By Jesse Savou, ARCSA A.P. / ASSE 21110 & 21120 Certified. Last updated September 23, 2022.
If you're thinking about rainwater harvesting (also known as roof-water harvesting!), you may be wondering about the best roofing materials for rainwater collection. Whether you're working with existing conditions, or spec-ing out a brand new roof, we've got expert advice for you.
The good news is *most* roofing materials are perfectly suitable for rainwater harvesting. While metal roofing or plastic sheeting (e.g. greenhouse roof) has the highest collection efficiency, most other surfaces are also a-ok. There's just a short list of rooftop surfaces that rainwater harvesters should avoid. Read down the list for info on roofing materials for rainwater harvesting.
Standing Seam Metal
If you're installing a new roof for the purpose of collecting rainwater, standing seam metal may very well be your best choice. A high quality enameled roofing material that is easy to clean, standing seam metal offers the highest collection efficiency available.
The American Rainwater Catchment Systems Association (ARCSA) advises that "standing seam roofs, powder coated or enameled, Galvalume (zinc + aluminum alloy) with non-toxic baked or enamel finish are appropriate for potable use with non-toxic finishes and appropriate components, tanks, and post-tank treatment."
If you're designing from scratch, standing seam metal may be the best choice for rainwater harvesting.
While it isn't the most affordable material out there, think about standing seam metal even for a garden shed or outbuilding. You can fill a 55-gallon barrel with less than single inch of rain falling on a 10' x 10' surface, so don't overlook those outbuildings when you think about opportunities to collect and store rainwater on your site. Check out our rainwater calculator to see how much water you can collect off your roof.
Corrugated metal also makes an efficient collection surface, but there is a precaution:
Most corrugated material is galvanized, which will leach some zinc into the rainwater. Many people collect from galvanized roofs with no problem, but this is something to be aware of. Collect a sample of rainwater and use a home test kit or send it to a lab to make sure zinc levels are below allowable limits. Zinc is an herbicide, so even if you won't be drinking the water, you'll want zinc levels to meet that standard to avoid hurting your plants.
HINT: Check out the spot where your downspout currently drains (or where rainwater releases from your galvanized roof). If the plants are dead in the spot where the water hits the ground, it may be a sign that zinc levels are high enough to impact plant growth. Consider coating the rooftop to keep zinc from leaching into your water. (See below for details on roof coatings.)
Asphalt Shingle / Bitumen / Composition Shingle
Got asphalt shingle (like most of us do)? Not a problem! Asphalt is inert, and generally safe for rainwater collection. That said, the adhesives used for installation are worth a mention. These adhesives do most of their off-gassing within a year of installation, and virtually all of it within three years, so if you have a brand new asphalt shingle roof (also known as composition shingle), you may wish to avoid irrigating edibles for the first few years. Are you going to be drinking the water? All potable systems must include treatment anyway, but send a sample to the lab to see what you need to treat for.
Most homes are topped with asphalt shingle. Generally speaking, it's safe to collect from this material, and to irrigate edible plants with your collected water.
Concrete Tile or Clay Tile Roofs
Clay and concrete tiles have lower collection efficiency than other roofing materials, simply because the tiles are porous and absorb so much water. This doesn't mean you can't harvest off them, though. With 623 gallons of high quality water available from a single inch of rain falling on 1000-square-foot surface, even an 85% collection efficiency (estimated for clay tile) will give you a lot of high quality water. Concrete tile may leave you with alkalinity in your water, but this isn't necessarily a problem. A simple home test kit will allow you to measure your pH.
Eco-conscious homeowners may be interested in solar panels as well as rainwater harvesting. The good news is solar panels make a wonderful collection surface, Water will roll off them nice and smoothly. While the water may also come into contact with your native roof surface, the solar panels will shed water cleanly. If adhesives are used for installation, it may be worth collecting a sample to see if there are toxins in the water.
Slate tile, painted tile, plastic sheeting, the list goes on and on. Most rooftop surfaces are just fine for rainwater harvesting. There are just a few roofing materials you should avoid...
Materials to Avoid
So what kinds of roofing materials do we need to watch out for when it comes to rainwater harvesting? Luckily the true black-list is short:
- Cedar Shake: Wood shingles are usually treated with fire retardants. This probably isn't something you want in your water. Have a sample tested to be sure. This water may be suitable for irrigation.
- Copper: The lucky few who can afford copper roofs may already know that copper is naturally resistant to algae/moss growth because it is an herbicide. Copper will leach into your rainwater, so if using for garden irrigation, copper isn't an ideal choice.
- Lead: Lead flashing is still available in some parts of the USA so test for lead if you're harvesting for drinking water.
- Biocides: Some rooftops are treated with biocides. Zinc- or copper-treated shingles are common in the Pacific Northwest to curb mold, moss, and algae growth on the roof. If you're harvesting for irrigation, make sure your biocide levels are low enough not to hurt your plants. (Check the spot where your downspouts drain onto existing landscape to get a sense of whether biocides are concentrated enough to harm the grass or other plants in that area.)
Elastomeric paints are specialty roof coatings specifically for rainwater harvesting. While the material isn't cheap and multiple coats are often required, this could be something to consider, especially for rooftops that aren't otherwise safe to collect from. The National Sanitation Foundation (NSF) provides a list of approved roof coatings for potable rainwater catchment systems.
*All water collected for potable use (e.g. human consumption) must be treated to meet drinking water standards.
Rooftop coatings exist just for rainwater harvesting, and can be used to make an otherwise-unsuitable material harvestable.
When in Doubt...
When in doubt collect a sample and send it to a lab to know what you're up against.
If you're collecting for potable use, you'll need to treat the water anyway, but it's essential to know what's in it to begin with to know what kind of treatment is most appropriate.
If you're collecting for garden irrigation (like so many people do!), you don't need to worry about organic pathogens... it's just the herbicides you need to look out for. You'll get a good hint about whether your water contains herbicidal compounds by paying attention to where your roofwater currently falls (e.g. where your downspouts release, or where the water falls from your roofline). If the plants in that area are dead, you may want to think twice. Otherwise, harvest away!
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