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A roof is more than a way to keep rain out of your home. In a rainwater harvesting system, it is the first large surface the water touches.
That makes roofing material important.
For most rainwater systems, smooth, clean, non-toxic roofing materials are the easiest to work with. Unpainted or suitable coated metal roofing is often a strong choice because it sheds water and debris well. Other roofs, including asphalt shingles, clay tile, concrete tile, and membrane roofing, can also collect rainwater, but the intended use of the water matters.
If you plan to drink the water, roofing material is only one part of the safety decision. Roof runoff can contain germs, metals, chemicals, dust, animal waste, and other contaminants. Drinking-water systems need suitable collection materials, pretreatment, treatment, regular testing, maintenance, and compliance with local requirements. The CDC does not consider collected rainwater automatically safe to drink.
What Makes a Roof Good for Rainwater Harvesting?
A good collection roof should do three basic things:
- Shed water quickly.
- Be easy to keep clean.
- Avoid adding unwanted chemicals or metals to the runoff.
Smooth surfaces usually have an advantage. Leaves, dust, moss, and bird droppings are less likely to stay trapped on a smooth roof than on a rough or porous surface.
The condition of the roof can matter as much as the basic material. A clean metal roof and a badly rusted metal roof should not be treated as the same catchment surface.
Also look at everything the water touches after it lands. That includes flashing, sealants, paint, gutters, solder, downspouts, screens, pipes, and the storage tank.
Roofing Materials at a Glance
| Roofing material | General fit for harvesting | Main concerns |
|---|---|---|
| Metal roofing | Often a good choice | Coatings, corrosion, galvanized surfaces, copper or lead components |
| Asphalt shingles | Often usable for non-potable systems | Granules, organic material, product-specific chemicals |
| Clay tile | Can be usable | Rough surfaces, dirt, moss, coatings |
| Concrete tile | Can be usable | Sediment, surface treatments, rough texture |
| Slate | Often usable if untreated and in good condition | Debris in joints, flashing materials |
| Membrane roofing | Depends strongly on the membrane | Plasticizers, coatings, adhesives, certification |
| Wood shingles or shakes | Usually a poor choice when water quality matters | Wood treatments, organic debris, moss |
| Green roofs | Better suited to specialized non-potable systems | Soil, nutrients, organic matter |
| Copper roofing | Poor choice for many rainwater uses | Copper can enter runoff |
| Lead-containing roofing or flashing | Avoid | Lead contamination |
| Asbestos-containing roofing | Avoid for new harvesting systems | Material condition and contamination concerns |
These are general guidelines. A roofing category alone does not tell you whether a specific product is suitable for your intended water use.
Metal Roofing
Metal is one of the simplest roofing surfaces for a rainwater harvesting system.
Its smooth surface usually drains quickly and is fairly easy to keep free of leaves and other debris. Studies of roof runoff have also found metal roofing can perform well compared with several rougher roofing surfaces.
But "metal roof" covers many different materials.
Uncoated metal
A suitable, corrosion-resistant metal roof can be a practical catchment for irrigation and other non-potable uses.
Non-potable means the water is not intended for drinking.
Check the entire roof system rather than just the panels. Screws, flashing, gutters, solder, and repairs may use different metals.
Galvanized roofing
Galvanized steel has a zinc coating that protects the steel from rust. Runoff may pick up zinc and other metals depending on the roof, its age, local conditions, and the product used.
That does not automatically make a galvanized roof unusable. It does mean you should not assume runoff quality from appearance alone.
Testing becomes especially important if the water will be used for higher-contact purposes.
Painted and coated metal
A painted roof may work well, but the coating matters.
Do not assume ordinary exterior roof paint is suitable for a rainwater catchment used for drinking water. Old coatings may also be harder to identify.
For a potable system, look for roofing materials and coatings specifically evaluated for rainwater catchment use. NSF P151 is a protocol used to evaluate components that directly contact harvested rainwater and determine whether they add contaminants above applicable health limits.
Copper roofing
Copper is different from ordinary steel or aluminum roofing.
Copper can enter roof runoff, especially when water is in contact with copper surfaces, gutters, or downspouts. EPA rainwater harvesting guidance identifies copper roofing components as a contamination concern.
For that reason, copper is not a good default choice when you are designing a roof specifically to collect water.
Asphalt Shingles
Many homes already have asphalt shingles, so replacing an otherwise sound roof just to install a rain barrel usually does not make sense.
Shingle roofs can be used for many non-potable rainwater systems, but they have some disadvantages.
The surface is rough. It can hold:
- Dust
- Pollen
- Bird droppings
- Leaves
- Moss
- Roofing granules
Some roofing products can also contribute chemicals or metals to runoff. The amount varies with the specific roofing material, its age, and environmental conditions. EPA guidance recommends considering runoff quality rather than assuming all asphalt shingles behave the same way.
For basic landscape watering, a well-maintained shingle roof may still be useful. Use good screening and a first-flush system where appropriate.
