A rainwater catchment system collects precipitation from roof surfaces, filters it, and stores it in tanks or cisterns for later use. In British Columbia, where rain falls heavily from October through March but barely at all from July through September, the sizing question is straightforward: how big does the tank need to be to bridge the dry season? The answer depends on three numbers -- your roof area, your annual rainfall, and your daily water demand.
This guide walks through the math, gives you the formulas, and provides reference tables for every major rainfall zone in BC. After 10 years of designing water systems on Vancouver Island, I have sized hundreds of these. The formula is simple. Getting the inputs right is what matters.
The Core Formula for Rainwater Catchment Sizing
Annual harvestable rainwater from a roof is calculated as:
Annual Harvest (litres) = Roof Area (m²) × Annual Rainfall (mm) × Runoff Coefficient
The runoff coefficient accounts for water lost to evaporation, splash, roof absorption, and first-flush diversion. For most roofs, use 0.85 as a conservative planning figure. Metal roofs perform better (0.95); cedar shakes perform worse (0.75).
Runoff Coefficients by Roof Type
| Roof Material | Runoff Coefficient | Notes |
|---|---|---|
| Metal (steel, aluminum) | 0.95 | Best performer -- smooth, non-absorbent |
| Concrete/clay tile | 0.90 | Good -- minor absorption |
| Asphalt shingle | 0.85 | Standard -- some absorption and granule loss |
| Cedar shake | 0.75 | Absorbs moisture, slower release |
| Green/living roof | 0.10-0.30 | Not practical for catchment -- absorbs most rainfall |
If you are building new and plan to use rainwater, metal roofing is the clear choice. It captures more water per square metre, sheds faster in light events, and does not leach chemicals into your supply.
Annual Rainfall by BC Region
BC's rainfall varies dramatically by location. These are annual totals from Environment Canada station data:
| Region | Annual Rainfall (mm) | Dry Season (approx. days) |
|---|---|---|
| Campbell River / North Island | 1,600 | 75 |
| Courtenay / Comox | 1,400 | 80 |
| Nanaimo / Central Island | 1,100 | 90 |
| Victoria / South Island | 900 | 100 |
| Vancouver / Lower Mainland | 1,200 | 70 |
| Interior BC (Kamloops area) | 400 | 120 |
Notice the pattern: the wetter areas (Campbell River, Courtenay) have shorter dry seasons but more total rainfall to harvest. The drier areas (Victoria, Interior) have longer dry seasons and less rainfall to capture -- you need proportionally larger tanks relative to roof area.
How Much Water Can Your Roof Capture?
Here is a quick reference table for a metal roof (coefficient 0.95) at common BC roof sizes:
| Roof (m²) | Campbell River (1,600 mm) | Nanaimo (1,100 mm) | Victoria (900 mm) |
|---|---|---|---|
| 50 | 76,000 L | 52,250 L | 42,750 L |
| 100 | 152,000 L | 104,500 L | 85,500 L |
| 150 | 228,000 L | 156,750 L | 128,250 L |
| 200 | 304,000 L | 209,000 L | 171,000 L |
| 300 | 456,000 L | 313,500 L | 256,500 L |
A 100 m² metal roof in Campbell River captures over 150,000 litres per year. That is a lot of water. The question is never "is there enough rain?" -- it is "can I store enough to last through summer?"
Per-Event Capture
For sizing gutters and first-flush diverters, it helps to know how much water arrives per rain event:
Litres per event = Roof Area (m²) × Rainfall (mm) × Coefficient
A 100 m² roof in a 25 mm rain event (common on the North Island) delivers: 100 × 25 × 0.95 = 2,375 litres in a few hours. Your gutters, downspouts, and first-flush diverter need to handle that flow rate.
Surface Catchment (Beyond the Roof)
Roofs are not the only catchment surface. Driveways, compacted areas, and sloped land all generate runoff that can be directed to storage. The coefficients are lower because surfaces are rougher and more porous:
| Surface | Runoff Coefficient |
|---|---|
| Asphalt/concrete | 0.90 |
| Compacted gravel road | 0.70 |
| Bare compacted soil | 0.60 |
| Grazed pasture (clay soil) | 0.45 |
| Grazed pasture (loam soil) | 0.35 |
| Forest / dense groundcover | 0.10-0.20 |
Surface runoff is typically directed to ponds or swales rather than tanks, because it carries sediment and organic matter that would foul a cistern. Roof water goes to tanks. Ground water goes to earthworks. Keep them separate in your design.
