Cast iron against pressed steel frames

Why the change

Cast iron is brittle. Strong in compression, poor in tension, and it cracks rather than bending under impact.

It is heavy, which matters on a roof and matters more when somebody has to lift it up a ladder.

It cannot take a modern double glazed unit without redesign, because the sections were never intended to hold a sealed unit and its weight.

Corrosion. Cast iron rusts, and it does so in layers that lift paint. Modern steel is galvanised and powder coated.

What is kept

The proportions and the visual language: slim sections, a single central glazing bar, a black finish, and a flush setting into the roof.

From the ground, a modern conservation rooflight and an original are very hard to tell apart, which is the point.

Restoring an original

Sometimes the right answer on a listed building, particularly where the original is intact and the listing description mentions it.

It means stripping, treating corrosion, repairing cracks and reglazing, usually with slim single glazing or a very slim sealed unit where the section allows. It is specialist work, it is not cheap, and it is a conservation decision rather than a glazing one.

Where an original exists, do not remove it without asking. It may be the reason the building is listed as it is.

Timber composites

Some manufacturers offer steel outside with a timber inner frame, giving the external appearance with a warmer internal finish and slightly better thermal performance.

What the original rooflights were

The conservation rooflight exists because of a real historic product. From the middle of the nineteenth century, cast iron rooflights were fitted in enormous numbers to workshops, warehouses, stables, farm buildings and the back slopes of ordinary houses.

They were cast rather than fabricated: a one piece frame, heavy, with a substantial section and a central glazing bar, glazed with putty into a rebate. They sat low against the slates or the plain tiles because the casting was shallow, and they were painted black or a dark colour.

That combination, low profile, upright proportion, central bar and a dark finish, is what a conservation officer is looking for today, and it is looking for it because those are the characteristics of the thing being imitated.

Cast iron in practice

Heavy. Cast iron is dense and the sections are thick. A surviving unit is a substantial lump of metal, and that has consequences for handling and for what the roof around it has to carry.

Brittle. Cast iron does not bend, it fractures. A dropped or struck frame cracks, and cracks in castings are difficult to repair invisibly.

Single glazed. Almost universally. The rebate was made for a single pane bedded in putty, and there is no cavity.

Long lived if maintained. Painted regularly and kept dry, cast iron lasts indefinitely. Neglected, it rusts, and rusting iron expands, which cracks the putty, admits more water and accelerates the process.

Pressed steel, which is what modern conservation units are

A modern conservation rooflight is fabricated from pressed or rolled steel sections rather than cast. That is a different manufacturing process producing a different object, and the differences all favour the modern one except in one respect.

Slimmer sections. Steel is stronger in tension and bending than cast iron, so the same structural job is done with less material. That is what allows a modern unit to carry a double glazed sealed unit while keeping the sightline slim enough to read correctly.

Lighter. Meaningfully so, which matters for handling and for the structure.

Double glazed. A modern conservation unit takes a sealed unit with a low emissivity coating, argon fill and a warm edge spacer, and clears Part L comfortably. There is no thermal penalty for choosing the conservation appearance.

Thermally broken. Steel conducts heat well, so a good conservation unit interrupts the path from inside to outside with a low conductivity section. Ask whether the profile is thermally broken, because not every product is.

Powder coated. A factory applied finish that does not need painting, against cast iron which needs repainting on a cycle.

The one respect in which cast iron wins is authenticity, and on a building where the original units survive that is not a small thing.

Where a surviving cast iron unit should be kept

Where it is original to a listed building, in reasonable condition, and part of what makes the roof what it is.

A conservation officer may well require it to be retained rather than replaced, and on a building of quality that is the right answer even where a modern unit would perform better.

What you are keeping is a single glazed hole in the roof. Its whole unit U-value is poor by any modern standard and no modern requirement applies to it, because retaining historic fabric is not the same as installing a controlled fitting.

Making a retained cast iron unit workable

Secondary glazing below it is the usual compromise, and it is usually what a conservation officer will accept. An independent glazed panel inside the reveal, ideally removable, gives you a cavity and a warmer inner surface without touching the historic unit at all.

Proposing it in the application rather than waiting to be asked makes for a smoother process.

