Best Roofing Materials for Rainwater Harvesting

rain running off a corrugated metal roof edge
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Collecting rainwater can be an effective and eco-friendly way to lower your water bill. Whether you’re planning to use it in the garden or to flush toilets in the home, even a small amount of rain can add up quickly. A 1,000-square-foot room can easily provide 550 gallons of water with less than an inch of rain. Even a small roof, like a 10x10 garden shed, can net you 60 gallons.

Safety

The goal of rainwater collection may be to lower your water bill, but you must place safety as a primary concern. Regardless of roofing material, all rainwater must be treated for human consumption. Even if you’re simply using it to water the plants, some roofing materials can shed minerals or chemicals that are toxic to the flora and fauna in your home or yard.

Before using water collected from your roof, test it with a home test kit, which is easy to find at any local home improvement store or online. Alternately, send a sample to a lab in your community.

One additional note about safety. Be sure to include an inlet screen in your rainwater collection system to keep out debris.

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rainwater collection system with screen on barrel under downspout

Efficiency

Nearly all roofs are safe for collecting water, especially when they go through a treatment system. However, some roofs are more efficient at letting water roll off without absorbing it.

Asphalt

The most common roofing material in American is asphalt or composite roofing. This is the standard shingle style you likely have. The biggest concern with asphalt roofing is the beads of gritty material they release. A good screen will take care of that, though. New composite roofing does release off-gassing of glues for the first few years, so test it after collection and perhaps avoid using it on the plants you’re eating until you know if it’s safe.

Corrugated Metal

If you’re lucky enough to have a corrugated metal roof or are thinking of investing in one, water will stream off efficiently. Galvanized metal, however, contains a high amount of zinc, which plants don’t like and humans need to be cautious about. So again, test.

rainwater flowing off a corrugated metal roof into a gutter system

Enameled Roofing Material

Also known as standing seam metal, this type of roofing is the safest, since it’s coated with non-toxic enamel or powder coating. Plus, it offers the highest level of efficiency. However, it’s an expensive option and you’ll still need to be mindful about testing and treatment.

Plastic Sheeting

Of course your home won’t be covered in plastic sheeting, but your greenhouse likely is, so don’t overlook it as an option for rainwater collection. Plastic is highly efficient at allowing water runoff.

Tile

Tile roofs, whether made of concrete or clay are a safe option since they are natural materials. However, rainwater collection is less efficient. Even at an estimated 85% efficiency, you can collect a high-quantity of the wet stuff, especially in areas frequented by rain.

Solar Panels

Solar panels themselves are sleek and made to repel water so collection efficiency is high. Of course, water will also runoff from other parts of the roof, so consider what material the roof is composed of.

Slate

As a natural material, slate works well with water. It’s another efficient option and the same rules apply regarding testing and treatment.

slate shingles on a roof

Materials to Avoid

The truth is, most roofing materials do a great job of allowing water to run off the surface of the roof and are safe to use. There are a few exceptions, though. Pay special attention where there are a high amount of chemicals in the production or installation of the materials. Specifically, cedar shake is problematic, which is unfortunate considering their generally high-cost to source and install. Wood shingles, however, are treated with fire retardants, which is obviously beneficial in another way. Nobody wants to be drinking those chemicals, though, even your plants.

Also avoid copper roofs, which are spendy and act as a natural herbicide to keep algae away. However, it’s also a bad combination for animals and edibles alike.

If you live in a wet climate, biocides are likely used on a variety of roofing materials. Typically, elements like zinc and copper do the job of minimizing algae, moss, and mold. However, use extreme caution when using water collected from these surfaces.

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