Sizing a rainwater tank properly
Most tanks are sized by the gap beside the house. The honest order is roof area first, then what you intend to use it for, and only then what fits.
A rainwater tank has two independent constraints, and people usually only think about one. Supply is how much water your roof delivers. Storage is how long you can go between deliveries. A big tank on a small roof stays empty; a small tank on a big roof overflows constantly and runs dry a week into a hot spell.
Step 1: how much does your roof actually collect?
The supply side is roof catchment area multiplied by rainfall. Two things trip people up.
It is the plan area that matters, not the sloped area. A pitched roof collects rain according to its footprint as seen from above.
Only the roof actually plumbed to the tank counts. If half the downpipes go to stormwater, you have half the catchment. This is the single most common reason a tank underperforms against what the owner expected: the tank is sized for the whole house and connected to one side of it.
You lose some to first flush, some to evaporation and wetting of a dry roof, and some to overflow in heavy downpours that arrive faster than the tank can take them. Any sizing that assumes you capture every millimetre is optimistic.
Step 2: what are you going to use it for?
This decides everything about how big the tank has to be, because it decides how much a dry spell hurts.
- Garden only. The tank can run empty without anyone noticing, and demand is naturally lowest when the tank is fullest. Small tanks work fine.
- Toilets and laundry. Demand is steady all year and does not care about the weather. This is where a tank has to carry the household through dry stretches — or have an automatic mains switch that quietly takes over.
- Whole of house, including hot water. Highest demand, and the point where reliability stops being optional. If it is also going to be drunk, read is tank water safe to drink before anything else.
Step 3: size on the dry spell, not the average
Annual rainfall is a misleading number. What matters is how the rain arrives. Two places with the same annual total behave completely differently if one gets it evenly across the year and the other gets most of it in three months.
The practical question is: how many days of your expected demand do you want to be able to cover with no rain at all? Multiply your daily use for the connected fixtures by that number, and you have a storage target that means something. In a summer-dry climate with a tank plumbed to toilets and laundry, that dry-spell figure does far more work than the annual average.
The parts that decide whether it works
- Gutter mesh keeps leaf litter out of the system, and out of the gutters where it blocks and dumps water over the side.
- A first flush diverter discards the initial run-off — the dust, bird droppings and debris that accumulated on the roof since the last rain. It is the cheapest water quality device you can fit, and it has to be emptied or it stops working.
- A screened inlet and a sealed, light-proof tank: screens keep mosquitoes and vermin out, and darkness stops algae. A tank you can see light through will grow things.
- An overflow that goes somewhere sensible. Every tank overflows; the question is whether it discharges to stormwater properly or undermines your footings.
- An air gap or approved backflow prevention wherever mains top-up meets tank water. This is not optional detail — it exists so tank water can never be pushed back into the drinking supply.
The pump is the part that fails
Tanks are largely inert; pumps are not. Two symptoms cover most complaints:
Short-cycling — the pump starts and stops repeatedly while a tap runs steadily. Usually a waterlogged pressure vessel, or a leak on the outlet side letting pressure bleed away.
Runs but delivers little or nothing — usually lost prime, a blocked strainer or foot valve, or the tank being lower than you think.
Pumps also need somewhere to live where their noise is not a problem, and a level, dry base.
Frequently asked questions
Does roof pitch change how much water I collect?
No. Collection is based on the plan area of the roof — its footprint seen from above — not the sloped surface area.
Why does my tank run dry even though it rains plenty?
The usual cause is that only part of the roof is connected. If half the downpipes still go to stormwater, you have half the catchment you assumed. After that, look at first flush, overflow during heavy falls, and how the rain is distributed across the year.
Why does my tank pump keep starting and stopping?
That is short-cycling, and it is usually a waterlogged pressure vessel or a small leak on the outlet side bleeding pressure away. Left alone it wears the pump out.
Do I need a first flush diverter?
It is the cheapest water quality improvement available on a tank, because it discards the dirtiest water off the roof. It only works if it is emptied between falls.
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)