When the rain pours and the runoff water goes to waste, you might wonder whether rainwater from the roof is safe for your plants. It’s definitely a good idea to minimise water wastage, but is water that’s been in contact with your home’s roof safe for edible and ornamental plants?
The answer depends on what your roof is made of. A range of roofing materials are used on residential homes. Some contaminate the water and some, such as asphalt, are inert. Naturally, you want to be sure that the rainwater is safe for use in the garden.
Even if you can’t use roof runoff to water your plants, it’s still fine for cleaning cars and outdoor furniture. Also, understanding roof leak emergencies is important to make sure that all the water is directed to the storage barrels.
Potential contaminants in roof runoff water
Studies have shown that rainwater from the roof contains contaminants such as:
- E. coli bacteria
- Poly-cyclic aromatic hydrocarbons (PAHs)
- Other pathogens
- Animal droppings
Depending on the roof material, the runoff water could also contain other contaminants.
These contaminants in roof runoff water are usually available in low quantities. With regards to watering your plants, there’s a low element of risk. Do note however that roof runoff water is not safe for drinking.
Is rainwater from an asphalt roof safe for plants?
Asphalt roofs are common, and most people prefer them for their durability. The inert nature of asphalt means it does not react with water. Thus, it does not leach contaminants into the water. Generally, water collected from this type of roof is considered safe for irrigating your garden plants.
Most shingle roofs are made of fibreglass and asphalt granules, and are considered safe when it comes to using water runoff.
If you have a galvanised iron roof, the water will definitely contain more zinc than that from a shingle or asphalt roof. Iron roofs have a zinc coating to prevent rusting, and the zinc can contaminate the water.
How to make roof water safe for vegetables
Clean water is a scarce commodity in many parts of the world. One way to make your garden and home sustainable is to collect rainwater whenever possible. The norm is to collect rainwater from the roof in water butts, then use it in the garden.
If the roof is built with common roofing materials, the water is usually considered safe for edible plants. The contaminants in the water usually occur in minimal quantities, which shouldn’t harm your plants. However, before using the water in your garden, a soil test is recommended, because excess zinc causes toxicity in plants.
Here are a few things you can do to make roof runoff water safe for watering your vegetables:
- Clean the water butt. Always keep water butts clean, and covered with a lid.
- Clean the gutters. The water runs off the roof and pours into the gutter, where it then flows into the barrel. Gutters can carry a lot of dirt and debris, which will in turn make the water dirty.
- Filter the water before it gets to the water butt. Filtering the water won’t remove metals, but it will prevent debris from getting into the water butt.
How to use roof runoff water on your vegetables
Follow these watering tips when using runoff water on your edible plants:
- Don’t use overhead irrigation. Instead, try drip irrigation, which ensures that the water does not splash onto the leaves and fruits.
- When harvest time is a few weeks away, stop using runoff water.
- Rinse fruit and vegetables in clean tap water before eating.
Choosing the right container for roof water
Not every barrel will be good enough for your roof water. The recommended option is an opaque water butt. The lack of light is necessary to prevent algae growth.
Ideally, you should fit some sort of filter to your water butt to stop debris getting in. This will also reduce the need for cleaning.
You also need to fit a lid to your water butt; again, to keep debris and light out.
Conserving water for use in the garden is a great way to minimise water usage, but you need to make sure that the water is not contaminated. By testing for zinc, keeping gutters and water storage containers clean, and watering carefully, you can put rainwater to good use and do your bit to conserve this precious resource.