Cistern Sizing: Bridging the Dry Season
The cistern (or tank) must hold enough water to cover your daily demand through the entire dry season. The formula:
Minimum Tank Volume (litres) = Daily Demand (L/day) × Dry Season Length (days)
Daily Demand Reference
| Use | Daily Demand |
|---|---|
| Household (per person, all uses) | 150-250 L/day |
| Garden drip irrigation (per 100 m²) | 50-100 L/day |
| Cattle (per head) | 50-80 L/day |
| Horses (per head) | 30-50 L/day |
| Sheep/goats (per head) | 8-12 L/day |
| Chickens (per head) | 0.5 L/day |
| Fire suppression reserve (rural) | 45,000 L minimum (BC Fire Code) |
Sizing Example: Family of Four, Campbell River Area
Household: 4 people × 175 L/day = 700 L/day
Garden (200 m²): 100 L/day
Dry season: 75 days
Minimum tank: (700 + 100) × 75 = 60,000 litres
Add a 50% safety margin (accounts for low-rainfall years and unexpected demand): 60,000 × 1.5 = 90,000 litres recommended
Round to standard tank sizes: two 50,000 L tanks, or one 50,000 L + two 25,000 L tanks.
Sizing Example: Small Farm, Nanaimo Area
Household: 2 people × 175 L/day = 350 L/day
Garden (500 m²): 375 L/day
Livestock (4 goats, 20 chickens): (4 × 10) + (20 × 0.5) = 50 L/day
Dry season: 90 days
Minimum tank: (350 + 375 + 50) × 90 = 69,750 litres
With safety margin: 105,000 litres recommended
This farm also needs a fire reserve of 45,000 litres on top of that, bringing total tank capacity to 150,000 litres. At this scale, a pond starts making more financial sense than tanks.
Standard Tank Sizes Available in BC
| Size (litres) | Typical Use |
|---|---|
| 1,000 | Small garden supplement |
| 2,500 | Greenhouse or small garden |
| 5,000 | Garden irrigation backup |
| 10,000 | Large garden or partial household |
| 20,000 | Small household supplement |
| 25,000 | Full household (short dry season) |
| 50,000 | Full household (longer dry season) |
Polyethylene (poly) tanks are the standard on Vancouver Island. They come in black or green, are UV-stabilized, and last 20-25 years. Concrete cisterns last longer but cost more and require crane placement. Underground fibreglass cisterns work where above-ground tanks are not practical.
Tank vs. Pond: When Does a Pond Make More Sense?
Once your storage requirement exceeds about 50,000-75,000 litres, run the numbers on a pond. A 100 m³ (100,000 litre) pond costs less per litre of storage than the equivalent tank capacity, especially if you have suitable clay soil on site.
| Factor | Tanks | Pond |
|---|---|---|
| Best for | Potable supply, small volumes | Irrigation, livestock, fire, large volumes |
| Water quality | Clean (filtered roof water) | Variable (surface runoff, sediment) |
| Regulatory | No water licence needed (roof collection) | Water licence typically required |
| Cost per litre (approx.) | $0.50-$1.50/L capacity | $0.05-$0.30/L capacity |
| Lifespan | 20-25 years (poly) | 50+ years (earthen) |
| Evaporation loss | None (enclosed) | Significant in summer |
Many properties benefit from both: tanks for potable household supply, a pond for irrigation and fire reserve. See our pond building guide for full details on siting and sizing a pond.
System Components
Gutters and Downspouts
Size gutters for peak flow. A 100 m² roof in a heavy North Island rain event (50 mm/hr) delivers 83 litres per minute. Standard 5-inch K-style gutters handle this. Use 6-inch for roofs over 150 m². Keep gutters clean -- leaf guards pay for themselves in reduced maintenance.
