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For most rainwater harvesting systems, a smooth metal roof is one of the best choices. Metal sheds water quickly, does not absorb much water, and is fairly easy to keep clean. It also tends to hold less dirt and organic material than rough roofing.
That does not mean every metal roof is suitable. Roof coatings, flashing, gutters, nearby chimneys, and the intended use of the water all matter. Rainwater can pick up germs and chemicals from any roof, so roof material is only one part of a safe collection system.
Best Roof Types for Rainwater Harvesting
A good rainwater catchment roof should have a smooth, durable surface that drains well and does not add unwanted chemicals to the runoff.
Metal Roofing
Metal roofing is often the best all-around choice for rainwater harvesting.
Common forms include:
- Standing-seam metal roofing
- Corrugated metal roofing
- Galvanized steel roofing
- Zinc-aluminum coated steel roofing
- Prepainted steel roofing with a suitable coating
Metal has several useful traits. Water runs off quickly. Leaves are less likely to stick to it. The surface is easy to inspect, and it usually produces less loose material than asphalt shingles.
Government rainwater guidance commonly lists metal roofing among suitable catchment surfaces.
However, check the entire roof rather than just the main roofing panels. Avoid using catchment areas that contain lead-based paint, unsuitable coatings, lead flashing, or other materials that could contaminate the water.
Clay and Terracotta Tile
Clay and terracotta tile can also work well.
The surface is durable, and water normally drains easily when the roof has a good slope. Tile roofs may collect more dust, moss, and organic material in joints and uneven areas than smooth sheet metal.
That means regular inspection and gutter cleaning become more important.
Terracotta tile is included among roof materials commonly used for rainwater collection.
Concrete Tile
Concrete tile can be used for rainwater harvesting, but it is usually not as simple to maintain as smooth metal.
Its rougher surface can hold:
- Dust
- Moss
- Pollen
- Bird droppings
- Small pieces of organic debris
New concrete tile may also affect early runoff. Some health guidance recommends discarding the first few runoff events from new concrete tile roofs.
Once established and properly maintained, a concrete tile roof can still provide useful water for many rainwater systems.
Slate
Slate is another good catchment surface.
It is hard, durable, and does not absorb much water. A slate roof can work well when it is clean and in good condition.
The joints and roof shape still need attention because leaves and debris can collect in valleys and around roof penetrations.
Polycarbonate and Fiberglass Roofing
Some polycarbonate and fiberglass sheet roofs can be used for rainwater collection. They are common on sheds, patios, greenhouses, and small outbuildings.
Before using them, identify the exact roofing material and any coating on it. This becomes more important when the water will be used for anything involving people, animals, or food.
Western Australian health guidance, for example, lists polycarbonate and fiberglass sheeting among roof surfaces from which rainwater can be collected.
Roof Types That Need More Caution
Some roofs can collect water but are less desirable because they hold debris, shed material, or may contain substances you do not want entering the tank.
Asphalt Shingles
Asphalt shingles are common, but they are usually not my first choice when designing a rainwater system from scratch.
Their rough surface can trap dirt and organic material. Shingles can also shed small mineral granules that end up in gutters, first-flush devices, filters, and tanks.
For simple non-potable uses such as ornamental garden irrigation, an existing shingle roof may still be useful if the roofing manufacturer and local guidance allow the intended use.
For drinking-water systems, do not assume that runoff from asphalt shingles is suitable just because it looks clear. The roofing material and whole treatment system need to be evaluated for that use.
Wood Shakes and Shingles
Wood roofs are generally more difficult rainwater catchments.
Their rough surface can hold moisture, dirt, leaves, moss, and animal waste. Some wood roofing may also contain preservatives or other treatments.
Rainwater intended for drinking should not be collected from preservative-treated timber roofing.
Bitumen-Coated Roofs
Avoid roof catchments coated with bituminous or tar-based products when water quality matters, especially for drinking-water systems.
Several public health and building authorities specifically warn against collecting drinking water from these surfaces.
Roofs With Lead Paint or Lead Flashing
Do not overlook flashing.
Flashing is the waterproof material installed around chimneys, walls, vents, and other roof joints.
A roof may have suitable metal or tile panels but still contain lead flashing. Lead-based paint is another concern.
