Preparing a base for a water tank
Water weighs a tonne per kilolitre. The base has to carry that, evenly, for decades.
A tank base has one job: carry a very heavy load evenly and keep it level. One litre of water weighs about one kilogram, so a 5,000 litre tank holds around five tonnes of water. A base that is slightly soft, sloped or lumpy will not hold that for long.
Why a poor base ruins a good tank
Poly and steel tanks are designed to sit on a flat, firm surface. Put one on a lump or a slope and the wall carries load it was never meant to. The result is a bulging wall, a cracked fitting or a seam that weeps.
Fittings at the base suffer first. The outlet and any bulkhead connection take the strain when the tank settles, so a leak at the outlet is often a base problem in disguise.
What a good base looks like
The surface must be level, firm and bigger than the tank footprint. Most manufacturers ask for a compacted pad or a concrete slab, and they publish their own requirement for each model. Follow the one that comes with your tank, because it affects the warranty.
- Level. Check it in both directions, not just one.
- Firm. Topsoil and fill settle. Natural, undisturbed ground compacts better.
- Clean. No sharp stones or stubble under a poly tank.
- Drained. Water must not pool or wash the pad out from underneath.
| Base type | Suits | Watch for |
|---|---|---|
| Compacted sand or crushed rock pad | Smaller poly tanks, where the maker allows it | Washout in heavy rain, and edges that crumble |
| Concrete slab | Larger tanks, soft or reactive ground | Needs correct thickness and edge support for the load |
| Raised stand or frame | Gravity-fed supply, or small elevated tanks | A full stand is a structural job, not a shed project |
Reactive clay and sloping sites need more thought
On reactive clay the ground moves with moisture, and a thin pad moves with it. On a slope, cutting into the hill and building out over fill is the classic way to lose a tank. If your site is either of these, speak to the tank supplier or an engineer before pouring anything.
Keep the area around the tank clear
Leave room to reach the inlet, the overflow and the outlet. Think about where the overflow water goes too. An overflow that runs straight onto the base is how a pad gets undermined.
A base checklist before delivery day
Do these checks in order before the tank arrives. A mistake caught now is cheap, and one caught after filling is not.
- Read the maker's base requirement for your model. It affects the warranty, so follow it over general advice.
- Check the ground. Natural, undisturbed ground compacts better than topsoil and fill, which settle.
- Check the site type. If it is reactive clay or a slope, speak to the tank supplier or an engineer before pouring anything.
- Measure the pad. It must be bigger than the tank footprint and level in both directions, not just one.
- Clear the surface. No sharp stones or stubble under a poly tank.
- Plan the drainage. Water must not pool or wash the pad out from underneath, so decide where the overflow goes.
- Leave room to work. You need to reach the inlet, the overflow and the outlet.
Large or elevated tanks may need council approval. See how to size a rainwater tank before you finalise the pad size.
Signs your tank base is failing
Many tank faults start at the base. This table links what you can see to the likely cause.
| What you see | Likely base cause | What to do |
|---|---|---|
| Bulging wall | The wall is carrying load on a lump or a slope | Contact the tank supplier |
| Seam that weeps | Uneven support has stressed the join | Contact the supplier, and do not repair it yourself |
| Leak at the outlet | The outlet takes the strain when the tank settles | Check the base before replacing the fitting |
| Cracked fitting | The tank has moved or settled | Licensed plumber, after the base is checked |
| Pad edges crumbling or washed out | Water is running under or over the pad | Fix the drainage and where the overflow goes |
| Tank looks tilted | Soft or uneven ground | Speak to the supplier or an engineer |
A leak at the outlet is often a base problem in disguise. Our guide to a leaking tank covers the other causes.
Frequently asked questions
What should a water tank sit on?
A level, firm surface larger than the tank footprint, usually a compacted pad or a concrete slab. Follow the requirement the manufacturer publishes for your model.
How much does a full water tank weigh?
About one tonne per thousand litres, so a 5,000 litre tank holds around five tonnes of water before you count the tank itself.
Can I put a rainwater tank straight on the ground?
Not safely on bare ground. Soft, sloped or stony ground can distort the tank wall and may void the warranty.
Do I need approval to install a tank base?
It depends on your council and the tank size. Check with your local council and your state or territory regulator before you build.
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)