The Critical Answer: Safety Depends Entirely on Roofing Material Behaviour
In Kenya, where water scarcity affects approximately 32% of the population and drought conditions cover 80% of the territory, rainwater harvesting has become more than just an alternative water source—it's a lifeline. With regions like Bomet County receiving over 1,200mm of rainfall annually, the potential for roof-based water collection is substantial. But a fundamental question remains: Can harvested rainwater be safely consumed, or is it only suitable for non-potable uses? The answer lies entirely in understanding how different roofing materials behave when they come into contact with rainwater.
The Reality of Rainwater Quality in Kenya
Research conducted in Nairobi's Embakasi area by the Kenya Engineer examined rainwater harvested from three roofing materials—corrugated iron sheets, clay tiles, and concrete tiles. The findings were revealing: all samples collected after the first run-off met World Health Organization (WHO) chemical and microbiological guidelines. However, turbidity levels exceeded the 1mg/L maximum allowable concentration for drinking purposes, indicating that all harvested rainwater requires some level of treatment, particularly chlorination, before consumption.
This aligns with Kenya's drinking water standard KS 459-1:2007, which sets clear parameters for potable water quality. The research established that while roof-harvested rainwater can be safe for domestic uses including laundry, toilet flushing, bathing, and general cleaning, achieving drinking water standards requires additional intervention.
Material Behaviour: The Decisive Factor
Corrugated Iron Sheets (Mabati): The Most Common Choice
Galvanized corrugated iron sheets represent Kenya's most widespread roofing material and are generally considered safe for rainwater harvesting. Research shows that rainwater collected from these roofs is relatively safe for domestic uses. However, material behaviour introduces an important consideration: zinc leaching.
Galvanized iron sheets are coated with zinc to prevent rust. When rainwater—which is naturally slightly acidic—contacts this surface, zinc can leach into the water. Studies on galvanized roofing materials show that zinc concentrations in harvested rainwater can exceed recommended levels, particularly from newer roofs or during the initial rainfall events.
The good news: zinc has low toxicity to humans and is actually an essential micronutrient in small amounts. The concern: excessive zinc can impart a metallic taste and, in large quantities over extended periods, may pose health risks. Testing harvested water for zinc levels provides certainty, and allowing first flush diversion helps minimize contamination.
Asbestos Roofing: A Critical Health Hazard
Kenya banned asbestos importation, manufacture, and use in 2006, yet many buildings constructed in the 1950s through 1970s still retain asbestos roofing. According to the 2023/24 Kenya Housing Survey, approximately 0.2% of houses nationwide still have asbestos roofs, with counties like Bomet (2.9%), Migori (1.4%), and Turkana (1.4%) showing higher percentages.
The material behaviour of asbestos cement roofing is particularly dangerous. Research published in 2022 analyzing asbestos roofing runoff found that water samples contained cadmium, iron, chromium, E. coli, and total and fecal coliform levels all exceeding WHO limits for drinking water. More concerning, the study estimated hazard quotients of 9.53 for children and 4.08 for adults via oral exposure to cadmium from asbestos roof runoff. The estimated cancer risks exceeded the 10⁻³ threshold, suggesting possible cancer health effects.
Recent studies from Hungary examining asbestos cement roof degradation documented a 75.4% increase in mercury concentration and a dramatic 127.3% increase in lead levels in harvested rainwater. Asbestos fibers released through weathering retain their carcinogenic potential, and contaminated rainwater poses severe health risks.
In large Nairobi residential estates including Bahati, Kimathi, and Ofafa Jericho, residents have historically collected drinking water from asbestos roofs. One resident told researchers: "Whenever it rains, we collect the rainwater, which saves us when taps run dry." This practice has contributed to health concerns, with Environment Cabinet Secretary Aden Duale correlating high cancer rates in Kenya to asbestos roofing exposure.
