Rainwater harvesting (RWH) is the practice of collecting and storing precipitation for later use, offering a valuable, independent water source. This method is particularly useful for structures without conventional roof drainage, such as sheds, greenhouses, or temporary remote setups. Collecting water without existing gutters requires creative, non-permanent solutions to capture and redirect the natural flow of water. The goal is to maximize the collection efficiency while ensuring the stored water remains relatively clean for non-potable applications like irrigation or washing.
Direct Catchment from the Drip Line
The most straightforward method for gutter-less collection utilizes the natural drip line, which is the path water follows as it runs off a roof’s edge. Wide-mouth barrels or large food-grade containers can be positioned directly beneath overhangs where the heaviest runoff occurs. To improve collection efficiency and reduce splash loss, a makeshift conveyance system should be installed directly below the roof edge.
A temporary splash block or diverter can be created using half-cut PVC piping or flexible plastic sheeting. This material should be angled slightly and placed just under the roof’s edge to guide the flow of water into the container’s opening. For systems requiring greater capacity, intermediate bulk containers (IBCs) holding 275 to 330 gallons are a common choice, which must be placed on a sturdy, level surface to support the significant weight of the filled tank. A simple mesh screen should cover the container opening to filter out large debris like leaves and insects.
Ground-Based and Surface Collection
Collecting water is also possible in areas lacking any permanent roof structure by utilizing ground-based and surface collection methods. A large, heavy-duty tarp or plastic sheeting can serve as a temporary catchment surface due to its lightweight and versatile nature. The sheeting must be stretched taut and secured at an angle, directing all collected rainfall toward the lowest point, where a container is positioned to receive the focused runoff.
Even small tarps, such as a 9-by-9-foot sheet, can collect a significant volume of water, potentially gathering around 36 gallons during a sufficient rain event. Another approach involves creating shallow trenches or swales in the ground, which should be lined with clean plastic to form a temporary reservoir or channel. This lined depression can funnel runoff from a wider area into a central, covered storage tank, which works particularly well for collecting rainfall from gentle slopes.
Necessary Filtration and Storage Components
Regardless of the collection method, a pre-storage treatment system is necessary to maintain water quality. The initial rainfall, known as the “first flush,” washes accumulated contaminants such as bird droppings, dust, and pollen off the collection surface. Diverting this initial, dirty water is accomplished using a first flush diverter, a mechanism that temporarily separates the first volume of runoff from the clean storage tank.
A simple DIY first flush diverter can be constructed from a vertical length of three-inch diameter PVC pipe connected to the collection line via a T-junction. The length of this pipe determines the volume of water diverted, with a general recommendation to divert the first five to ten gallons, depending on the size of the catchment area. Once the diverter pipe fills, a buoyant object, such as a sealed float or a plastic bottle, rises to plug the inlet, forcing the cleaner subsequent rain into the main storage system.
Storage containers must be tightly sealed, constructed of food-grade plastic, and stabilized with UV inhibitors to protect against degradation from sunlight. Dark-colored containers are preferable because they limit light penetration, inhibiting the growth of algae within the stored water. Elevating the storage container on cinder blocks or a sturdy platform uses gravity to create the necessary water pressure for simple spigot use or a gravity-fed irrigation system.
Health and Regulatory Guidelines
Rainwater collected from rooftops is considered non-potable and should not be consumed for drinking, cooking, or bathing without proper purification and testing. Even when visually clear, collected water can contain various contaminants, including germs and chemicals leached from roofing materials, airborne pollutants, or stored debris. For most systems, the collected water is best reserved for non-potable outdoor uses, such as garden irrigation, landscape watering, or washing vehicles.
If a dual plumbing system is used for indoor non-potable applications, such as toilet flushing, the rainwater system must be kept entirely separate from the potable household water supply to prevent cross-contamination. Individuals must investigate local regulations before installing any collection system, as rainwater harvesting laws vary significantly across states and municipalities. While many regions encourage rainwater collection, some jurisdictions impose restrictions on the maximum storage volume or require permits for systems exceeding a certain size.