Steel or aluminium conservation rooflights

Victorian and Edwardian outbuildings, stables, mills and coach houses were often lit by cast iron rooflights, a thin frame holding small panes with a bar between them, bedded into the slates. The frames were slim because iron is strong, and the glass was cut into narrow panes because large sheets were costly. That slim, divided look is what planners and owners are trying to recover when they ask for a cast iron style rooflight.

Modern conservation rooflights imitate it in two metals. Steel, usually galvanised or stainless, is the nearest cousin to iron. It can be rolled into thin, strong sections with sharp corners, and it takes a paint finish well. Aluminium is lighter and is extruded rather than rolled, so the section can be shaped to a profile with a fine upstand and a drip. Nothing on the market is cast iron in the original sense, and a good supplier will say so plainly.

Steel has the edge on slimness. Because it is stiffer, a steel frame can be thinner for the same span, and a steel glazing bar can be a fine line only a few millimetres wide. Viewed from the road, the frame reads as a dark edge hardly thicker than the slate beside it. For a listed building with a prominent roof, that difference can decide a consent.

Aluminium sections have to be a little deeper to match the strength, and the thermal break that keeps the frame warm adds to the depth. The best aluminium conservation rooflights hide that in the part of the frame below the glass line, so what you see from outside is still slim. Check the manufacturer's section drawing, not the photograph. Two units that look alike in a picture can differ by several millimetres at the edge, and an officer will see it on the drawing.

The glazing bar is the other tell. A true foundry style has a bar that is narrow, square-edged and slightly raised, running from eaves to ridge. Both metals can reproduce it, and some aluminium systems use a stuck-on bar to imitate the bar between two separate panes. A bar that is part of the structure looks different in raking light from one that is applied on the face of the glass. Our page on whether conservation rooflights have glazing bars covers the options.

The metal underneath matters less than the finish on top, because the finish is what meets the weather.

Steel

Steel is protected by hot-dip galvanising, a zinc coating, and usually a paint or powder coat over it. Stainless steel resists corrosion without a coating, but it is pricier and has a distinct sheen that is often painted or matted back. A scratch that exposes bare steel can start rust, so the finish needs to be good from the factory and kept intact during fitting. On an old roof, a slightly weathered, matt dark grey is the look to aim for.

Aluminium

Aluminium does not rust. Its surface forms a thin oxide layer, and a powder coat or anodised finish adds colour and protection. Powder coat is available in a wide range, which helps where an officer wants a specific shade to match old metalwork. Anodised finishes are harder wearing but come in a narrower set of tones. Where a roof sits near the Exe estuary or the south Devon coast, salt in the air is a reason to ask for a marine-grade coating whichever metal you choose.

Colour deserves its own mention. Listed buildings and conservation areas often attract a condition that the frame be a dark, matt colour, and a gloss finish can read as plastic in sunlight. Ask for a sample finish and look at it outside, against slate, in daylight, before the order is placed. Chips of colour on a desk are not a reliable guide.

Steel is heavier than aluminium, and a large steel rooflight can be an awkward lift to carry onto a scaffold and bed into a roof slope. On a pitched slate roof the unit is small, and the weight is borne by the rafters either side, so for a standard conservation size the difference is manageable. On a long, low slope, or a building with old or frail timbers, the lighter aluminium unit puts less load on the structure.

This is where the survey earns its keep. If the rafters are old, small in section or have been notched in the past, we say so, and the choice of metal follows from what the roof can carry. Steel suits timbers that are sound, with trimming done in new doubled-up timber either side. Aluminium suits a roof where you would prefer to disturb as little as possible.

The way the rooflight meets the slates is the same for both metals. The frame sits in a bed of flashing, usually lead or a dark metal, dressed over and under the slates around it, and the weathering is done to suit the covering. See conservation rooflights on slate roofs for the flashing detail.

Metal conducts heat, so a frame made entirely of steel or aluminium would be cold to touch, and moisture in the room's air would condense on it. The industry answer is a thermal break, a strip of low-conducting material, commonly polyamide, set between the inside and outside of the frame so heat does not pass straight through.