A first-flush diverter sends the first portion of runoff from a storm away from the storage tank. This early runoff often carries more roof dirt and debris than the water that follows. CDC guidance recommends considering first-flush diversion as one way to improve collected rainwater quality.
Clay and Concrete Tile
Clay and concrete roofs can collect plenty of water, but their surfaces can be harder to keep clean than smooth sheet metal.
Tile roofs may have:
- Rough or porous surfaces
- Small pockets where debris collects
- Moss or lichen growth
- Surface coatings or treatments
- Mortar and flashing that also contact the runoff
None of these automatically rules out rainwater harvesting.
For garden irrigation or similar non-potable uses, a sound and clean tile roof may work well when paired with leaf screens and suitable pretreatment.
For drinking-water systems, identify the actual tile, coatings, flashing, and other materials rather than assuming "clay" or "concrete" means the runoff is safe.
Slate Roofing
Natural slate can make a useful catchment surface if it is in good condition and has not been treated with unsuitable coatings.
Slate itself is only part of the system.
Pay attention to old flashing, repairs, sealants, gutters, and solder. Older roofs may contain materials that would not be chosen for a new drinking-water catchment today.
Do not disturb an old roof simply to inspect hidden materials unless it can be done safely. Roof work and unknown older construction materials are good reasons to use a qualified roofer.
Membrane and Flat Roofing
Flat and low-slope roofs often use materials such as:
- EPDM
- TPO
- PVC membranes
- Modified bitumen
- Liquid roof coatings
You cannot judge these roofs by appearance alone.
Some roofing membranes and coatings are made or certified for rainwater collection applications. Others are designed only to keep a building dry.
If you are planning a potable system, check the exact membrane, coating, adhesives, primers, and repair materials. NSF maintains certifications for some rainwater catchment components under NSF P151. Certification can apply to a particular product, color, thickness, or installation condition rather than every product made from the same general material.
Bitumen-based surfaces deserve extra caution. Drinking-water guidance has noted that bitumen-based roof coatings may leach unwanted substances or affect taste and odor.
Wood Shingles and Shakes
Wood roofs are usually harder to manage as rainwater catchments.
The surface can trap organic material and support moss or other growth. More importantly, some wood roofing has been treated with preservatives, stains, fire retardants, or other chemicals.
If you do not know how an existing wood roof was treated, do not assume its runoff is suitable for household water uses.
A wood roof may still feed a carefully planned non-potable system in some situations, but it is rarely the first material to choose when building a roof specifically for rainwater collection.
Green Roofs
A green roof works very differently from a normal roof.
Rain passes through vegetation and growing media before it leaves the roof. This can change the amount, timing, and chemistry of the runoff.
Water from a green roof may contain additional nutrients and organic material. EPA guidance generally treats it as more appropriate for non-potable applications.
If the main goal is collecting relatively clean roof runoff, a conventional smooth catchment surface is usually easier to manage.
Roofing Materials to Avoid or Treat With Extra Caution
Some materials create enough concern that they should not be treated like an ordinary catchment roof.
Lead
Planning around practical considerations for rainwater safe to collect from asphalt shingle roofs helps coordinate roof drainage that protects the foundation before storage.
Avoid lead roofing, lead flashing, and lead-containing solder in a new rainwater system.
Lead is a health concern, and CDC guidance specifically notes that roofing and plumbing materials can introduce lead and other chemicals into collected rainwater.
If you have an older roof and do not know what the flashing or solder contains, have the materials identified before planning potable use.
Copper
Large copper roof areas, copper gutters, and copper downspouts can increase copper in runoff.
This is especially important when the collected water will contact food, people, animals, or sensitive plants.
Asbestos-containing roofing
Older buildings may have asbestos-cement roofing.
Do not cut, drill, sand, pressure-wash, or otherwise disturb suspected asbestos roofing as part of a DIY rainwater project. Material identification and any required work should be handled under applicable local rules by qualified professionals.
Unknown roof coatings
An old painted or coated roof is difficult to evaluate when no records exist.
Do not rely on color, age, or appearance to decide that a coating is suitable for drinking-water collection.
The Roof Is Only One Source of Contamination
Choosing the right roof helps, but it does not make the collected water clean.
Rain can pick up contaminants before it even reaches the roof. Once it lands, it can collect:
- Dust
- Smoke particles
- Pollen
- Bird and animal droppings
- Leaves
- Insects
- Roofing particles
- Metals
- Chemicals
CDC guidance notes that roofing, gutters, piping, storage materials, and other system surfaces can all affect rainwater quality.
This is why a good rainwater system treats the entire path from roof to end use as one system.
Match the Roof to How You Will Use the Water
The right roofing material depends partly on what happens to the water after collection.
Ornamental landscape irrigation
This is usually one of the simplest uses.
A clean, sound roof with screens and basic debris control may be enough for many systems. Still, avoid roofs known to contain lead, toxic treatments, or other unsuitable materials.
