Wet and dry rainwater tank systems
The difference is whether the downpipe stays full of water between storms.
A wet system keeps its downpipes full of water. A dry system lets them drain empty after rain. If your tank sits beside the downpipe, choose dry. If it sits away from the house, you are looking at a wet (charged) system.
What a dry system is
In a dry system, each downpipe runs straight into the tank, or into a short pipe that falls all the way to it. Nothing holds water once the rain stops. The pipe empties and stays empty.
It is the simplest layout. Fewer parts hold water, so there is less to go wrong.
What a wet system is
A wet system is also called a charged system. The downpipes drop below ground and run along to a tank that can stand some distance from the house. The pipe stays full between storms, and rain from one downpipe pushes water along to the tank.
That lets you collect from several downpipes with one tank. It also lets you put the tank where it fits, not where the downpipe happens to be.
Wet and dry side by side
| Dry system | Wet (charged) system | |
|---|---|---|
| Downpipe | Drains empty after rain | Stays full of water |
| Tank position | Beside the downpipe | Can be away from the house |
| Downpipes served | Usually one tank per downpipe | Several downpipes into one tank |
| Main risk | Limited by where the tank fits | Standing water, leaks you cannot see |
The catch with wet systems
Water that sits in a pipe goes stale. If the pipe is open anywhere, it can breed mosquitoes. Leaves and grit settle in the low points too. You have to seal every joint, and you need a way to clear the system out.
The pipe also has to cope with being full. A weak joint that is fine in a dry system can weep in a wet one. Before you commit, read how a tank is plumbed from roof to house.
Which one is worth it
Dry is the better choice when the tank fits next to a downpipe. It is cheaper to build and easier to keep clean. A wet system earns its place when the site leaves you no other option, or when you want one big tank instead of several small ones.
Either way, the tank still needs a screened inlet, a first flush diverter and a proper overflow. A wet system also changes how the tank fills, so check sizing against the roof area you actually connect.
How to decide between a wet and a dry system
Start with the site. The layout follows from where the tank can go.
- Find the downpipes you want to collect from. One downpipe usually means a dry system with a tank beside it.
- Check where the tank fits. If it fits beside the downpipe, choose dry.
- Count the downpipes. If you want several feeding one tank, a wet system is the way to do it.
- Look at the ground. A wet system needs underground pipes laid to the tank.
- Think about upkeep. Dry is cheaper to build and easier to keep clean.
- Ask a licensed plumber to confirm. Laying underground stormwater pipework is licensed work.
Looking after a wet system
A wet system holds water in the pipe between storms, so it needs a little more care than a dry one.
| Risk | Why it happens | What to do |
|---|---|---|
| Mosquitoes | Standing water in an open pipe | Seal every joint and screen all openings |
| Stale water | Water sits between storms | Make sure you have a way to clear the system out |
| Silt and leaves | They settle in the low points | Fit a first flush diverter and keep the screens clear |
| Weeping joints | A weak joint is fine in a dry system but can weep when full | Have a licensed plumber check the joints |
| Hidden leaks | The pipes are underground | Watch the tank level over a few dry days |
Whatever layout you choose, the tank still needs a proper overflow.
Frequently asked questions
What is a wet system on a rainwater tank?
A wet system, also called a charged system, keeps its downpipes full between storms. It lets the tank sit away from the house and collect from several downpipes.
What is a dry rainwater tank system?
A dry system sends each downpipe straight into the tank, so the pipe drains empty once the rain stops. It is the simpler layout.
Is a wet system better than a dry one?
Not by default. Dry is simpler and easier to keep clean. A wet system is worth it when the tank cannot sit beside a downpipe.
Do wet systems attract mosquitoes?
They can, because water sits in the pipe. Sealed joints and screened openings are what keep mosquitoes out.
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)