Most aluminium conservation rooflights carry a thermal break as standard. Steel is harder to break, because the frame is thin and the sections are rolled, so steel units often rely on the sealed glass unit and a thin insulating liner to control cold spots. The result is that a typical aluminium unit achieves a better frame performance, while steel can be perfectly serviceable where the frame area is small.

The glass does most of the work. A double-glazed sealed unit with a low-emissivity coating and a warm-edge spacer cuts heat loss across the whole rooflight and keeps the glass inner pane warmer, which reduces condensation. Approved Document L sets limiting U-values for new and replacement rooflights, with special consideration where a building is listed. We deal with that in can conservation rooflights be double glazed. Whichever metal you choose, the insulation around the frame and down the lining is what stops condensation showing at the sill.

Both metals can last for decades on a roof, and neither is fit and forget. Steel rewards a look every few years. The aim is to spot a chip in the coating early, before water gets under it, and touch it in with a matching paint. Aluminium asks less, mostly a wash with water and a mild cleaner to remove salt and grime, because dirt left on a powder-coated surface dulls it over time.

The glass unit is usually the first part to reach the end of its life, in either frame, because the seals of a sealed unit age. Choose a frame that lets the unit be replaced without removing the whole rooflight from the roof, and ask the supplier whether that is so. Changing a sealed unit alone, frame retained, is not notifiable to Building Control, but a whole new rooflight is.

A few questions are worth putting to any supplier before you commit.

• What is the frame section, in millimetres, at the glass edge and at the bar?
• Is the thermal break continuous, and what is it made of?
• What finish is applied, and what is the guarantee on the coating?
• Is the glazing bar structural or applied to the glass?
• Can the sealed unit be replaced in place?

The answers show quickly which products are carefully made and which are catalogue items with a heritage label. We put the chosen specification into the quote, so you can compare it against what an officer has agreed.

These are the usual pairings, though the survey can change them.

A Grade II listed house with a visible slate roof

Steel is worth considering for its slim frame, and the officer may prefer it. If the rafters are sound and the budget allows, it is the nearer match. Our guide to conservation rooflights for listed buildings covers the consent side.

A Victorian terrace or villa in a conservation area

Aluminium is often the practical choice. It holds a dark powder-coated finish on a damp coastal roof, and the thermal break keeps the room above a modest temperature. Rules vary by council, so see conservation rooflights in Devon conservation areas.

A converted barn, coach house or stable

Either metal suits. Aluminium is easier on long slopes and when the room beneath is a living space that needs a warm frame. Steel gives the nearest echo of the iron lights the building may once have had.

An opening rooflight for ventilation

Aluminium has more options, including units that open on a hinge or electrically and sit discreetly. Steel opening lights tend to be bespoke and take longer.

The same rule runs through all of these. Pick the metal after the structure and the finish are understood, not before.

Steel units are more often made to order, with more fabrication and finishing by hand, and that tends to push the price up and the lead time out. Aluminium units are produced in more standard sizes with a wider choice of coatings, which brings the price down for common openings. A bespoke size, an opening mechanism, a special colour or a higher glass specification raises either one.

The metal is only part of the bill. Scaffolding, slate work, structural trimming, the lining and the finishing around the opening all add to it, and on a listed building the drawings and any consent conditions take time too. Our costs page sets out what drives the price, and a survey turns that into a written figure for your roof.

Whichever frame you choose, you see it from the room. A conservation rooflight is slim, so the lining beneath it will be the dominant feature of the opening. That is where the finish schedule comes in. It records the depth of the lining, whether it will be straight or splayed, and whether the reveal is plaster, lime or boarded and painted. It also names the paint line, where the painted lining meets the metal frame.

A steel frame with a dark, fine edge looks sharpest against a crisp white plaster return. An aluminium frame, with a slightly deeper inner section, can take a small shadow gap or a flexible sealant line. In an old lime ceiling, the same lime finish is carried to the frame, and the joint between the plaster and the metal is flexed with a thin sealant bead rather than rigid filler, because the two materials move at different rates.

It is worth looking at the sample frame in the room where it will go, held up to the ceiling, before ordering. The metal looks different against warm lime plaster than it does on a showroom board. If you want to compare the options for your own house, ask for a quote and a finish schedule, and tell us the roof covering and whether the building is listed or in a conservation area. The whole range is on our conservation rooflight installation page.