Best Roofing Materials for Rainwater Harvesting

If you plan to collect rainwater from your roof, you’ll need to know the best roofing materials for rainwater harvesting. Most roofing materials are perfectly suitable for rainwater harvesting, though some have higher collection efficiency than others.

What is Rainwater Harvesting?

Rainwater harvesting is the process of collecting water runoff so it can be stored for things like watering plants, cleaning, washing cars, and washing clothes. Your roof provides an effective surface for rainwater collecting. Rain runs off the roof, collects in the gutters, and moves through the downspouts into a storage container like a rain barrel or cistern. 

Rainwater collection has multiple uses and helps reduce water runoff and improves water quality and sustainability by preventing erosion and flooding in rivers and streams.

Best Roof Types for Rainwater Harvesting

There are several roofing materials that can be used to collect rainwater. Here are a few of the most popular.

Galvanized Metal

Galvanized roof panels are sheets of metal coated with zinc oxide to prevent and reduce the effects of rust. Galvanized metal roofs have at least a 95% collection efficiency, with the other 5% being the percentage of water that evaporates or is absorbed by the material. It is an affordable option depending on the thickness of the steel panels.

Because this material is made with a rust-resistant zinc coating, it risks seeping into the rainwater. Zinc can stunt plant growth, and if people consume too much zinc, it can cause nausea, dizziness, headaches, and vomiting. The water can still safely be used for cleaning and watering plants but should be tested to make sure the zinc levels are not high. 

Here are some of the benefits of galvanized metal roofing:

• Affordable
• Heat-resistant
• 60-year lifespan
• Rust-resistant

Standing Seam Metal

Corrugated metal roofing, particularly standing seam metal, is an excellent but costly option for collecting rainwater. Standing seam metal panels have hidden fasteners that interlock with clamps, so they don’t require screws. Since the pieces fit together “seamlessly,” they have a high collection efficiency of 95% or higher. Just be sure the metal has a non-toxic finish.

Other benefits of standing seam metal roofs are:

• Weather-tight
• Durable
• Fire resistant
• Low maintenance
• Energy efficient

Asphalt Shingles

Most homes have asphalt shingles, which are safe for rainwater harvesting. Asphalt shingles have a runoff collection efficiency of 90%. The biggest issue with collecting water from an asphalt roof is the grit that can be loosened from the shingle and get into the water. Adding a screen to the top of the collection tank will filter out any asphalt gravel.

You shouldn’t collect rainwater for the first three years after installing a new asphalt roof. The adhesives will need plenty of time to air out any harmful chemicals. Older shingles made before the 1980s can contain asbestos, which is also dangerous.

Some benefits of an asphalt shingle roof include:

• Versatile
• Variety with designs
• Affordable
• Recyclable
• Easy to repair and install
• Sound-proof
• Algae-resistant
• Wind-resistant

Concrete Tile

Concrete tiles are made with cement, sand, limestone, and water. They are porous and absorb water, so their collection efficiency is only about 85%.

Concrete tiles might raise the alkalinity in your rainwater harvest, but you can use a home test kit to check the pH levels.

Here are some of the benefits of using concrete tiles:

• Durable
• Resistant to extreme weather
• Lasts for over 50 years
• Insulates heat 

Clay Tile

Terracotta roofs are a fantastic surface for collecting rainwater. However, clay is porous and has a lower collection efficiency at 85% to 90%. But even 85% provides plenty of high-quality rainwater.

Because clay tile roofs absorb water, they tend to cultivate moss or algae, which means they will need to be pressure washed at least once a year.

Other benefits of clay tiles include the following:

• Made from natural materials
• Water-resistant
• Lasts for over 75 years
• insulate heat

Solar Panels

If you’re considering rainwater harvesting, chances are you’re environmentally conscious, so why not go the extra step and install solar panels? Not only do they offer clean energy, but they make a great rain collection surface. A 500 Watt-Peak solar panel can collect about 608 gallons of clean water a year.

