Roof Materials for Rainwater Harvesting
Your roof material is a critical factor in rainwater harvesting — it determines both how much water you can collect and whether that water is safe for potable use. Different roofing surfaces have different runoff coefficients (the percentage of rain that actually flows off the roof rather than being absorbed or lost to splash). The difference between the best and worst materials can mean over 9,028 gallons per year in harvest potential for a standard 1,500-square-foot roof.
Material Comparison
The table below ranks all six common roof materials by collection efficiency. The harvest column shows estimated annual collection from a 1,500-square-foot roof in a state receiving approximately 50 inches of rainfall per year (such as Georgia). Click any material for a detailed guide covering efficiency, potability, maintenance, and best practices.
| Material | Efficiency | Potable Safe | Annual Harvest (1,500 sq ft) |
|---|---|---|---|
| Metal Roofing | 95% | Yes | 42,881 gal |
| Slate | 90% | Yes | 40,622 gal |
| Clay Tile | 85% | Yes | 38,366 gal |
| Concrete Tile | 85% | Yes | 38,366 gal |
| Asphalt Shingle | 80% | No | 36,108 gal |
| Wood Shake | 75% | No | 33,853 gal |
Understanding Collection Efficiency
Collection efficiency (also called the runoff coefficient) represents the percentage of rainfall that actually flows off your roof and into your gutters. The remaining percentage is lost to absorption, surface tension, splash, and evaporation. Smooth, non-porous surfaces like metal achieve the highest efficiency (95%), while textured or porous materials like wood shake score lowest (75%).
This 20-percentage-point spread has real financial implications. On a 1,500-square-foot roof receiving 40 inches of annual rainfall, the difference between metal (95%) and wood shake (75%) is about 7,476 gallons per year. Over a 10-year system lifespan, that adds up to 74,760 gallons — a significant amount of free water.
Efficiency also varies with rainfall intensity. Light mist and drizzle may not produce enough runoff to overcome surface tension on rough materials, while heavy downpours exceed the absorption capacity of even porous surfaces. The efficiency figures we use represent averages across typical rainfall patterns.
Potable vs. Non-Potable Collection
If you plan to use harvested rainwater for drinking or cooking, your roof material matters critically. Metal, clay tile, concrete tile, and slate are considered safe for potable collection because they do not leach harmful chemicals into the water. Metal roofs (galvanized steel, aluminum, copper, or coated steel) are the gold standard for potable rainwater harvesting due to their high efficiency and inert surface.
Asphalt shingle and wood shake roofs are not recommended for potable water collection. Asphalt shingles contain petroleum-based compounds and granulated minerals that leach into runoff, particularly during the first flush after dry periods. Wood shake can harbor mold, mildew, and bacteria, and may be treated with preservatives or fire retardants that contaminate rainwater. These materials are perfectly suitable for non-potable uses like garden irrigation, lawn watering, toilet flushing, and general outdoor use.
Regardless of roof material, all potable rainwater systems require multi-stage treatment including sediment filtration, carbon filtration, and UV sterilization. The roof material determines whether this treatment can produce safe drinking water — it cannot compensate for chemical leaching from inappropriate surfaces.
Choosing the Right Roof for Rainwater Harvesting
For most homeowners, the roof material decision is made when the house is built or the roof is replaced — changing your roof solely for rainwater harvesting is rarely cost-effective. However, if you are planning a roof replacement, considering rainwater collection in your material selection can provide long-term benefits. Metal roofing has become increasingly popular for residential use, and its 40-70 year lifespan (compared to 20-30 years for asphalt shingle) partially offsets its higher initial cost.
If you already have an asphalt shingle or wood shake roof, you can still harvest effectively for non-potable uses — these materials represent 80% of US residential roofs, and millions of homeowners successfully collect rainwater from them for garden irrigation, lawn watering, and other outdoor applications. The key is matching your treatment level to your intended use: simple debris screening for outdoor use, sediment filtration for toilet flushing, and full multi-stage treatment for potable use (only with potable-safe roofing).