First-Flush Diverter
The first 1-2 mm of rainfall washes dust, bird droppings, pollen, and debris off the roof. A first-flush diverter captures this dirty water and diverts it away from the tank. Size the diverter at 1 litre per m² of roof area. A 100 m² roof needs a 100-litre first-flush chamber.
Filtration
At minimum, install a leaf screen at the downspout entry and a sediment filter before the tank. For potable use, add a 5-micron cartridge filter plus UV sterilization. Non-potable use (irrigation, livestock) only needs screening and settling.
Tank Placement
Place tanks as high as practical to enable gravity-feed to use points. Every metre of elevation gives you approximately 10 kPa (1.4 psi) of pressure. A tank 3 metres above your garden tap delivers adequate drip-irrigation pressure without a pump.
If gravity-feed is not possible, a small pressure pump (0.5-1 HP) serves most household systems. Solar-powered pumps work well for remote or off-grid properties.
Do You Need a Permit for Rainwater Collection in BC?
Collecting rainwater from your own roof into tanks does not require a water licence under BC's Water Sustainability Act. Roof precipitation is not considered a "stream" and you are not "diverting" from a natural watercourse. This is the simplest, most regulation-free way to secure water on your property.
However:
- Building a pond to store rainwater (even rain-fed) typically requires a water licence
- Diverting surface runoff from land into storage may require a licence
- Any structure intercepting groundwater or a stream requires a licence plus possibly a change approval
Roof-to-tank collection stays entirely outside the regulatory framework. That alone makes it worth prioritizing.
Combining Catchment with Other Water Sources
Most properties perform best with a layered approach:
- Roof catchment to tanks: Clean water for household, no licence needed
- Surface catchment to pond: Bulk storage for irrigation and fire (water licence required)
- Swales for soil recharge: Passive infiltration to recharge groundwater and support trees
- Well or municipal as backup: Emergency supply for extended drought years
Use our Water Planner to model all sources together, or the Water Budget tool to see month-by-month supply versus demand across the year.
Common Mistakes in BC Rainwater Systems
- Undersizing for the dry season: Sizing only for average monthly demand instead of the full dry period. July through September, you draw down without refill.
- Forgetting fire reserve: BC Fire Code requires 45,000 litres for rural properties. This is not optional and must not be drawn from for daily use.
- Poor gutter maintenance: Blocked gutters overflow and bypass the tank. One blocked downspout can waste 25% of your catchment.
- No first-flush diverter: Contaminated water enters the tank after every dry spell. The first 2 mm of rain is the dirtiest.
- Tank placed too low: Then you need a pump for everything. Site the tank at elevation if terrain allows.
- Single tank, no redundancy: One tank failure means zero supply. Two smaller tanks provide backup.
Quick Sizing Reference by Region
For a family of four (700 L/day household + 100 L/day garden), metal roof (0.95 coefficient):
| Region | Min. Roof for Full Supply | Tank Size Needed |
|---|---|---|
| Campbell River (1,600 mm, 75 days dry) | 40 m² | 60,000 L |
| Courtenay (1,400 mm, 80 days dry) | 46 m² | 64,000 L |
| Nanaimo (1,100 mm, 90 days dry) | 65 m² | 72,000 L |
| Victoria (900 mm, 100 days dry) | 88 m² | 80,000 L |
| Interior BC (400 mm, 120 days dry) | 190 m² | 96,000 L |
"Min. Roof for Full Supply" is the roof area needed to harvest enough annual rainfall to meet the 800 L/day demand. The Interior is tough -- 400 mm of rain simply cannot supply a full household from a normal-sized roof without supplemental sources.
Use Our Free Calculator
Our Water Capture Calculator runs these formulas automatically. Enter your roof area, location, roof type, and demand -- it gives you annual harvest, recommended cistern size, and a water security score. The Water Budget Planner shows monthly supply versus demand so you can see exactly when you run short.
Sources & References
- Environment Canada historical climate data -- climate.weather.gc.ca
- BC Water Sustainability Act -- bclaws.gov.bc.ca
- Brad Lancaster, Rainwater Harvesting for Drylands and Beyond, Vol. 1-3
- BC Fire Code -- minimum water supply for fire suppression
- Canadian Mortgage and Housing Corporation (CMHC) -- residential water use statistics
Need help with this on your property?
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