Repair rather than replacement of the frame itself. Rust is cut out or treated, putty is renewed, and the whole thing is repainted properly. That is specialist work and it is entirely feasible.

Reglazing with a slim laminated pane rather than plain float glass, where the officer permits it, improves safety without altering the appearance.

Expect condensation. A single glazed cast iron unit in a heated room will run with water on cold mornings and there is no specification that changes it. Ventilation and, where possible, secondary glazing are the answers.

Where a modern pressed steel unit is right

Any new opening on a listed building or in an Article 4 area, where the appearance has to satisfy a conservation officer and the performance has to satisfy Part L.

Any replacement of a modern rooflight or a plastic dome on a designated property, which is a heritage improvement and worth saying so explicitly in the application.

And any situation where a surviving cast iron unit has deteriorated beyond sensible repair, though that is a conversation with the conservation officer rather than a decision to take alone.

What to look for in a modern conservation unit

Profile height above the covering. The characteristic that matters most, and the manufacturer’s section drawing showing it is the document that decides an application.

Upright proportion. Taller than wide. Where more glazed area is wanted, two units side by side rather than one wide one.

A genuine central glazing bar, structural rather than an applied bar stuck to the glass. On a designated building an officer may well look.

Matt black or a very dark finish, not gloss.

A thermal break in the profile, and a warm edge spacer in the sealed unit.

Living with steel

Powder coated steel does not need painting, and it does need looking at every few years for damage to the coating.

Exposed steel corrodes where timber and aluminium do not, and a small chip left alone becomes a rust bloom. Touching it up promptly is a five minute job; leaving it is how a frame deteriorates.

On a coastal or exposed site, where salt is in the air, that check matters considerably more, and specifying a higher durability coating at the point of order is worth the small premium.

The accessory question

Worth raising because it catches people out on both. Neither cast iron nor modern pressed steel units carry the clip in blind ranges that mass market roof windows do.

Blinds become a separate made to measure item fitted to the reveal, which costs more, never sits as neatly, and lacks the side channels that make a blackout blind genuinely black out.

Budget for it at the start rather than discovering it at the end, and where blackout matters in a room getting a conservation rooflight, discuss it before the unit is chosen.

The original, and what it was

Nineteenth century rooflights were cast iron: molten metal poured into a mould, producing a frame in one piece with the glazing bar integral to it.

Casting gives crisp detail and allows sections that are slim where they can be and thicker where load requires it, which is why originals have a particular character: fine bars, a subtly modelled surround, and no visible joints.

Cast iron is strong in compression and brittle in tension, which is why originals sometimes crack rather than bend when a roof moves, and why a cracked one is a repair rather than a straightening job.

Pressed steel

The modern equivalent, and what most conservation rooflights sold today are made from.

Steel sheet is pressed and folded into the profile, then welded and finished. It is a fabrication rather than a casting, so the section is uniform in thickness and the corners are formed rather than modelled.

The result is lighter, considerably cheaper, and dimensionally consistent, which matters when units are made to bespoke sizes.

Visually it is close but not identical. Cast detail has a softness at the arrises that pressed work does not, and a conservation officer familiar with originals can tell them apart at close range.

Where the difference matters

On a highly graded listed building where original rooflights survive elsewhere on the roof, matching the existing may genuinely require a cast unit or a faithful reproduction.

On most listed and conservation area work, a good pressed steel conservation rooflight is what is specified and what is accepted, because from the ground the low profile and the single glazing bar are what read, not the metallurgy.

Corrosion, and what protects each

Both rust. The protection is the finish, and the vulnerability is at cut edges, fixing holes and any damage.

Modern units are galvanised or otherwise treated before painting, which is a substantial improvement on the originals. A twenty year old conservation rooflight with a sound finish is doing well; an original that has been repainted regularly for a century is doing better than either.

Near the coast, ask what the treatment specification is rather than only what colour the paint is.

Repairing an original

Worth doing where one survives. Original rooflights are part of what is listed, and removing one to fit a modern equivalent may not be permitted.

Repair means needle gunning back to sound metal, treating, repainting, and reglazing, and specialist firms do this work. It is more expensive than replacement and it is frequently the only option.