Rainwater Harvesting & Tank Sizing: The Complete Walkthrough

This guide covers rainwater harvesting & tank sizing, rated a moderate task, typically taking 1–2 days setup.

Our printable card on rainwater harvesting & tank sizing covers the essentials at a glance — here's the fuller walkthrough, with the reasoning behind each step.

It's worth understanding why this skill matters before working through the method itself. At a glance, it's rated moderate, with a difficulty of moderate; time of 1–2 days setup; cost of moderate–high — details that matter more once you're actually planning around them than they do on a first read. The rest of this article works through exactly what each of those means day to day, in more depth than a single printable card has room for, and with the reasoning behind each recommendation rather than just the recommendation itself.

How This Applies Across Property Types

This skill applies whether you're working with a suburban block or a larger rural property, though the scale and specific materials will differ. Where climate or regional conditions change the approach, that's called out in the method above rather than assumed — but as a general rule, Australian conditions (heat, water restrictions in many areas, and UV exposure on materials left outdoors) are worth factoring in even where a source guide doesn't explicitly mention them.

Roof to tank to garden

A basic system is simple: roof catchment, gutters, a first-flush diverter to discard the initial dirty runoff, then the tank itself, with an outlet feeding the garden or household use.

Before You Start

Keep these points in mind:

• A tank sized to your roof area and rainfall
• Gutters and downpipes in good condition
• A first-flush diverter
• A leaf/debris screen
• A pump if water needs to be pressurised for use

Each of these compounds over a season — small misses early tend to show up later — a tank sized to your roof area and rainfall A pump if water needs to be pressurised for use Treat this list as a routine to build rather than a one-off checklist, and it becomes second nature within a season or two.

Step by Step

Worth paying attention to:

• 1. Calculate roughly available water: roof area (m²) × rainfall (mm) × 0.8 (accounting for losses) = litres collectable per rain event.
• 2. Size the tank to your intended use — garden-only needs are much smaller than full household backup.
• 3. Install gutter screens to keep leaves and debris out.
• 4. Fit a first-flush diverter so the initial dirty runoff from the roof doesn't enter the tank.
• 5. Connect the tank outlet to garden taps directly (gravity) or via a pump for pressurised use.

Getting these details right matters more than it might seem — 1. Calculate roughly available water: roof area (m²) × rainfall (mm) × 0.8 (accounting for losses) = litres collectable per rain event. 5. Connect the tank outlet to garden taps directly (gravity) or via a pump for pressurised use. Treat this list as a routine to build rather than a one-off checklist, and it becomes second nature within a season or two.

When Things Go Wrong

Keep these points in mind:

• Tank empties faster than expected — check for leaks, or recalculate actual usage against catchment.
• Poor water quality — check gutter screens and first-flush diverter are working, and that the roof material is suitable for water collection.
• Mosquitoes breeding in the tank — ensure all inlets and the tank itself are properly screened and sealed.
• Low pressure at outlets — a gravity-fed system depends on tank height; a small pump solves this for most household uses.

Each of these compounds over a season — small misses early tend to show up later — tank empties faster than expected — check for leaks, or recalculate actual usage against catchment. Low pressure at outlets — a gravity-fed system depends on tank height; a small pump solves this for most household uses. Treat this list as a routine to build rather than a one-off checklist, and it becomes second nature within a season or two.

Timing and Maintenance

Best time. Install before the wettest season in your region so the tank can fill from the start.

Duration. A basic setup can be done in a weekend; larger systems with pumps and plumbing take longer.

Maintenance. Clean gutter screens and first-flush diverters a few times a year; check the tank for sediment periodically.

Signs of success. Consistent water availability through dry spells, without running the tank dry between rain events.

It's worth reading these as a set rather than in isolation; they reinforce each other.

Tips & variations

A few notes worth having alongside the method above: Drip Irrigation Setup, Greywater Systems, Undersizing the tank for the intended use, Skipping the first-flush diverter, leading to poor water quality. These come from the same printable card and are easy to miss on a first read.

Related Guides

A few related guides worth reading alongside this one:

• Drip Irrigation Setup
• Greywater Systems

For the condensed printable version, head to the main guide page linked below. If something's already gone wrong, our Troubleshooting Directory indexes common problems by symptom across all 219 guides. As with every guide on this site, this is general information based on our own research and experience, not professional advice — your specific soil, climate, and circumstances will always vary from ours to some degree.

Our Rainwater Harvesting & Catchment Calculator is a useful companion to this specific skill.

Frequently Asked Questions

→ View the printable A4 guide for rainwater harvesting tank sizing