Every time it rains, a greenhouse roof sheds hundreds of litres of clean, soft, chlorine-free water straight onto the ground. Plants love rainwater more than anything coming out of a municipal tap, and capturing it costs almost nothing once the system is in place. For anyone gardening through a dry prairie August or paying for metered city water, a rainwater system pays for itself fast.
This guide covers how much water you can realistically harvest, how to size your storage, the filtration you actually need, and how to plumb a simple gravity-fed setup that keeps your plants watered without a pump.
How much water your roof can actually catch
The yield is bigger than most people expect, and the math is straightforward. For every millimetre of rain that falls on one square metre of roof, you collect almost exactly one litre of water.
The formula: roof footprint (m²) x rainfall (mm) x 0.9 collection efficiency.
Take a 13.5' x 12' greenhouse. That footprint is about 15 square metres. The roof catches rain based on its footprint, not its sloped surface area, so 15 m² is your number.
Now apply real rainfall. Toronto averages about 60 mm of rain in June. So: 15 x 60 x 0.9 = roughly 810 litres from a single average June. A wetter month, or a region like coastal BC that sees 150 mm in November, pushes that past 2,000 litres. Over a full year, a backyard greenhouse roof in most of southern Canada will shed 8,000 to 12,000 litres. That's a lot of free irrigation.
The lesson: even a modest roof keeps a backyard garden watered through normal weather. Your limit isn't how much falls, it's how much you can store between rains.
Sizing your storage tank
Storage is where most systems are under-built. A single 200L rain barrel fills in one good storm and then overflows, wasting the rest. Size for the dry stretch between rains, not for a single downpour.
A practical way to size it: estimate your peak summer water use, then store at least two weeks of it. A 162 sq ft greenhouse packed with tomatoes and cucumbers in July can drink 40 to 60 litres a day in hot weather. Two weeks of that is 600 to 850 litres.
- Small setup (200-400L): One or two linked rain barrels. Fine for seedlings and shoulder-season growing, but you'll run dry in a summer heatwave.
- Mid setup (1,000L): A single IBC tote (the 1,000L caged plastic tanks you can buy used for $100-150). This is the sweet spot for most backyard greenhouses and handles a two-week dry spell with room to spare.
- Large setup (2,000L+): Multiple linked totes or a dedicated cistern, for serious year-round growers or drought-prone regions.
Link multiple barrels at the base with bulkhead fittings so they fill and drain as one connected volume. Always plumb an overflow outlet near the top that directs excess water away from your foundation.
Filtration: keep it simple, keep it clean
Rainwater off a polycarbonate roof is remarkably clean compared to runoff from asphalt shingles, but you still want to keep debris and algae out of the tank.
- First-flush diverter. This simple device dumps the first few litres of each rainfall, which carry the dust, pollen, and bird droppings that accumulated on the roof between storms. It's the single most effective upgrade for water quality.
- A mesh gutter screen or leaf guard. Stops leaves and large debris before they reach the downpipe.
- An opaque or dark tank. Sunlight grows algae. A black or dark green tank, or one wrapped to block light, stays clean far longer than a translucent one.
- A fine inlet screen. A simple mesh over the tank inlet keeps mosquitoes from breeding, which matters across most of Canada in summer.
You do not need elaborate filtration for irrigation water. Drinking-water treatment is a different project entirely. For watering plants, a first-flush diverter and a dark, screened tank covers it.
Plumbing a gravity-fed system
The elegant thing about rainwater for a greenhouse is that you can often skip the pump. Water flows downhill, and a tank raised even half a metre gives you enough pressure for a watering can or a low-flow drip line.
A basic gravity setup, start to finish:
- Gutter the roof. Most greenhouse kits have an eave channel or accept a clip-on gutter along the lower edge of each roof slope. Run it to one corner.
- Drop a downpipe into the first-flush diverter, then into the top of your tank.
- Raise the tank on a sturdy, level platform. Cinder blocks or a built-up gravel pad work. A 1,000L full tank weighs over a tonne, so the base must be solid and level.
- Fit a tap and hose outlet near the base for filling cans, plus a separate higher outlet feeding a drip line if you want automated watering.
- Run a drip line or soaker hose along your benches. Gravity pressure is low, so use drip emitters rated for low-pressure or a simple soaker hose, not a sprinkler.
For a greenhouse like the Drivhus, with its 13.5' x 12' footprint and clean polycarbonate roof, a single IBC tote at one corner and a gravity drip line covers most of a growing season's watering with zero electricity. You can see the structure it bolts onto at the Drivhus product page.
Winterizing in a freezing climate
Across most of Canada, a rainwater system has to be put to bed before the hard freeze, usually mid-to-late October depending on your zone. Frozen water expands and cracks tanks, splits fittings, and ruptures pipes.
- Drain tanks completely before the first sustained sub-zero stretch.
- Disconnect and drain all hoses, drip lines, and the first-flush diverter.
- Leave the tank's outlets open over winter so any meltwater drains instead of pooling and freezing.
- Store IBC totes with the lid cracked to prevent vacuum collapse and to let them dry out.
- If you heat your greenhouse through winter, you can keep a small indoor reservoir going, but the outdoor collection side still needs to come offline.
In coastal BC and other mild zones you may run the system most of the year, but even there a hard frost snap can do damage, so watch the forecast.
Final thoughts
A rainwater system turns your greenhouse roof from a passive shelter into a working part of your garden's water supply. Start with the yield math for your region, size your tank for the dry stretch rather than the storm, add a first-flush diverter and a dark screened tank, and let gravity do the rest. Done right, you'll water an entire summer of tomatoes on rain you'd otherwise have watched run down the driveway, and your plants will be healthier for the soft, untreated water. Set it up in spring, winterize it in fall, and it'll serve you for years.