Lead can enter runoff and should not be part of a roof catchment intended to supply drinking water.
If you do not know what an older roof or flashing contains, identify the material before relying on the runoff for higher-quality uses.
Does Roof Shape Matter?
Yes, although roof material usually gets more attention.
A moderately sloped roof generally makes rainwater harvesting easier because gravity moves water toward the gutters quickly.
The important features are:
- Water drains completely
- Roof valleys do not constantly hold leaves
- Gutters can handle runoff
- Downspouts can be routed to the tank
- There are few places where dirty water can pool
A simple gable or shed roof can be especially easy to work with because runoff is concentrated along predictable roof edges.
A complicated roof with many valleys can still harvest rainwater, but it may require more gutters, downspouts, screens, and cleaning.
Can You Harvest Rainwater From a Flat Roof?
Yes, but drainage deserves extra attention.
A flat roof is normally built with at least some drainage slope, even if it looks flat from the ground.
Problems develop when water sits on the roof. Ponding can allow dust, leaves, animal waste, algae, and other material to remain in contact with the water longer.
Before using a flat roof as a catchment, make sure:
- Drains work correctly
- Water does not remain pooled for long periods
- The roofing membrane is suitable for the intended water use
- Drains and strainers can be cleaned
- Overflow paths are adequate
Do not assume a waterproof roofing membrane is automatically suitable as a drinking-water catchment.
A Clean Roof Matters as Much as Roof Material
Even an ideal metal roof can produce poor-quality runoff if it is covered with contamination.
Rainwater can wash material from the roof into your tank, including:
- Bird and animal droppings
- Dust
- Leaves
- Pollen
- Insects
- Soot
- Airborne chemicals
CDC guidance notes that rainwater can pick up germs and chemicals both from the air and from roofing, gutters, piping, and storage materials.
Consider selecting the right roof is most effective at collecting rainwater to assess the catchment surface’s effect on usable collection volume.
Trees also make a difference. Heavy tree cover can mean more leaves, seeds, insects, bird activity, and organic matter on the roof.
Keeping branches away from the catchment area can reduce this load.
Watch for Chimneys and Roof-Mounted Equipment
The best roof surface may still have sections you should leave out of the collection system.
Be cautious about runoff near:
- Wood-burning chimney flues
- Roof-mounted heating equipment
- Evaporative coolers
- Equipment discharge pipes
- Chemically treated timber
- Areas exposed to chemical spraying
Public health guidance recommends avoiding some of these areas because contaminants can enter the roof runoff.
You may be able to collect from another roof section rather than abandoning the entire roof.
Use a First-Flush Diverter
No roof stays perfectly clean between storms.
A first-flush diverter sends the first portion of runoff from a storm away from the storage tank. That first water tends to pick up a high amount of dust, droppings, and other material sitting on the roof.
After the first flush is diverted, later runoff is allowed into the tank.
CDC recommends considering a first-flush device to improve collected rainwater quality. The amount diverted should be matched to the roof catchment rather than chosen at random.
A first-flush diverter helps reduce contamination. It does not make rainwater safe to drink by itself.
Add Screens Before Water Reaches the Tank
A roof works best as part of a complete collection path:
roof → gutter → leaf screen → first flush → tank inlet → storage
Leaf screens or rain heads remove larger material before it gets farther into the system.
They can catch:
- Leaves
- Twigs
- Seeds
- Large insects
- Pieces of roof debris
The tank inlet and overflow should also be screened to reduce insect and animal entry.
These simple stages reduce tank sediment and make later maintenance easier.
Match the Roof to How You Will Use the Water
There is no single roof standard that fits every rainwater use.
For ornamental garden irrigation, you may accept a catchment that you would not choose for household water.
As the chance of people swallowing the water increases, the system needs more careful material selection, treatment, testing, and maintenance.
For example:
Garden Water
A wide range of clean roof surfaces may work for watering ornamental plants.
You should still avoid obvious chemical contamination because those substances can enter your soil or irrigation system.
Toilet Flushing and Outdoor Cleaning
Water quality requirements are higher than they are for simply filling a decorative pond, but these uses are still generally considered non-potable.
Non-potable means water that is not intended for drinking.