In August 2024, the Kenyan government issued a three-month directive for asbestos removal from all roofs. By January 2025, the Cabinet formally approved a nationwide asbestos removal initiative from all public and private facilities. The material is now classified as hazardous waste under Legal Notice No.121 of the Environmental Management and Coordination (Waste Management) Regulations 2006.
Verdict: Asbestos roofing is absolutely unsuitable for rainwater harvesting under any circumstances.
Stone-Coated Tiles: Proceed with Caution
Stone-coated tiles present a more nuanced situation. Genuine brands like Decra roofing tiles are considered safe for rainwater harvesting according to industry sources in Kenya. However, concerns exist about non-genuine stone-coated tile brands that do not guarantee water safety.
The material behaviour varies significantly between manufacturers. Without consistent quality standards and testing, property owners cannot assume all stone-coated tiles are equally safe for water collection intended for drinking purposes.
Clay Tiles and Concrete Tiles: Generally Safe Options
Research from Embakasi demonstrated that both clay tiles and concrete tiles produce harvested rainwater meeting WHO chemical and microbiological parameters (after first flush). These materials exhibit stable behaviour with minimal leaching of harmful substances.
However, the porous nature of clay and concrete means some water absorption occurs, slightly reducing collection efficiency compared to metal roofing. Their primary advantage lies in their inert material composition, which doesn't introduce toxic chemicals into harvested water.
Painted and Artificially Colored Roofs: Avoid for Drinking Water
Roofs with artificial coloring or certain paint coatings can leach toxic chemicals into harvested rainwater. The material behaviour of painted surfaces depends entirely on the paint composition, with some formulations containing heavy metals or other contaminants that render the water unsafe for consumption.
Thatched Roofs: Unsuitable for Harvesting
Traditional thatched roofing deteriorates over time, affecting both water quality and color. The rotting organic material introduces biological contamination, making these roofs completely unsuitable as catchment surfaces for rainwater harvesting.
Beyond Material: The First Flush Factor
Regardless of roofing material, the first flush of rainfall carries the highest contamination load. Dust, debris, bird droppings, and accumulated pollutants wash off during initial rainfall, making this water particularly unsuitable for consumption.
Kenyan rainwater harvesting experts strongly recommend first flush diversion systems. These simple mechanisms—either manual or mechanical—divert the initial rainfall away from storage tanks. Organizations like Dig Deep, working in Bomet County, employ hydraulic first flush systems as a safeguard, ensuring contaminated water doesn't enter storage tanks.
Research from Kisumu County specifically focused on developing first flush diversion systems for roof rainwater harvesting, recognizing this as a critical component for water safety. Major Kenyan water tank suppliers like Kentainers include first flush/rain diverters as standard components in their rainwater harvesting packages.
Storage Material Matters Too
While roofing material determines initial water quality, storage tank material affects long-term safety. Research examining water quality changes across different storage materials found quality deterioration following this pattern: steel tank stored rainwater showed the greatest deterioration, followed by plastic tank stored rainwater, with rainwater before storage showing the best quality.
The study concluded that water stored in both plastic and steel tanks ranged from "poor" to "unfit for drinking" without proper treatment and monitoring. This emphasizes that material behaviour extends beyond the roof to every component contacting the harvested water.
Treatment: The Non-Negotiable Requirement
Even with safe roofing materials and proper first flush systems, research consistently shows that harvested rainwater requires treatment before drinking. The turbidity levels found in all Kenyan studies exceeded drinking water standards, and bacterial contamination (E. coli and total coliforms) appeared across different roof types.
Chlorination represents the most accessible and effective treatment method for Kenyan households. Boiling provides an additional safeguard. More advanced filtration systems offer enhanced protection but come with higher costs.
Testing: Know What You're Drinking
For households using harvested rainwater as drinking water, testing provides essential information. The Nairobi City Water and Sewerage Company (NCWSC) Kabete Laboratories and other certified facilities conduct water quality analysis according to Kenya Standards for Drinking Water (KS459-2007).
