Roof rainwater for vegetables: first flush, which roofs work, and mosquitoes
Roof rainwater suits a vegetable bed if you divert 0.4-0.8 litres per square metre of roof first, water the soil not the leaves, spare salad greens, and seal the tank.
- The first water off a roof is the dirtiest, carrying dust, metals and bird or animal droppings; divert about 0.4-0.8 litres per square metre of roof.
- Coated metal, clay tile, concrete tile and plastic roofs give the cleanest water; avoid copper, bare galvanised metal, treated wood and lead-based paint.
- Apply rainwater to the soil at the base, never on leaves or fruit, in the morning, and use tap water for salad greens and leafy herbs if you can.
- A rain tank needs a tight lid, screened inlets and a weekly check, or it becomes an Aedes mosquito nursery.
Roof rainwater is fine for watering vegetables, and plants like it because it is soft. It takes three habits: divert the first flush that washes dust and droppings off the roof, water the soil at the base so it doesn’t touch what you eat, and seal the tank against mosquitoes. This lesson draws on a Rutgers Extension study that tested real rain-barrel water, plus guidance from Purdue and the University of Maine, with a worked example for one side of a house roof.
Getting beds ready for the wet season is in the rainy-season garden lesson. This one is about collecting the water.
What comes off the roof
Rain itself is fairly clean, but running over a roof and gutter it picks up whatever built up during the dry spell. Rutgers summarises several studies showing roof runoff can carry pathogens, zinc, lead and polycyclic aromatic hydrocarbons (PAHs), and that the first flush of runoff carries the highest contaminant levels.
The Rutgers team fitted 12 rain barrels to houses with asphalt-shingle roofs, half suburban and half urban, and sampled them 3-7 days after rain from July to October.
- Heavy metals were well below the irrigation limits. Zinc was higher in the urban barrels, probably from tyre dust and industry.
- PAHs were not detected at all.
- E. coli, the marker for faecal contamination from warm-blooded animals, exceeded the single-sample irrigation limit of 235 per 100 mL in 9% of samples. The likely sources were birds and squirrels on the roofs or in nearby trees.
- Drinking-water standards: every sample failed for coliforms.
Rutgers concluded the water can be used on a vegetable garden, but with precautions, not casually. The University of Maine answers more bluntly that the risk is hard to gauge: the amounts found seem small, but nobody can call it zero.
Which roofs work
| Roof | OK for vegetable water? | Why |
|---|---|---|
| Coated metal, plastic, clay tile, concrete tile | Yes | Purdue lists these as giving the cleanest water |
| Asphalt shingle in good condition | Yes | Purdue says suitable if in good condition; the Rutgers study used this type |
| Untreated metal such as bare galvanised sheet | No | Can leach zinc |
| Copper | No | Purdue advises against it |
| Treated wood | No | Purdue advises against it |
| Anything with lead-based paint | No | Lead gets into the water |
If you’re unsure what your roof or its paint is, use the rainwater on ornamentals and lawn, and tap water on the vegetables.
How much to divert: a worked example
The tool for this is a first flush diverter: a vertical pipe that fills with the first runoff, after which the cleaner water flows on into the tank. A small hole or valve at the bottom lets it drain slowly so it’s ready for the next rain.
Purdue sizes diverters at about 10 gallons per 1,000 square feet of roof, and a Rutgers Climate Institute sheet gives a rule of thumb of 1-2 gallons per 100 square feet. In metric that is roughly 0.4-0.8 litres per square metre of roof.
Say one side of the roof drains into a single gutter, 5 m wide and 8 m long, feeding a 200-litre tank.
- Roof area. 5 × 8 = 40 m².
- First flush to divert. 40 × 0.4 = 16 litres, up to 40 × 0.8 = 32 litres. If birds roost on the roof or leaves pile up, use the higher figure.
- Diverter pipe size. Per the Rutgers sheet, one foot of 6-inch PVC pipe holds about 1.5 gallons, roughly 5.7 litres. For 32 litres you need about 32 ÷ 5.7 ≈ 5.6 feet, or about 1.7 m, of 6-inch pipe.
