at the glass. Both materials take double glazed sealed units, and we are happy to specify a laminated inner layer where you want one. Part K does not require laminated glass in a dwelling roof, but the extra layer holds together if the glass is struck.
Neither material is better in every case. The right one is the one that suits your roof, your council and your budget, and the quote shows both where that helps.
What councils usually accept
Councils are generally concerned with how a unit looks and sits in the roof, so a flush steel or aluminium conservation rooflight with a central bar is the type most often considered in a sensitive area. We cannot promise an outcome, because each authority decides for itself. Check the council's guidance and the register, as explained in the front roof answer and the note on glazing bars.
At the survey we look into what the council has said about your address, and the written quote shows it. To compare costs, see conservation rooflight cost, and for the overall service look at conservation rooflight installation. Notifiable jobs have their Building Control notice sent by us, and our 10-year workmanship guarantee is stated in the same document.
If an officer asks which material you intend, you can answer from the job plan, which names the metal, the finish and the glass. A clear answer at the start tends to save a round of questions later.