Testing parameters should include:
- Physical: pH, turbidity, color
- Chemical: heavy metals including zinc, lead, cadmium, iron, copper, aluminum
- Bacteriological: E. coli and total coliform count
Regional Advantages: Why Rainwater Harvesting Works in Kenya
Despite the material behaviour challenges, rainwater harvesting offers distinct advantages in many Kenyan contexts:
Superior to Alternative Sources: In regions like the Rift Valley, groundwater often contains high fluoride levels causing bone deformities. Borehole water frequently exhibits high salinity. By comparison, properly harvested and treated rainwater provides better baseline quality.
Reliability: Unlike boreholes requiring pumps that break down, gravity-fed rainwater systems need minimal maintenance. Research tracking failed water points across rural Africa shows tens of thousands of abandoned boreholes, while properly designed rainwater harvesting systems continue functioning.
Cost-Effectiveness: The average rural Kenyan household spends approximately Ksh 3,800 ($38) monthly coping with distant or unreliable water supplies, compared to Ksh 446 ($4.46) for piped water in Nairobi. Rainwater harvesting systems, once installed, dramatically reduce these ongoing costs.
The Bottom Line: Material Behaviour Determines Safety
Can a roof be safely used for rainwater harvesting, even for drinking? Yes—but only if material behaviour supports it:
Safe with Proper Treatment:
- Galvanized corrugated iron sheets (monitor for zinc)
- Genuine stone-coated tiles (verify manufacturer)
- Clay tiles
- Concrete tiles
Absolutely Unsafe:
- Asbestos roofing (banned, hazardous)
- Thatched roofing (biological contamination)
- Painted/artificially colored roofs (potential chemical leaching)
Essential Requirements Regardless of Material:
- First flush diversion system
- Regular roof cleaning and maintenance
- Proper storage tank material and maintenance
- Water treatment (minimum: chlorination)
- Periodic water quality testing
Policy and Future Directions
Kenya's 2016 Water Act prioritizes rainwater harvesting regulation, and the National Water Harvesting and Storage Authority has been proposed to encourage storage tank programs. However, challenges remain:
- Lack of design standard drawings for ferro-cement tanks
- Insufficient policy guidelines for quality standards
- Limited public awareness of material-specific risks
- High upfront costs for proper systems
The 2024-2025 government directive to remove all asbestos roofing represents a critical public health measure. As this material is eliminated from Kenya's building stock, the overall safety of rainwater harvesting will improve significantly.
Practical Recommendations for Kenyan Households
- Assess Your Roof: Identify your roofing material. If asbestos, prioritize replacement following NEMA guidelines.
- Install First Flush Systems: Don't skip this component. The most contaminated water must be diverted.
- Test Your Water: Especially important for galvanized roofs and stone-coated tiles. Testing reveals actual contamination levels rather than relying on assumptions.
- Treat Before Drinking: Chlorination is non-negotiable. Consider boiling during disease outbreaks.
- Maintain Regularly: Clean gutters, inspect roofs, check storage tanks for algae or contamination.
- Consider Your Context: Rainwater harvesting makes excellent sense in areas with adequate rainfall, poor groundwater quality, or unreliable piped systems. Urban areas with reliable municipal water might use harvested rainwater for non-potable purposes, reducing costs and water stress.
Conclusion
The safety of roof-harvested rainwater for drinking depends entirely on material behaviour. In Kenya's context, where water scarcity drives innovative solutions, understanding how roofing materials interact with rainwater determines whether families access safe drinking water or inadvertently expose themselves to health risks.
The evidence is clear: some materials, properly managed, can safely support rainwater harvesting for drinking. Others pose unacceptable risks that no amount of treatment can fully mitigate. As Kenya continues addressing its water challenges, informed material choices, proper system design, and consistent treatment protocols will determine whether rainwater harvesting fulfills its promise as a sustainable, safe water source for millions of Kenyans.
For more information on construction best practices and sustainable building in Kenya, visit makaobora.com
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