Emergency Rainwater Collection & Use

When municipal water supply fails, rainwater can become a critical supplementary or primary source. Unlike streams or groundwater, rain begins as relatively clean atmospheric water — though what it picks up on the way down and on its path to your collection container determines whether it is safe to use. This guide covers how to set up rainwater collection quickly, how to treat it for different uses, and the important limits of rainwater as an emergency water source.

Setting Up Rainwater Collection Quickly

You do not need an elaborate permanent system to collect significant amounts of rainwater. Here are approaches from simplest to most capable:

Open Container Collection

The most basic method: place clean buckets, pots, rubbish bins, or any large clean container outdoors in open rain. This catches rain directly from the sky without any roof contamination.

• Simple, immediate, no setup required.
• Yields are limited by container size.
• Covered partially (but not sealed) to prevent debris while allowing rain in.
• Must be covered after collection to prevent mosquito breeding.

Tarp or Sheet Collection

A large tarp, plastic sheeting, or even a clean canvas sheet stretched between poles collects rain over a wider area and channels it to a single container.

1. Suspend the tarp at a slight angle so water runs toward one corner.
2. Create a channelling point — a funnel of tarp material — at the low corner.
3. Place a clean collection container under the drain point.
4. Secure against wind.

A 3m × 3m (9 square metre) tarp during a 25mm (1 inch) rain event collects approximately 200 litres — potentially several days' water supply for a family.

Roof and Gutter Collection

Gutters and downpipes already exist on most homes and channel rainwater efficiently. Redirecting a downpipe into a storage container is the most productive quick setup.

1. Use a downpipe diverter (available at garden centres — a T-connector that diverts flow to a hose or container) or temporarily redirect the downpipe manually.
2. Place storage containers — water butts, rubbish bins with lids, large buckets — under the diverted downpipe.
3. Implement first-flush diversion (see below).

Roof Material Considerations

Not all roof materials are equal for rainwater collection. Some introduce contaminants:

Roof Material	Suitability for Drinking Water Collection	Concern
Glazed clay or ceramic tiles	Good	Minimal contamination
Plain concrete tiles	Acceptable	Slight pH elevation; sediment possible
Galvanised steel	Caution	Zinc leaching, especially new or rusty
Colorbond / painted steel	Generally good	Avoid if paint is peeling or unknown composition
Asphalt shingles	Caution	Hydrocarbon compounds, especially new shingles
Bitumen / felt	Poor	Hydrocarbons; use for non-potable only
Copper gutters	Poor	Copper leaching into water
Lead flashing	Poor	Lead contamination; do not use this collection
Green / living roof	Poor	Biological contamination from soil and plants

For emergency collection where roof type is unknown or poor, use open-area tarp collection to bypass the roof entirely.

The First-Flush Diverter

The first water to run off a roof during any rain event carries the highest concentration of contaminants — accumulated bird droppings, dust, particulates, insect debris, atmospheric pollutants, and anything deposited on the roof since the last rain.

A first-flush diverter captures and discards the first volume of water (typically 1 litre per 10 square metres of roof area) and diverts the subsequent cleaner flow to the storage container.

How to improvise a first-flush diverter:

1. Place a small container (a 1-litre bottle or similar) at the start of your collection path.
2. Allow it to fill with the first flush water — this diverts the dirtiest water away.
3. Only connect the main collection container once the initial dirty flow has cleared.
4. After rain, empty the diverter container and reset for the next event.

This single practice significantly improves the quality of collected rainwater.

Storage Containers

Proper storage is critical — improperly stored rainwater can become more contaminated than when it was collected.

Container requirements:

• Food-grade material — HDPE plastic (marked with recycling symbol #2), food-grade stainless steel, food-grade fiberglass
• Covered — sealed or tight-fitting lid to prevent mosquito breeding, debris, and evaporation
• Opaque or dark coloured — prevents algae growth (light promotes algae)
• Clean — washed with a dilute bleach solution before first use; rinse thoroughly
• Not previously used for chemicals — never use containers that held petrol, pesticides, or cleaning chemicals

Container sizing guide:

Family Size	Minimum Storage Target (2 weeks)	Recommended Container
1–2 people	150–200 litres	200L water butt or 4× 50L containers
3–4 people	280–350 litres	350L IBC tote or 2× 200L barrels
5–6 people	420–500 litres	500L IBC tote or multiple barrels

Yield Calculation

To estimate how much water you can collect:

Formula: Collection area (m²) × rainfall depth (mm) × 0.8 (efficiency factor) = litres collected

Example: 50 m² of roof, 20mm of rain → 50 × 20 × 0.8 = 800 litres

Imperial formula: Roof area (sq ft) × rainfall (inches) × 0.623 = US gallons

Example: 100 sq ft of roof, 1 inch of rain → 100 × 1 × 0.623 = 62 gallons

The 0.8 factor accounts for losses from first-flush diversion, evaporation, and collection inefficiency.

