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What is a Soil Pipe?

Paul Scott
Paul Scott

A soil pipe is a PVC or cast iron pipe used in plumbing installations to remove soiled or contaminated water from toilets. Soil pipes differ from normal waste water removal pipes in that they are generally of a larger diameter and are designed specifically to remove solid waste from toilets. Most plumbing installations consist of a combination of soil and waste water lines and each has its own particular characteristics and requirements. Most of these involve pipe sizes, fittings, and routing constraints specific to each type although they both eventually vent into a common main sewerage line.

The nature of soil water waste makes its removal from any installation in a safe and completely contained fashion technically tricky but critical from a health and safety viewpoint. Soil water typically contains large quantities of paper and fecal solids which are introduced into the waste removal system with fairly large amounts of flush water. Normal waste water from hand basins, baths, and kitchen basins contain nowhere near these amounts of solids and are not subjected to rapid water flow increases.

PVC pipes are often used as soil pipes.
PVC pipes are often used as soil pipes.

Soil water also requires that a soil pipe be substantially larger than a regular waste water pipe to accommodate the larger volumes. The average inside diameter of a domestic soil pipe is around four inches (about 10 cm) and that of a typical waste water pipe is 1½ inches (about 3.8 cm). Soil pipe system fittings also need to offer as few internal obstructions or snags as possible to prevent lint and paper build-up from blocking the pipes. Due to their size, soil pipe routings can present problems when toilets are not placed in rooms bordering on the outside walls of the building. Toilets in interior rooms may need specialist soil pipe assemblies that make use of smaller pipes and specially designed flush systems.

Soil pipes are used to remove soiled or contaminated water from toilets.
Soil pipes are used to remove soiled or contaminated water from toilets.

Soil pipe layouts also need to adhere to strict installation parameters such as adequate venting, correct falls, or inclinations of the pipes and the installation of traps to prevent sewer gas from entering the building. In addition, pipe and joint materials should be of the correct grades. Soil water stacks that accept several soil pipes also need to be correctly designed and installed to prevent the soil water from backing up and flooding the toilets. The manner in which waste water and soil pipes enter manifolds for introduction into sewer lines is also subject to strict design restrictions for the same reason.

Discussion Comments


If you're building your own house, this is definitely not something you want to skimp on. In fact, I wouldn't touch the drainage pipes at all, I would get a contractor in to make sure it was all done properly.

There's nothing worse than having a problem with this system and needing to fix it after the fact. Better to spend a little bit more on it in the first place and get that peace of mind.


@bythewell - In theory that can work but I'm not convinced it's any better than connecting to a general system with soil drain pipes. I've visited the local sewerage processing facility with a class and they are able to purify the waste almost completely using bacteria and dedicated machinery.

If you were doing this at home it would be quite expensive and you would never be completely sure of getting it right which can be dangerous.

I do think that our sewerage system isn't perfect, but it's not terrible either.


I'm more of a fan of houses installing one of those systems that will process this kind of waste so that it becomes harmless and can be used in the garden.

I've seen designs that will even take things like soiled disposable diapers and gradually break them down until the only thing that remains of the diaper is the plastic and the rest can be used as compost.

I mean, it's just not natural and it's not good for anyone to funnel all our waste into one place and, more often than not, contaminate water supplies with it.

Plus, if you do everything on your own property there's no need for specialized sewer pipes.

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    • PVC pipes are often used as soil pipes.
      By: thejimcox
      PVC pipes are often used as soil pipes.
    • Soil pipes are used to remove soiled or contaminated water from toilets.
      By: afxhome
      Soil pipes are used to remove soiled or contaminated water from toilets.