Why do older Melbourne homes keep getting blocked drains?

Older homes across Melbourne’s east block up more often than newer ones, and the reason is underground rather than inside the house.

The drains under a house built in the 1950s or 1960s are a different material to the ones under a house built in the 1990s. They were laid in short lengths with a joint every metre or so, and each of those joints is a place the pipe can open up.

Once one of them does, the same drain tends to block again on a cycle. Clearing it restores flow for a while, but the opening that caused it is still there. Our residential plumbing services cover this across the older suburbs, where it is a repair problem rather than a cleaning problem.

What is under an older Melbourne home

Most pre-1980s homes have earthenware drains, a material also sold as vitrified clay or terracotta. All three names describe the same thing: pipe made from clay fired at high temperature until it hardens into a dense ceramic.

It was supplied in short lengths, generally somewhere between 600mm and 900mm. One end of each length is flared, the next pipe is pushed into it, and the gap between them is packed and sealed with mortar.

Clay was the standard material for household drains in Australia until around 1980, when PVC began replacing it. PVC comes in much longer lengths and its joints are chemically welded rather than mortared, which is why newer homes have far fewer weak points underground.

The practical difference is the number of joints. A thirty-metre clay drain can carry forty or more mortared joints. The same run in PVC might have a handful.

Why the joints open up

Two things work on those joints, and both of them take decades rather than years.

The first is the mortar itself, which degrades with age and with constant exposure to wastewater. The second is the ground it sits in. Melbourne is built largely on reactive clay soil, which expands when it takes up water and contracts as it dries out.

That means the ground around a drain rises and falls with the seasons, every year, for the life of the house. Clay pipe does not flex, so the movement is taken up at the joints, and over thirty or forty years that is enough to open gaps the mortar no longer bridges.

Nothing has to go wrong for this to happen. It is the expected end state of a clay drain in Melbourne soil, which is why the age of the house predicts the problem better than how the plumbing has been treated.

What gets in once a joint opens

Tree roots are the most common. A drain leaking slightly at a joint puts moisture into the surrounding soil, and roots grow towards it and through the gap. This is covered in more detail in our article on why drains block every spring.

Fine soil and grit also wash in through the same opening. That builds up along the bottom of the pipe and reduces how much the drain can carry, which is why an older line can run slowly even with nothing obviously stuck in it.

The more serious outcome is the pipe cracking or partly collapsing at that point. Once a section has dropped out of line, waste no longer runs away cleanly and the blockages become more frequent.

Why the same drain blocks again and again

When a drain is machine cleared, a cutting head runs down the line and removes whatever is inside it. Flow comes back straight away and the drain tests clear, which is the correct emergency response and usually the right first move.

What that does not do is change the pipe. The open joint or crack is exactly as it was, and roots outside the pipe are undamaged, so the same defect admits the same growth. In practice the blockage tends to return within twelve to eighteen months.

This is also why a recurring blockage is worth treating differently to a one-off. If you are not sure whether you are dealing with a local blockage or the main drain, our article on telling a blocked drain from a blocked sewer sets out the checks.

What ends the cycle

There are two steps, and the order matters. The first is a CCTV inspection, where a camera on a flexible rod is fed through the drain so the condition of the pipe can be seen directly. That locates the defect, shows how bad it is, and records the state of the rest of the line.

The second step is repairing the pipe itself. Pipe relining is the usual method on clay drains: a resin-saturated liner is drawn into the existing pipe and cured in place, forming a new sealed pipe inside the old one. It seals the open joints and cracks along the way, and it is done through existing access points rather than by digging up the yard.

Relining is licensed drainage work in Victoria. The Building and Plumbing Commission lists relining of sanitary and stormwater drains within the drainage class of plumbing work (Building and Plumbing Commission — Drainage work). It is not work a property owner can carry out themselves.

Clay pipe takes a liner well, because the resin bonds firmly to the clay wall once the line has been cleaned and prepared. The exception is a pipe that has fully collapsed, which a camera inspection identifies clearly.

Is your home in the group most at risk?

The more of these that apply, the more likely it is that the drain rather than the household is the problem:

• The house was built before about 1980 and the drains have never been replaced.
• There are established trees or large shrubs within a few metres of where the drain runs.
• The same drain has blocked more than twice, with a year or two between each one.
• Fittings drain slowly even when nothing is blocked outright.
• There is a strip of lawn over the drain line that stays greener than the grass around it.

None of these confirm a damaged pipe on their own. Together they are a reasonable case for looking inside the line before paying for another clear.

Contact local plumbers about a drain that keeps blocking

A drain that has blocked repeatedly is giving you the same information each time, and the cost of finding out why is lower than the cost of continuing to clear it.

Beyond blocked drains and pipe relining, our team handles hot water, burst pipes, leaking taps and general maintenance for homes across Melbourne’s eastern suburbs. Submit a quote request or call us on 1300 292 731.