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.

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

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.

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.

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.

For the condensed printable version, head to the main guide page linked below.

→ View the printable A4 guide for rainwater harvesting tank sizing