- Rain needed to fill the tank. 1 mm of rain on 1 m² gives 1 litre, so this roof yields 40 litres per millimetre. Diverting 32 litres takes about 0.8 mm; the 200-litre tank needs another 5 mm. About 6 mm of rain fills it. Real yields are a bit lower because of splash and evaporation.
The first rain after a long dry spell carries the most dust and droppings; Purdue notes that this is when contaminants build up on the roof. If you don’t have a diverter yet, at least let the first few rains of the season run to waste before connecting the gutter to the tank.
Watering safely
Rutgers and Purdue agree on the essentials.
- Water the soil at the base, not the leaves or fruit. Use a watering can without the rose, or drip line.
- Water in the morning only, so plant surfaces dry and sunlight helps kill germs.
- Leave as long as you can between watering and harvest. Don’t pick right after watering.
- For salad greens and leafy herbs, use tap water if possible. The University of Maine also suggests rainwater for fruiting crops rather than leafy greens, since most contaminants are a risk from surface residue and don’t move easily through the plant.
- Always wash produce in tap water, never in harvested rainwater. See the washing lesson.
Tank water is for the soil, not the leaves: water at the base, in the morning, and wash produce in tap water.
Tank care and mosquitoes
An open rain tank is a perfect Aedes egg-laying site. WHO advises covering, emptying and cleaning household water containers every week. For a rain tank, that means:
- A tight lid, fine mesh over the inlet, and mesh over the overflow as well.
- A dark or covered tank. Purdue notes this keeps light out and limits algae.
- A weekly larvae check. Shine a torch on the surface; if you see larvae, use the water on plants until the tank is empty, then clean it.
- Clear gutters and screens after rain, and clean the tank before the season. Purdue suggests a dilute vinegar or bleach solution.
Rutgers gives a bleach dose to cut germs in the barrel: plain unscented household bleach at 5-6%, about 1 ounce (roughly 30 mL) per 55-gallon (roughly 200-litre) barrel, once a month, perhaps twice a month in very wet spells, then wait about 24 hours before using the water. The same sheet notes household bleach is not labelled for water treatment. Read the label, and never mix it with other cleaners.
Common mistakes
- Running the gutter straight into the tank with no diverter. The dirtiest water all ends up stored.
- Pouring tank water over whole lettuces in the evening. It wets leaves eaten raw, and they stay wet overnight.
- Washing produce in, or drinking, tank water. It doesn’t meet drinking standards.
- Collecting from bare galvanised or old painted roofs for the vegetable bed.
- Leaving the lid off. It becomes a mosquito nursery within a week.
Sources
- Rain Barrels Part IV: Testing and Applying Harvested Water to Irrigate a Vegetable Garden — Rutgers Cooperative Extension
- Rainwater Harvesting for Homes and Community Gardens — Purdue Extension
- Is it safe to use rainwater harvested from an asphalt shingled roof? — University of Maine Cooperative Extension
- Rainwater harvesting, first flush diverter sizing — Rutgers Climate Institute
- Dengue and severe dengue — WHO
FAQ
Can I drink the water from my rain tank?
This article is about watering plants only. Barrel water mostly met irrigation limits, but in the Rutgers study every sample failed the drinking-water standard for coliforms. Don't drink it, and don't wash produce with it.
My roof is old bare galvanised sheet. Can I use the water?
Purdue Extension advises against harvesting from untreated metal roofs such as bare galvanised sheet, because they can leach zinc. Use that water on ornamentals and use tap water on vegetables.
Can I add oil or dish soap to the tank to stop mosquito larvae?
Better not; that water ends up on your vegetables. The safer route is a tight lid, mesh over the inlet and overflow, using the water regularly, and checking for larvae every week.
General information, not medical advice. If you are pregnant, take regular medication, or have a liver or kidney condition, ask a doctor or pharmacist before taking herbs beyond normal food amounts.