Galvanised vs Copper Pipes: When Replacement Is Actually Due

What galvanised pipe is, and how it dies

Galvanised pipe is mild steel tube hot-dipped in zinc, inside and out. The zinc is not paint. It corrodes preferentially, sacrificing itself to protect the steel underneath, and while it lasts the pipe is genuinely excellent. The problem is that the internal zinc is in contact with moving water for its entire life, and once it has been consumed the steel is exposed.

From there it goes one way. Iron oxide occupies several times the volume of the steel it came from, so the corrosion does not thin the wall evenly, it grows lumps inward. Those tubercles narrow the waterway, catch more debris, and accelerate. A 20mm nominal pipe that started with a full bore can finish with a few millimetres of clear passage through the middle of a solid-looking length of steel.

Threads fail first, and there is a mechanical reason for it. Cutting a thread machines the galvanising straight off the outside of the pipe, so every screwed fitting is a spot where the protection was removed on the day of installation. That is why leaks in a galvanised system appear in clusters at elbows, tees and unions rather than in the middle of a straight run. Hot lines go before cold lines, because heat accelerates every part of the process.

How to tell what you have in 5 minutes

You can settle this yourself before anyone quotes on anything. Find exposed pipework at the water meter, at the hot water unit, under a suspended timber floor, in the roof space, or where the supply comes out of the wall at the laundry trough. Then run these checks.

The symptoms that mean the pipe, and the ones that do not

This is where money gets wasted, because low pressure is the headline symptom of galvanised pipe and it is also the headline symptom of about 5 other things that cost far less to fix. There is a simple test that separates them.

Static pressure is what a system holds with nothing running. Dynamic pressure is what it delivers under flow. A corroded galvanised system frequently shows perfectly acceptable static pressure, because a narrow pipe still fills up eventually. Open a single tap and it can look fine. Open a second and a third and the flow collapses, because the restriction is now the limiting factor. That collapse under load is the signature of a restricted bore, and it is what genuinely points at the pipe.

1. Run one cold tap on its own and note the flow.
2. Open a second and a third outlet, including a shower.
3. If the first tap barely changes, the pipe is not your restriction. If it drops to a trickle, it is.

When the flow drops equally everywhere and does not change with the number of outlets, look elsewhere before spending anything on pipework. A pressure limiting valve that has failed closed, a stop tap at the meter that was never fully reopened after a repair, a clogged whole-of-house filter cartridge, a failed tempering valve on the hot side, or genuinely low street pressure all present the same way and none of them are solved by re-piping the house.

Copper, PEX, or a mix of both

Copper is the material most people ask for, and it deserves the reputation. It tolerates heat, it can be run exposed without protection, it holds its shape, and a properly installed copper system will comfortably outlast the mortgage. The costs are labour and access: copper is rigid, so every bend, every joist and every stud wall has to be opened up and worked around.

PEX changes that calculation in a character home, and it is the point most articles on this subject skip. PEX is flexible, so a continuous run can be pulled through a wall cavity, under a floor or across a roof space from one access hole to another. In a house with lath-and-plaster walls, ornate cornices, pressed metal ceilings or original skirtings, the damage avoided is often worth more than the difference in material price. Reinstating decorative plaster is skilled, slow work.

• Copper: the default for exposed runs, plant rooms, the hot water unit connections, and anywhere the pipe can be seen or knocked.
• PEX: the low-damage option for internal re-runs through finished walls, floors and ceilings. It must be installed as a matched system, with the manufacturer's own fittings and crimping tool, and protected from UV and from rodents.
• Polyethylene: the usual choice for the buried service run between the meter and the house, because it comes in a single continuous coil with no underground joints.
• Never galvanised. Repairing galvanised with more galvanised puts new steel into a system already producing the debris that will block it.

A mixed approach is normal and correct: polyethylene from the meter, copper at the hot water unit and anywhere exposed, PEX for the concealed distribution. What matters is that every junction between dissimilar metals is handled properly, which is the next section.

Mixed metals: why a half-finished job is worse than none

Put copper and galvanised steel in direct contact with water flowing between them and you have built a battery. The zinc and then the steel become the anode and give up material to protect the copper, so corrosion at that junction runs several times faster than it would have on either metal alone. This is galvanic corrosion, and it is the reason a well-meaning partial repair often produces a leak within a couple of years, right at the new joint.

Direction matters as well. Copper upstream of galvanised is the worst arrangement, because dissolved copper carried along by the flow deposits on the steel downstream and sets up the same reaction at every point it lands. That is why a new copper service run teed straight into an old galvanised internal system can make the internal pipework fail faster than leaving it alone would have.

Staging the work, and who owns which pipe

The boundary in South Australia is the water meter. SA Water owns and maintains the main in the street and the meter itself, and everything on the property side of that meter belongs to the owner, including the buried service run up to the house. A leak before the meter is SA Water's to fix. A leak after it is yours. Establishing which side a fault sits on is the first thing worth doing, because it decides who pays before anyone discusses materials.

Inside the boundary, stage the work by sequence rather than by symptom. Replacing whichever branch leaked most recently leaves new copper hanging off old steel and creates the galvanic problem described above. The order that works is: the service run from the meter first, then the hot water unit and its distribution, then the wet areas one at a time, with a dielectric transition at each remaining boundary until the last of the steel comes out.

• Photograph every run before the walls or floors close, with a tape measure in frame. It costs nothing and it is worth a great deal to the next tradesperson, and to a buyer.
• Replace isolation valves as you go, so future work on a single wet area does not mean shutting the whole house down.
• Keep the Certificate of Compliance for the work. It belongs with the house papers.
• If the only galvanised left is an outside garden line or a single laundry branch, replace that and stop. A whole-house re-pipe on a house with 1 tired branch is money spent years too early.

Questions we get asked

References

• SA Water: water meters, supply and responsibility
• Standards Australia: AS/NZS 3500 Plumbing and drainage
• Consumer and Business Services SA: plumbing, gas and electrical licensing