Rainwater Catchment FAQ

To meet our region’s rising demand for fresh water we need to both conserve water and develop alternative water supplies. People have been capturing and storing rainwater for thousands of years, in part, because rainwater is usually the cleanest source for fresh water. Rainwater catchment systems can be as simple as a rain barrel placed under a downspout to store water for a few roses, or large enough to supply an entire garden and orchard with water during Southern Oregon’s dry summer growing season.

Is it legal to harvest rainwater in Oregon and Jackson County?

Yes! Rain falling on your roof and stored in a barrel or tank belongs to you. The county or municipality where you live may require a building permit.

How much rain do we get in Jackson County?

The county has many micro-climates, so rainfall varies quite a bit. The average rainfall recorded in Medford is about 18” per year, but parts of the county get a little less, and some parts receive twice as much!

How many gallons of rainwater can my roof collect?

About 6/10ths of a gallon per square foot of roof surface. Multiply your roof size by .623, and then multiply that number by the average number of inches of rainfall you receive. If you subtract a little—about 5%--for rain that blows or splashes off the roof, that’s how much water you could collect. It’s usually much more than people realize.

Does rainwater need to be filtered before use?

That depends on what how you plan to use the water you collect and store. If for irrigation only, you need to screen out leaves and other debris before it reaches the storage tank, and perhaps use a filter to keep particles from clogging drip system emitters. If the water is used for drinking, it needs to meet safe drinking water standards, and that usually requires a small particle filter and a UV filter.

How much does a rain water catchment system cost?

So much depends on your system complexity and storage tank size. A simple system involving a rain barrel and gravity used to water a few tomato plants can be installed for a few hundred dollars, assuming you already have gutters and downspouts. Larger storage, pumps, and filtration increase the cost considerably. Storage tanks are usually the most expensive component—accounting for about 70% of the total system cost. Ball park costs for tanks are about $1 per gallon of storage, although as tanks get larger, per gallon costs can be lower. It’s best to consult with a rainwater catchment systems designer to learn what’s possible for your situation.

How do I make the best use of rain that falls on my land?

Rainwater harvesting management practices include planting appropriate ground covers, grading roads and building sites, using swales and rain gardens to reduce runoff and erosion, and let rain soak into the soil to support plant life and replenish the aquifer. These are all very low cost-per-gallon harvesting methods. A rainwater catchment system that harvests water from a suitable surface like a roof allows you to store water for later use.

What’s the best tank material for water storage?

Polypropylene and vinyl lined steel tanks are commonly used for rainwater. They go up in an hour or a day—which is very convenient. The poly tanks are rated for about 30 years, and may last longer if protected from sunlight under a deck or roof. Steel tanks can last for a very long time, but the vinyl liners need periodic replacement. These tanks are almost always designed for above ground storage, although reinforced poly and fiberglass tanks can be used below grade. In most cases the cost per gallon decreases as tank size increases…up to a point where transportation of a large tank becomes very costly. From a material perspective, a ferro cement tank often has a lower cost per gallon than comparable polypro tanks, fiberglass tanks, or vinyl lined steel tanks. If you have lots of time and want to save money, making ferro cement tanks of various sizes and shapes can be cost effective—and you can pretty much design any size or shape you want up to 30,000 gallons without engineering. You can also purchase pre-made 2000 gallon concrete water tanks that measure about 12’ x 8’ x 6’. Masonry tank materials like concrete and ferro cement may leak, but the leaks tend to be minor and easily repaired. In ancient times people used mortared stone and brick tanks lined with clay and lime plasters, and although they also tended to leak, they worked pretty well!

How do you access a tank or cistern for maintenance?

Accessibility has two aspects—the outside of the tank for inspection, and the inside of the tank, for inspection and cleaning. The access hatch needs to be locked as a safety precaution.

What are environmental impacts of each tank material’s manufacture and eventual disposal?

There isn’t an easy answer for this—every tank storage material has a down-side, whether in manufacture or disposal. All the plastic tanks are derivedfrom petroleum.  Steel tanks have a vinyl plastic liner, and both steel and vinyl have their own environmental stories. Concrete is essentially human-made stone. The Portland cement portion of concrete or ferro cement tank (about 1/6th by weight) has a very high energy footprint (the rest is sand and gravel), but concrete can last indefinitely. These tanks are steel reinforced to resist cracking due to settling or seismic events, and cracks can be repaired. The most common water sealing material that coats the inside of a concrete or ferro cement tank is a lime-cement-acrylic plaster or paint. That too has some environmental cost. None of these materials are designed to be “disposable,” so while they all carry a high environmental cost, with care, they all last a very long time.

What is the best roof to collect rainwater from?

Short answer: the material that will deposit the least amount of crud into your system as the rain runs off! Painted or galvanized steel roofs usually deliver the cleanest water to the system’s filtration system. You’ll still need to screen out debris (leaves, pollen, dead bugs and birds, etc.) with gutter guards or a roof washer, and a first flush device can divert and dispose of most of the nitrates and microbes that accumulate on your roof. Steel roofs are considered the best since they don’t release roof particulate into the tank like composite roofs, and there are fewer nooks and crannies for moss and bacteria to grow on the roof surface, which can be a problem with tile and shake roofs.

How much water do people use?

In North America most households use over 100 gallons of water per person per day. Even with water efficient appliances and practices, washing dishes, clothes, flushing toilets, taking showers, and irrigating lawns and gardens adds up! How will you use the rainwater you store? If it’s only for seasonal gardening, or to have on reserve to fight a fire, you need much less than if you want rainwater to supply all of your water needs. For example, a twenty-tree orchard and a half-acre garden can use between 20,000 and 30,000 gallons of water in a season, depending on irrigation techniques and efficiency. Smaller gardens need less water.

Which is better—one large tank or several smaller ones?

Both can be very effective components of a rainwater catchment system, and both have advantages. There are places where a single tank is best, and other times when multiple tanks do a better job. That said, multiple tanks let you isolate a problem should one tank develop a leak, or allow one to be a “settling tank” for the system, where debris that gets past your first flush screens can accumulate for periodic cleanout, passing cleaner water on to the other tanks. Multiple tanks linked in series also allow people to add storage as they can afford, or as their needs grow.

What is the return of investment for a rainwater catchment system?

That depends on what you put into the calculation. A property owner with irrigation rights or access to a good well or city water might wait for a long time before a simple rainwater harvesting system pays for itself. It could be thirty years or longer! On the other hand, should water rates shoot up, or if the water master cuts back on allotments, or electricity prices increase, or the well goes dry or becomes contaminated, the rainwater payback comparison looks better. Where city water rates are high, or where people must truck in water, a rainwater catchment system could pay for itself in five or seven years. For most of us who consider factors like water security and independence, disaster resilience, the likely energy cost increase to run pumps, environmental cost of lowering a groundwater table from over pumping--rainwater harvesting makes sense as a supplemental or back-up system.