Roof catchment, rainfall pattern and intended use — and why a tank plumbed to the house is sized on dry spells, not on averages.
Home › Tanks
Rainwater tanks and pumps
A tank is sized by roof area and what you intend to use it for, not by the space in the yard: and the pump fails long before the tank does.
Roof area first, then use, then space
Tank sizing goes wrong when people start from the gap beside the house. The honest order is: how much water your roof actually collects (roof catchment area against local rainfall), what you are going to use it for (garden only, toilets and laundry, or whole-of-house), and only then what fits.
A tank plumbed to toilets and laundry needs to keep supplying through a dry spell, which usually means either a bigger tank or an automatic mains switch. A garden-only tank can run empty without anyone caring. Those are different tanks.
The first flush diverter is the cheapest water quality device you can fit. The first water off a roof carries the dust, leaf litter and bird droppings that accumulated since the last rain; a diverter dumps that volume before the rest enters the tank. Combined with gutter mesh and a screened inlet, it is most of what keeps a tank clean.
Pumps are the part that fails. A pump that starts and stops repeatedly while a tap runs steadily is short-cycling, usually a failed pressure vessel or a leak on the outlet side. A pump that runs but delivers nothing has usually lost prime or has a blocked foot valve or strainer.
If anyone in the house might drink it, read water quality first. We never call tank water potable on this site: that is a decision for a NATA-accredited laboratory test and your state health guidance, not for a web page.
Tanks guides
Surface or submersible, how to size on flow and head, and why tank pumps short-cycle or burn out.
Submersible or surface, why bore yield matters more than power, and the faults that stop a bore delivering.
Float valves, blocked overflow pipes and where the water should go.
Switch or controller, sizing on flow, and why house pumps cycle or hammer.
Five parts, a simple garden use, and the point where a licensed plumber is needed.
A full tank weighs a tonne per kilolitre. What the base must do and why they fail.
How a mains top-up valve works, why it sticks and where licensed work starts.
How a diverter keeps the dirtiest roof water out of your tank, and what it cannot do.
Signs of sludge, the two ways to clean and why you should never climb in.
What filters, UV and chlorine each do, and why testing comes first.
The path from roof to tap, part by part, and which jobs need a licensed plumber.
What to check every few months, every year and every few years.
The common sizes, what decides the one you need, and how to match gutters.
Find the source of a leak, what you can check and when to call for help.
What lilac pipe marks, where you see it and why it stays separate.
Tank level, prime, power and short-cycling, and where licensed work starts.
How the main tank materials compare and which suits which site.
Whether the downpipe stays full, and what that means for where the tank goes.
What a toilet and laundry connection needs, and why a licensed plumber must do it.
Stop using the water, remove the animal safely, clean the tank and test before drinking.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NHMRC. Australian Drinking Water Guidelines
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)