Vegetable gardens
Water used around food deserves more care.
Runoff can contain germs and chemicals even when the roof itself is suitable. CDC recommends limiting uses that increase the chance of swallowing untreated rainwater and suggests using a safer water supply for edible plants when possible.
Where local guidance permits harvested rainwater for food gardens, irrigation near the soil rather than spraying edible parts can reduce direct contact. Water quality and local requirements still matter.
Toilet flushing, laundry, and household reuse
Indoor non-potable systems involve more than roof selection.
They may require dedicated plumbing, backflow protection, treatment, labeling, and other safeguards. Rainwater must not be allowed to contaminate a potable plumbing system.
Check your local plumbing and health requirements before making these connections.
Drinking and cooking
This is the highest-risk use.
A good roof does not make roof runoff potable.
Potable means safe for drinking.
A drinking-water rainwater system should be designed around the whole path:
roof catchment → debris control → first flush or other pretreatment → storage → treatment → distribution → testing and maintenance.
CDC recommends regular testing for appropriate germs and chemicals when collected rainwater is used for drinking, cooking, or bathing. Local health authorities can advise which tests and treatment requirements apply in your area.
Materials that contact drinking water should also be suitable for that use. NSF notes that some rainwater catchment components can be evaluated under NSF P151, while tanks and plumbing components may be evaluated under NSF/ANSI/CAN 61 for drinking-water contact.
Check More Than the Main Roofing Surface
When evaluating an existing roof, follow the water.
Check every material between the point where rain lands and the tank inlet.
That can include:
- Roof panels, shingles, or tiles
- Paint and protective coatings
- Flashing
- Valleys
- Roof repair compounds
- Sealants
- Screws and fasteners
- Gutters
- Gutter guards
- Downspouts
- Soldered joints
- Diverters
- Screens
- Pipes
A suitable roof combined with lead flashing or an unknown coating is not the same as a fully suitable catchment system.
Roof Condition Matters
Even a good roofing material can become a poor catchment if it is badly maintained.
Before connecting it to a tank, look for:
- Heavy moss or algae
- Large amounts of bird waste
- Peeling paint
- Rust or corrosion
- Crumbling shingles
- Loose roofing granules
- Standing water
- Chemical spills
- Recent roof treatments
- Damaged gutters
Do not climb onto a roof just to clean a rainwater system unless you have the equipment and experience to work safely at height. Many roofs can be inspected from the ground or serviced by a roofer.
Also check the roof again after repairs. A new sealant, coating, flashing material, or pest treatment can change what contacts your rainwater.
A Practical Choice for a New Rainwater Catchment
If rainwater harvesting is an important goal while designing a new roof, a smooth roofing surface made from materials suitable for the intended water use is usually the easiest option.
Metal roofing is often practical because it sheds water and debris well. But do not stop at the words "metal roof."
Confirm:
- What metal and coating will contact the water.
- What flashing and fasteners will be used.
- Whether gutters and downspouts are compatible.
- Whether the materials are suitable for your intended water use.
- Whether any required certification applies to the exact product and installation.
- How you will screen debris and manage first flush.
- How the stored water will be treated if it will be used for higher-contact purposes.
Designing the whole collection path at the same time is much easier than trying to correct incompatible materials after the roof and tank are installed.
Frequently Asked Questions
What is the best roofing material for collecting rainwater?
Smooth metal roofing is often one of the easiest materials for rainwater harvesting because it sheds water and debris well. However, coatings, flashing, fasteners, and gutters also need to be suitable for the intended use.
Can I collect rainwater from asphalt shingles?
Yes, asphalt shingle roofs are commonly used for non-potable rainwater collection. Their rough surface can hold more debris, and runoff quality can vary by product and roof condition. Use good screening and consider first-flush diversion.
Can rainwater from a metal roof be used for drinking?
Not automatically. Even runoff from a suitable metal roof can contain germs, airborne pollution, animal waste, or contaminants from other system components. Drinking-water use requires appropriate collection materials, treatment, testing, maintenance, and compliance with local requirements.
Should I avoid copper gutters?
Copper can enter roof runoff, so copper gutters and downspouts deserve caution when rainwater is being collected for household, food-related, or drinking-water uses. A different compatible gutter material is often easier to manage.
Are clay tiles good for rainwater harvesting?
Clay tiles can be used for rainwater collection, especially for non-potable purposes. Their rough surfaces and joints may hold more dirt, moss, and organic matter than smooth roofing, so maintenance and pretreatment are important.
Does a first-flush diverter make any roof safe?
No. A first-flush diverter can remove some of the dirt and contaminants washed from a roof early in a storm, but it cannot correct unsuitable roofing materials or make untreated rainwater safe to drink.
Do I need a special roof coating for potable rainwater?
If a coating will contact water intended for drinking, choose a product specifically evaluated for rainwater catchment use rather than assuming an ordinary roof coating is suitable. Certifications such as NSF P151 can help identify evaluated catchment components.
