What is cast iron, and where will I find it in a house?
Iron with enough carbon in it to be cast rather than worked, which makes it strong in compression, hard, and brittle. Where it turns up in British houses, how it fails, and what you can and cannot do to it.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 5 min
- Safety
- warning
Short answer
Cast iron is poured into a mould rather than rolled or forged, so it takes complex shapes cheaply but cannot be bent. It is brittle: it cracks rather than deforms. In houses it appears as soil pipes and rainwater goods, radiators, baths, fireplaces and inserts, air bricks, drain covers and old structural columns. It rusts slowly and forgivingly, and it usually fails from impact, frost or being over-tightened.
It is the material people most often misidentify and most often mishandle, because it looks like steel and behaves nothing like it. If you have a Victorian or Edwardian house, a surprising amount of what is still working after a century is cast iron, and most of what breaks does so because someone treated it as though it would bend.
Safety
Step by step
- Understand what makes it different.It has a high carbon content, which makes it fluid when molten and hard when set. That hardness comes with brittleness: it is very strong in compression and poor in tension, so it snaps rather than bending.
- Learn to recognise it.Thick sections, considerable weight, a grey fracture surface, and often visible mould seams or a slightly grainy as-cast texture. It is magnetic. Tapped, it gives a dull thud rather than the ring of steel.
- Know where it is in a house.Soil and rainwater pipes and gutters on older properties, column and panel radiators, roll-top baths, fireplaces and grate inserts, air bricks and coal-hole covers, boot scrapers, gully gratings, and structural columns in Victorian commercial conversions.
- Understand how it corrodes.It rusts, but slowly and mostly at the surface, because the graphite in it slows the process. Century-old sound cast iron guttering is common. What kills it is water sitting in a joint or a socket and freezing, or a hidden face that was never painted.
- Respect the joints.Traditional cast iron drainage is socket and spigot, caulked with yarn and lead or with a proprietary compound. Modern replacement systems use rubber-sealed couplings. Neither is a threaded joint, and neither responds well to force.
- Never over-tighten a fixing into it.The material has almost no give. A bolt through a cast ear or lug will snap the ear before the bolt yields, and a cracked cast component usually cannot be repaired in place.
- Drill it carefully or not at all.Cast iron drills reasonably well with a sharp HSS bit at low speed and no hammer action, and it produces fine dark dust rather than curls. Hammer action will crack it. If the component is under load, do not drill it at all.
- Get help with weight and height.Cast iron gutters, downpipes, baths and radiators are two-person items at minimum, and removing them at height is genuinely dangerous. This is where a lot of DIY cast iron work goes wrong.
Tips
- Cast iron rainwater goods are usually worth repairing rather than replacing. The metal is often sound and the failure is a joint, a bracket or the paint, all of which are fixable.
- The right paint system matters more than the paint colour. A rust-inhibiting primer suitable for iron, then a compatible topcoat, and paint the back faces and inside of gutters where you can reach them.
- Cast iron radiators are heavy enough to need the floor considered, particularly on a suspended timber floor. A full one weighs far more than an equivalent steel panel.
Common mistakes
- Levering or springing a cast iron component into place — It has no elastic range worth speaking of. What would bend in steel simply cracks, often invisibly, and the crack finds you later as a leak.
- Burning or dry-sanding old paint off cast iron — Pre-1992 paint on older ironwork is very likely lead-based. Heat and dry abrasion make it airborne. Use a method appropriate to lead paint, or have it done.
- Replacing one cast iron gutter section with plastic — The profiles and the drop do not match, the joint is a permanent weak point, and it looks wrong. If a section must go, replace it with cast iron or a cast iron effect system designed to join to it.
If it doesn't work
A cast iron gutter dripping at a joint
Cause: Failed jointing compound, or corrosion in the socket holding water — Fix: Clean the joint back thoroughly, re-bed it with a suitable gutter sealant designed for metal, and re-bolt. Do not over-tighten the bolt: the ear will go before the bolt does.
A split running down a cast iron downpipe
Cause: Water trapped in the pipe freezing and expanding — Fix: Replace the section. A split casting cannot be reliably repaired in place, and the cause — a blockage holding water — has to be cleared or it will split the next one.
A cast iron radiator weeping at a joint after being moved
Cause: The section joints have been disturbed, or a nipple has been overstressed — Fix: This is specialist work. Cast iron radiators are rebuilt and pressure tested by people who do it, and a weeping section joint is not a sealant problem.
Questions people ask
Can cast iron be welded?
It can, by people who specialise in it, using specific techniques and controlled preheating and cooling. It is not ordinary welding, and an amateur attempt usually cracks the casting further.
Is cast iron guttering worth keeping?
Usually yes, on an older house. It lasts far longer than plastic, it looks right, and on a listed building or in a conservation area replacing it with plastic may not be permitted at all.
How do I tell cast iron guttering from plastic from the ground?
Look at the brackets and the joints — cast iron is bolted at each joint with a visible bolt through the overlap, and it is usually painted rather than coloured through. It also rings differently when tapped, and it does not flex when a ladder rests near it.
What is ductile iron?
A modern cast iron with the graphite in spheroidal rather than flake form, which makes it far tougher and able to deform rather than snap. Modern drainage covers and pipe fittings are often ductile iron rather than grey cast iron.