After a solid downpour, it’s natural to check your tank and expect it to be full. But if the level hasn't changed much — or at all — it can be confusing and a little stressful.
Many homeowners jump to the conclusion that something must be wrong with their rainwater harvesting setup. But in most cases, there’s no fault in the system at all. The issue comes down to a mismatch between rainfall expectations and reality.
Rain doesn’t always mean rapid refill
Even when it feels like a lot of rain, it might not be enough to noticeably boost your tank levels. Here’s why:
• The rain might have been brief or light overall
• Your roof collection area may be smaller than you realise
• A portion of the rainfall is lost in the first flush (it clears dirt and debris before entering your tank)
• Gutters and filters could be blocked or slow-flowing
So while it might feel like "that was a big storm", in terms of how much actually makes it into your tank, the numbers often tell a different story.
A quick calculation: what your roof can actually collect
Here’s a simple formula to estimate how much water you should collect during a rain event:
1mm of rain on 1m² of roof = 1 litre of water
So if you have a 150m² roof and it rains 10mm, you could harvest up to 1,500 litres— if everything is clean, flowing, and connected properly.
That might sound like a lot, but if you’ve got a 25,000-litre tank, it’s only a 6% increase in volume. It’s completely normal not to see a dramatic change unless the rainfall is sustained over time.
Don’t overlook the basics: clean gutters and filters
One of the most common reasons for low water collection isn’t system failure—it’s blocked or dirty components.
A few things to check regularly:
• Are your gutters full of leaves or moss?
• Is your first-flush diverter working and emptying properly?
• Are your filters or screens clogged with grime?
• Have any downpipes come loose?
If water can’t flow freely from your roof to your tank, your harvesting efficiency drops quickly — no matter how much rain you get.
When to be concerned
If you’ve had multiple decent rainfalls and your tank still seems unusually low, it may be time to investigate further.
Watch for:
• Disconnected or cracked pipework
• Blocked filters you’ve missed
• An undersized roof area relative to your household demand
Even then, many issues are minor and easy to fix once identified.
Let’s take a look if you're not sure
We're always happy to inspect a system and give honest, straightforward feedback. Sometimes it just takes a fresh set of eyes (and a quick check) to confirm everything’s working exactly as it should.
Book a system check now.
Final thought: prep before the next storm
The best time to check your setup isn’t after the rain — it’s before. Make sure your gutters are clean, your filters are clear, and your expectations are based on your actual roof and rainfall figures.
