The material of your roof matters when you are collecting rainwater. Different roofing surfaces release different substances into the water that runs off them, and some of those substances can affect the taste, smell, or safety of the water.
This guide explains how common roofing materials can affect water quality. It does not tell you which materials are safe for drinking water, because that depends on your specific roof, its condition, your local environment, and the quality of the water itself. The only reliable way to know if your collected rainwater is safe to drink is to have it tested.
How roofing materials affect water quality
When rain runs off a roof, it picks up small amounts of material from the roof surface. The type and amount of material depends on:
- The roofing material itself. Different materials release different substances into the water.
- The age and condition of the roof. Older or damaged roofs may release more material than new, well-maintained roofs.
- The climate and environment. Roofs in coastal areas, for example, may pick up salt and other airborne contaminants. Roofs near industrial or agricultural sources may pick up other pollutants.
- The amount of rain. A heavy storm will wash off more material than a light drizzle.
The substances that can be released from roofing materials include metals, organic compounds, and other chemicals. Some of these are harmless in the small amounts that are released from a roof. Others can affect the taste or safety of the water.
Common roofing materials
Corrugated iron (steel)
Corrugated iron is one of the most common roofing materials in New Zealand. It is a metal roof, and like all metal roofs, it can release small amounts of metal into the water that runs off it. The amount released depends on the condition of the roof and the type of coating it has.
If the roof is in good condition with an intact coating, the amount of metal released is typically small. If the roof is old, rusted, or has a damaged coating, more metal may be released, which can affect the taste of the water and, in some cases, its safety.
VERIFY: whether corrugated iron roofs are generally considered suitable for collecting drinking water, and what levels of metal release are acceptable — check a health authority’s rainwater guidance, BRANZ, or a water testing lab before stating a specific verdict.
Concrete tiles
Concrete tiles are another common roofing material in New Zealand. They are made from cement, which is alkaline. When rain runs off a concrete tile roof, it can pick up small amounts of alkaline substances, which can raise the pH of the water. This can give the water a slightly bitter or soapy taste, which some people notice.
The amount of alkaline substance released depends on the age and condition of the tiles and the amount of rain. Newer tiles may release more than older tiles that have been weathered.
VERIFY: whether concrete tile roofs are generally considered suitable for collecting drinking water, and what levels of alkalinity are acceptable — check a health authority’s rainwater guidance, BRANZ, or a water testing lab before stating a specific verdict.
Asphalt shingles
Asphalt shingles are less common in New Zealand but are used on some properties. They are made from a combination of asphalt, fiberglass, and other materials. When rain runs off an asphalt shingle roof, it can pick up organic compounds and other substances from the asphalt and other materials.
These substances can affect the taste and smell of the water, and in some cases, their safety. The amount released depends on the age and condition of the shingles and the amount of rain.
VERIFY: whether asphalt shingle roofs are generally considered suitable for collecting drinking water, and what levels of organic compound release are acceptable — check a health authority’s rainwater guidance, BRANZ, or a water testing lab before stating a specific verdict.
Slate
Slate is a natural stone roofing material that is durable and long-lasting. It is generally less reactive than other roofing materials, meaning it is less likely to release significant amounts of substances into the water that runs off it.
However, the specific type of slate and its condition can affect the amount of material released. Older or damaged slate may release more than new, well-maintained slate.
VERIFY: whether slate roofs are generally considered suitable for collecting drinking water, and what levels of material release are acceptable — check a health authority’s rainwater guidance, BRANZ, or a water testing lab before stating a specific verdict.
Metal (aluminium, zinc, or other alloys)
Metal roofs made from aluminium, zinc, or other alloys are becoming more common in New Zealand. Like all metal roofs, they can release small amounts of metal into the water that runs off them. The amount released depends on the specific alloy, the type of coating, and the condition of the roof.
Some alloys or coatings may release more metal than others. The water may need to be tested to confirm it is safe for drinking, especially if the roof is new or the coating is unknown.
VERIFY: whether metal alloy roofs are generally considered suitable for collecting drinking water, and what levels of metal release are acceptable — check a health authority’s rainwater guidance, BRANZ, or a water testing lab before stating a specific verdict.
What to do if you are unsure
If you are not sure whether your roof is suitable for collecting drinking water, the safest approach is to have the water tested. A water testing lab can check for common contaminants, including metals, bacteria, and chemical compounds. The results will tell you whether the water is safe for drinking or whether it needs treatment.
VERIFY: the specific contaminants that should be tested for when assessing rainwater from a particular roofing material, and whether any standard or health authority guidance specifies a testing protocol — check a water testing lab’s recommendations or a health authority’s guidance on rainwater quality before stating a specific list of tests.
You can also ask your supplier or a plumber for advice on whether your roofing material is suitable for drinking water collection. They may be able to tell you whether the material is generally considered safe and whether any additional treatment is needed.
Treatment options
Even if your roof is a suitable surface for collecting drinking water, the water may still need treatment before it is safe to drink. Common treatment options include:
- Filtration — removes particles, sediment, and some contaminants.
- UV treatment — kills bacteria and other microorganisms.
- Chlorination — adds a small amount of chlorine to kill microorganisms.
- Aeration — helps remove volatile organic compounds and improves the taste of the water.
The right treatment depends on the quality of the water and the intended use. If you are using rainwater for drinking, it is worth investing in a treatment system that is appropriate for your water quality.
Summary
- Different roofing materials release different substances into the water that runs off them.
- The amount of material released depends on the material, the condition of the roof, the local environment, and the amount of rain.
- Some substances can affect the taste, smell, or safety of the water.
- The only reliable way to know if your collected rainwater is safe to drink is to have it tested.
- Even with a suitable roof, the water may need treatment before it is safe to drink.