Other benefits of solar panels are:

• Affordable
• Saves on energy costs
• Low maintenance
• Reduces carbon emissions

Note: if the solar panels are installed with adhesives, you should test the water for possible toxins.

What Roofing Materials Are Not Good for Collecting Rainwater?

Not all roofing materials are ideal for collecting rainwater. Some can be loaded with dangerous contaminants that can affect your health.

Cedar Shake 

Wooden roof surfaces tend to grow algae, moss, and mold that can deteriorate your wooden shakes or shingles. Manufacturers usually treat wood shingles with fire retardants or spray chemicals that prevent mold and algae growth. These are not something you want in your collected rainwater.

Copper 

Copper is a natural pesticide and fungicide that will contaminate your rainwater if used for harvesting. Water that has been polluted with copper should not be used for drinking or for watering plants.

Lead 

Lead is a toxic chemical that shouldn’t be consumed but is still used for roofing material like flashing in some parts of the US. You should test for lead before harvesting rainwater from your roof.

Do Roof Coatings Help Rainwater Harvesting?

Specialty roof coatings like elastomeric paints are used to make otherwise unsafe roofing materials safe for rainwater harvesting. The material requires many coats and is fairly expensive, but it might be something to consider if you have a roof unsuitable for water collection.

You can always visit the National Sanitation Foundation for a list of approved roof coatings for rainwater harvesting.

How Do You Calculate Rainwater Collected From Your Roof?

You need to consider several factors when calculating your annual water collection efficiency.

1. Measure your collection area
2. Measure how much rain your home receives annually
3. Consider the runoff coefficient of your roof

Once you have these factors, you can use this formula to calculate how much rainwater your roof will collect:

Roof Area x Annual Rainfall x Runoff Coefficient

However, if you don’t have time to take all of these measurements and only want a quick estimate, then you can use this rule of thumb:

For every 1,000 square feet of roofing, the collection system will harvest roughly 600 gallons of water.

Is Water Collected From a Roof Safe?

Harvesting rainwater from your roof is generally safe so long as you don’t collect it from toxic roofing materials. Rainwater collected for the purpose of drinking should be purified. Filtering your rainwater for watering plants is not necessary but will benefit plant growth. 

Rainwater can be contaminated by bacteria and parasites, so it should be boiled and treated before drinking to filter out any pollutants. To avoid getting sick, do not use harvested rainwater for drinking, cooking, brushing your teeth, or rinsing fruits and vegetables.

Uses for Rainwater Collected From a Roof

• Sprinkler and irrigation systems
• Watering lawns and gardens
• Washing vehicles, driveways, and sidewalks
• Refill fountains, swimming pools, and ponds
• Cleaning clothes or appliances (toilets, sinks, etc…)

Benefits of Rainwater Harvesting

Harvesting rainwater provides several benefits for homeowners and the environment:

• Environmentally-Friendly: Collecting rainwater is great for the local environment and helps conserve water. Rainwater is also much better for gardens and plants because the water isn’t treated with any harsh chemicals like chlorine.
• Alleviate Emergencies: You can use your harvested rainwater as a backup water source in emergencies. These emergencies can include water shortages, water contamination, damage to your plumbing, or natural disasters.
• Free Source of Water: Rainwater harvesting is a free resource you can control, which is ideal if you live somewhere with water restrictions. It also cuts down on your water bill because you are using less water from the utility company.
• Multiple Uses: Rainwater can be used for a variety of things, including indoor and outdoor irrigation, cleaning, watering plants, and refilling ponds. Depending on the type of roofing material, you can even use it for drinking water, so long as it’s filtered and purified before consumption.
• Easy to Use: Most rainwater collection systems are simple to install and easy to use. The equipment is inexpensive, low maintenance, and easy to retrofit to your home.

FAQ About the Best Roofing for Rainwater Harvesting

When to Hire a Pro 

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With RoofGnome, you can find local contractors in your area who are licensed and insured and offer standard roofing warranties! There’s no need to pull out the ladder and DIY when you can hire a professional roofing contractor with experience and skill to get the job done right.

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