Rainwater harvesting is the practice of collecting and storing precipitation that falls onto a surface, typically a building’s rooftop, for later use. This process allows homeowners to capture a valuable resource that would otherwise become stormwater runoff, reducing reliance on municipal or well water sources. Setting up a basic home system involves channeling rainfall from the roof into a dedicated storage container, providing a sustainable source for non-potable applications like irrigation. This guide outlines the necessary planning, components, installation steps, and safety measures for establishing a functional rainwater collection system.
Initial Planning and Legal Considerations
Before purchasing any equipment, every prospective collector must first investigate local ordinances and state laws concerning rainwater usage. While rainwater harvesting is now permitted in all fifty U.S. states, regulations vary widely, often imposing restrictions on system size or the intended use of the collected water. For instance, some jurisdictions may limit the total storage capacity to a small volume, such as 110 to 360 gallons, for non-potable residential use before requiring a permit.
The next planning step involves sizing the system to match both the available supply and the expected demand. The potential collection volume is approximated by multiplying the roof’s horizontal square footage by the average rainfall. One inch of rain falling on a 1,000 square foot roof yields approximately 623 gallons, though efficiency factors (75% to 85%) must be applied. This calculation determines the appropriate storage tank size, ensuring it meets needs during dry spells but prevents stagnation.
Essential Components of a Catchment System
A functional rainwater harvesting setup relies on four primary physical components working in sequence. The system begins with the catchment surface, most often the home’s roof, which should be made of a non-toxic material like metal or certain shingles to minimize contamination. Water then moves through the conveyance system, consisting of properly sloped gutters and downspouts that channel the collected rainfall toward the storage unit.
The pre-storage filtration system is the most contamination-sensitive component and typically includes a leaf screen and a first-flush diverter. A leaf screen sits at the top of the downspout to block large debris like leaves and twigs. The first-flush diverter is a specialized mechanism that automatically routes the initial pulse of rainfall—which contains the highest concentration of contaminants like dust and pollutants—away from the main storage tank.
The storage container, or cistern, holds the treated water and may be an above-ground barrel or a large subterranean tank. These containers should be opaque, made from food-grade plastic or other approved materials, and completely sealed to prevent sunlight exposure. Blocking light is a design requirement that inhibits the growth of algae, which would otherwise degrade water quality and potentially clog downstream components.
Step-by-Step System Installation
Installation begins by selecting a stable, level site for the storage container, preferably near a downspout that handles a significant portion of the roof’s runoff. Since a full tank can weigh several tons (a 500-gallon tank weighs over two tons), the container must be placed on a solid foundation, such as a concrete slab or compacted gravel pad, and secured to prevent movement.
Next, the downspout is modified to interrupt the flow and divert the water into the system components. This typically involves cutting the downspout and installing a connection kit or a specialized rainwater diverter that feeds directly into the pre-filtration mechanisms. The first-flush diverter is plumbed in line, often as a vertical section of pipe, to physically separate the initial, dirty water from the cleaner flow that follows.
The primary inlet pipe from the diverter is connected to the storage tank, often entering through a screened basket at the top to catch any remaining fine debris. An overflow pipe must be installed near the top of the cistern to safely manage excess water during heavy rain events, preventing damage to the system and foundation flooding. This overflow line should be the same size as the inlet pipe to handle the maximum flow rate and must be directed away from the building’s foundation, preferably into a drainage area.
Ongoing Maintenance and Water Safety
Consistent maintenance is required to ensure the system remains efficient and the collected water is safe for its intended non-potable use. The catchment surface and gutters should be inspected and cleaned at least twice a year, or more frequently if there are overhanging trees, to remove accumulated debris that can contaminate the water. Clogged gutters reduce the volume of water collected and can cause water to pool, creating a breeding ground for insects.
The first-flush diverter needs to be regularly emptied and rinsed to ensure it is ready to divert the next batch of initial runoff. Filters, such as leaf screens and fine-mesh filters at the tank inlet, must be checked monthly, especially during the rainy season, to prevent blockages that could impede water flow. These pre-filtration elements are the first line of defense against sediment accumulation and biological contamination.
Periodic internal cleaning, perhaps every 15 to 20 years, is necessary to remove sediment (sludge) that settles at the bottom of the sealed storage tank. Sealing the entire system is paramount for public health, as the tank’s lid, inlet, and overflow pipes must all be screened and tightly sealed to prevent access by mosquitoes and other pests. If the water is intended for any application that could involve human contact, like vegetable gardening, occasional testing for bacterial indicators, such as E. coli, is a prudent safety measure.