Understanding plumbing when planning a new home or renovation.

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Plumbing systems consist of the water pipes which connect from the water supply (usually the town system or utility provider, or from your own source of water such as rainwater tanks or bore) to where it’s required at taps and toilets.

Pressure forces water along the pipes – either pressure from the town system, pressure created by a pump from your bore or tank, or your own raised tank. Water pipes must be able to withstand the pressure of the supply system. Water pipes are available in different grades to withstand various pressures. They are usually 12 to 25 millimetres (half to one inch) in diameter and made from various materials including copper, galvanised steel and various plastics. Because water is forced along the pipes under pressure water pipes don’t have to be installed at a specific level and can go up or down.
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The amount of water you get out of a tap or fitting in your house will depend on:

• The pressure in the town system. If your property is at the top of a hill you will have a lower water pressure than properties at the bottom of the hill. In addition the water pressure of town systems could vary during the course of the day depending on how many people are using water. So early in the day when everyone is using their bathrooms the water pressure could be lower than at other times.
• The size of the pipes feeding the fitting. Long pipes with a small diameter cause extra friction which reduces the flow.
• How many other fittings are drawing water in your house at the same time. I’m sure we’ve all experienced being in the shower and the water has suddenly got warmer when someone flushed the toilet taking the cold water.
• The actual fitting. Some fittings are designed to limit the amount of water exiting from the tap. In fact in some water poor regions it’s law that you must only fit low flow fittings. This can be really irritating if you’re trying to fill a bathtub with water.
• The water pressure will be less in the upper floors of a building. This might not be noticeable for a two storey house unless your area experiences very low water pressure.

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The water supply pipes must have at least one shutoff valve so that the water can be turned off when repairs are required or there’s a leak. But it’s good practice to install valves within the house, ensuring they’re easily accessible. In more than one instance we’ve had pipes burst flooding the house, and the sooner you can cut the water supply the less water will flood your house.

The other component of the plumbing system are the wastewater pipes (sewer pipes) which take wastewater to the town system or your septic tank. These are gravity pipes and must always flow downhill. Drainpipes vary from 40 to 100 millimetres (one and half to four inches) in diameter. Each waste outlet should have a trap, which is a U-shaped pipe which permanently holds water, preventing smelly sewer gases from entering the room. Waste pipes need air to allow the wastewater to move otherwise an air-lock will be created. At the higher end of the system a vent pipe connects to the atmosphere. Since this will release smelly air these vents are usually placed above the roof, although a non- return valve can be fitted which only allows air to enter, while preventing air from escaping.

It’s always advisable to use good quality plumbing pipes and fittings. There have been several cases of imported tap fittings leaching lead into the water supply. Of course lead is poisonous. Also I’ve had flexible hoses feeding taps suddenly burst because they were substandard, flooding the house.
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Generally there are codes which govern plumbing materials, and plumbing design and installation. 

This is an extract from 'An Introduction to Building and Renovating Houses: Volume 1' from the chapter 'Understanding Building Processes' written by Paul Netscher.
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Paul Netscher has written 2 easy to read books 'An Introduction to Building and Renovating Houses - Volumes 1 and 2'. An Introduction to Building and Renovating Houses Volume 1 deals with Hiring Contractors, Managing Construction and Finishing Your Home.  and Designing your ideal home Volume 2 deals with Finding Your Ideal Property and Designing Your Dream Home. 
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