Repairs: drainage

The basics

Normal minimum drain diameters are:

  • 100mm for maximum 10 dwellings; and
  • 150mm for more than 10 residential units.

Clay pipes

Older drainage systems (Victorian or thereabouts) are most likely to comprise salt-glazed clayware pipes of 100mm, 150mm or 215mm nominal bore diameters (i.e. 4 inch, 6 inch or 9 inch).

In many respects these pipes have stood the test of time but salt-glazed pipework suffers from several problems. For example, if not carefully bedded in a pea shingle or similar, the pipes are prone to moving out of alignment and the joints gradually open up with age so that they no longer remain watertight.

This can give rise to a number of problems:

  • Paper and solids flushing down the drain may get stuck on the angled edges at the pipe sockets where the pipes are no longer nicely aligned. This means a head of water is built up, and this increased pressure can cause more significant failure to joints elsewhere or to pipe connections into chambers and so on.
  • Water seeping through defective joints may cause localised increased wetting to soil around foundations. This can be a particular problem in clay soils where volume changes lead to heave/shrink and subsidence or settlement in the property, or where the fine material in the soil can be washed out causing settlement to occur.
  • If the pipes are not at a suitable depth below ground level, the pipes themselves can become prone to fracturing and breakdown due to vehicle traffic from driveways that were never intended for the volume or weight of modern vehicles.

Cast iron pipework

Sometimes cast iron pipework is used underground. This has considerable strength, but over a period of time can rust. The rust expanding into the pipework can cause blockages, thinning of the pipe and eventual collapse/failure.

Pitch fibre pipework

Pitch fibre (bitumen-impregnated asbestos cement) pipework was commonly used in the 1940s, 1950s and 1960s. The pipework was cheap but does not seem to have performed well or stood the test of time. The pipes are prone to deforming and collapse.