Plumbing separation and backflow protection may be required when rainwater plumbing is installed near a public drinking-water system. Requirements vary by location.
Drinking and Cooking
Roof type alone cannot make rainwater potable.
Potable means suitable for drinking.
CDC says rainwater is not necessarily safe to drink because it can contain germs and chemicals even when it looks clean. If rainwater is used for drinking, cooking, or bathing, CDC recommends regular testing for relevant germs and chemicals and treatment suited to the contaminants present.
A drinking-water rainwater system should be treated as a whole system involving:
- Suitable roof and gutter materials
- Clean collection surfaces
- Leaf and debris control
- First-flush diversion
- Suitable storage
- Appropriate treatment stages
- Regular maintenance
- Current laboratory testing
- Applicable local health and plumbing requirements
A single filter or UV unit does not prove that roof runoff is safe to drink.
What Is the Best Roof if You Are Building From Scratch?
If rainwater harvesting is an important part of a new building, a simple sloped metal roof made with catchment-compatible materials is usually a strong choice.
Aim for a design with:
- Large, easy-to-clean roof areas
- Simple drainage
- Few valleys
- Accessible gutters
- Enough downspouts for the roof
- No lead flashing
- No unsuitable roof coatings
- No treated wood draining into the catchment
- No chimney contamination over the collection area
You do not need a special-looking "rainwater roof." A simple roof that drains cleanly and can be inspected and maintained is usually better than a complicated roof with many collection points.
Roof Area Determines How Much Water You Can Catch
Roof material affects runoff quality and efficiency, but catchment area and rainfall determine how much water is available.
A large tile roof may collect much more water overall than a tiny metal shed roof simply because it has more catchment area.
Actual collection is always lower than the theoretical amount of rain falling on the roof. Some water is lost through:
- First-flush diversion
- Splash
- Gutter overflow
- Wetting of the roof
- Leaks
- Debris
- Tank overflow
So when planning storage capacity, do not size the tank based only on roof material.
The Best Practical Choice
For most new rainwater harvesting systems, smooth metal roofing is the easiest roof type to work with. Slate, clay tile, concrete tile, and some sheet roofing materials can also perform well.
Whatever roof you have, look beyond the main surface. The quality of your collected water also depends on flashing, paint, gutters, nearby trees, chimneys, roof-mounted equipment, first-flush control, tank design, and maintenance.
For drinking-water use, take the conservative route. Confirm that every catchment material is suitable, use appropriate treatment, maintain the whole system, arrange current laboratory testing, and follow your local health and plumbing requirements.
Frequently Asked Questions
Is a metal roof better than shingles for rainwater harvesting?
Usually, yes. Smooth metal drains quickly, is easy to inspect, and tends to shed less loose material than asphalt shingles. The metal, coating, flashing, and gutters still need to be suitable for the intended use.
Can I collect rainwater from an asphalt shingle roof?
An existing asphalt shingle roof may be usable for some non-potable purposes, depending on the roofing material and intended use. For drinking-water use, do not assume shingle runoff is suitable without checking the roofing materials and evaluating the complete treatment and testing system.
Can I collect rainwater from a tile roof?
Yes. Clay, terracotta, concrete, and other suitable tile roofs can be used for rainwater harvesting. Their rough surfaces and joints may require more cleaning than smooth metal.
Can I harvest rainwater from a flat roof?
Yes, if it drains properly and the roof membrane is suitable for the intended water use. Avoid areas where water regularly ponds or collects heavy debris.
Does a first-flush diverter make roof water safe to drink?
No. A first-flush diverter removes some of the dirt and contamination carried by the first runoff from a storm. It does not remove every germ or chemical and does not make rainwater potable by itself.
Is rainwater from a metal roof safe to drink?
Not automatically. Rainwater can contain germs and chemicals from the air, roof, gutters, pipes, and storage system. Drinking-water use requires suitable collection materials, treatment, maintenance, current testing, and compliance with applicable local requirements.
Should I replace my roof just to harvest rainwater?
Usually not if the existing roof is in good condition and suitable for your intended use. It often makes more sense to improve gutters, screens, first-flush diversion, storage, and maintenance. If you are already replacing the roof, rainwater harvesting can be considered when choosing the new roofing and flashing materials.
