WHAT SIZE WATER TANK DO YOU ACTUALLY NEED?

Step 1: measure your catchment

Catchment is the roof area that actually drains to the tank, measured as the footprint from above, not the sloped surface area. A 12 metre by 8 metre shed roof is 96 square metres of catchment, whatever its pitch.

Only count the roof sections that are plumbed to the tank. Half a house roof with gutters running the other way is not catchment. If you want more water, connecting another roof is nearly always cheaper than buying a bigger tank.

Step 2: find your rainfall

Use your local annual rainfall in millimetres. The Bureau of Meteorology publishes long term averages by station and it is worth using the nearest station to you rather than a regional figure, because rainfall in WA changes fast with distance from the coast.

As a rough sense of scale: the South West and Great Southern are the wettest parts of the state, the Wheatbelt and Mid West are considerably drier, and the difference between them can be more than double. Do not size a Northam tank using Bunbury rainfall.

Step 3: calculate what you can collect

The formula is simple.

Catchment area in square metres, multiplied by annual rainfall in millimetres, equals litres collected per year at 100 percent efficiency.

Then take off losses. Some rain never reaches the tank: it wets the roof, blows off, or goes out the first flush diverter. Allow around 15 to 20 percent for that, so multiply your result by about 0.8 to get a realistic figure.

Worked example: a 200 square metre roof with 600 mm of annual rainfall collects 200 x 600 = 120,000 litres per year at best, and about 96,000 litres realistically.

Step 4: size the tank to your dry season

This is the step most people skip. The tank does not have to hold a year of water. It has to hold enough to bridge the gap between the end of the wet season and the start of the next one.

Work out your daily use, multiply it by the number of days in your dry stretch, and that is your minimum useful capacity.

Worked example: a household using 400 litres a day across a six month dry season needs about 400 x 180 = 72,000 litres of storage. If the roof above only collects 96,000 litres a year, a 72,000 litre tank is well matched. A 150,000 litre tank would never fill.

That is the balance to aim for: storage a bit under what the roof can supply in a good year, and at least equal to what you use across the dry.

Rules of thumb, and their limits

Some quick sanity checks: - If your tank overflows for weeks every winter and runs dry every February, it is too small for your roof - If your tank has never been more than half full, either the catchment is too small or the tank is oversized - Two smaller tanks can beat one large one, because you can isolate and service one while the other stays in supply - Firefighting supply is usually a separate requirement on top of your household number, and it must actually be available in summer

Stock water is a different calculation again, driven by head count and season rather than by roof area, and dams or bores often carry part of it.

Things that change the answer

Site factors that matter as much as the arithmetic: - Truck access, because a tank you cannot deliver is not an option - Ground conditions and the cost of preparing a level compacted pad - Height and distance from the catchment, which affects the plumbing - Where the overflow can safely discharge - Whether the water is for drinking, which sets the liner grade and the mesh requirements

Send us your roof dimensions and your daily use and we will do the sums with you rather than sell you the biggest tank in the catalogue.