Treatment Before Drinking

Raw collected rainwater is not safe to drink without treatment, regardless of how clean it appears. Treatment requirements depend on use:

For Drinking and Food Preparation

Apply a multi-step approach: filter, then disinfect.

Step 1 — Sediment Filtration: Pass water through a clean cloth, coffee filter, or commercial sediment filter to remove particles. This protects the disinfection step from interference.

Step 2 — Disinfection options:

Method	Dose / Procedure	Contact Time	Effectiveness
Boiling	Full rolling boil for 1 minute	Immediate	Excellent (all biological)
Household bleach (5–8% sodium hypochlorite)	8 drops per litre; 16 if cloudy	30 minutes	Good (bacteria, viruses)
Chlorine dioxide tablets	1 tablet per litre	4 hours	Excellent including Cryptosporidium
UV purifier (Steripen)	Per device instructions	60–90 seconds	Excellent (all biological)
Gravity filter (Berkey, Sawyer)	Pass through filter	Per flow rate	Excellent if filter maintained

For extended use, chlorine dioxide tablets or a gravity filter system are the most practical.

For Sanitation and Hygiene (Non-Drinking)

Untreated collected rainwater is generally acceptable for:

• Toilet flushing
• Laundry (if clothes are being washed in a water body anyway)
• Outdoor plant watering
• Washing hands if followed by alcohol hand sanitiser

For body washing, hair washing, and wound cleaning, at least a basic level of disinfection is recommended.

Cooking vs Drinking vs Irrigation

Use	Treatment Required
Drinking	Full treatment: filter + disinfect
Cooking (food in contact with water)	Full treatment
Washing produce (will be eaten raw)	Full treatment
Washing produce (will be cooked)	Basic — boiling the food will treat
Body washing / bathing	Filtered; disinfected preferred
Washing dishes	Filtered; final rinse with treated water
Toilet flushing	None
Garden irrigation	None

Legal Status of Rainwater Collection

Laws on rainwater collection vary significantly:

Region	Legal Status
Most of UK	Legal; encouraged; no permit needed for domestic use
Most EU countries	Legal for domestic and garden use
Most Australian states	Legal; some states offer rebates for tanks
Most US states	Legal and increasing encouragement
Colorado (USA)	Historically restricted; now allows limited collection
Utah (USA)	Permit required for large systems

In virtually all jurisdictions, small-scale emergency collection from your own roof is legal. Check local rules for permanent tank installation.

Contamination After Industrial Accidents or Wildfire Smoke

Two scenarios significantly reduce rainwater safety:

After wildfire smoke: Rainwater falling through wildfire smoke absorbs particulates, heavy metals (from burning structures), and volatile organic compounds (VOCs). This water should be used for non-potable purposes only until air quality returns to normal. The risk is highest in the first 1–3 rain events after a major wildfire in your region.

After industrial accident or chemical spill: Airborne chemical contamination (from factory fire, chemical plant accident, or atmospheric release) can deposit onto collection surfaces. If you are aware of any nearby industrial incident, suspend potable rainwater collection until authorities confirm air quality is safe.

⚠️ Rainwater collected during or immediately after an industrial accident, large fire, or wildfire event may contain toxic chemical contaminants that standard purification methods cannot remove. Err on the side of caution and use stored or bottled water until the all-clear is given.

Quick Reference

Question	Answer
How much rain needed for drinking water for 4 people for a day?	~5 litres from approximately 0.625 m² at 1cm rain depth
Can I drink collected rain without treatment?	No — always filter and disinfect for drinking
Is roof-collected water safer than stream water?	Generally yes, except with contaminated roof materials
Does boiling work for rainwater?	Yes for biological contamination — not for chemical
What container can I use for storage?	Food-grade, opaque, covered — no previous chemical use
Is rainwater legal to collect?	Legal in most countries for domestic emergency use
Wildfire smoke in area — is rainwater safe?	Use for non-potable only until air quality normalises
First rainfall after a dry period — more or less contaminated?	More — first flush carries accumulated roof deposits

Rainwater collection bridges the gap between your stored water running out and normal supply being restored. Even modest collection capability — a tarp, some containers, and purification tablets — can provide the critical difference between adequate hydration and a water crisis during an extended emergency.