Special Reports
Tips on Capturing Rainwater

By Philip S. Wenz
May 31, 2008

Until recently, many cities have required that homeowners connect their
roof downspouts to their municipal storm water sewers to control flooding
and to deliver rainwater - which is polluted with roof dirt and bird feces
- to sewage treatment plants. But as cities have grown, their storm
sewers, which are often combined with sanitary sewers, are becoming loaded
to capacity and can flood during heavy rains, dumping sewage into streets
and waterways.

Meanwhile, the threat of a long-term drought hangs over much of
California, including the Bay Area.

These flood/drought conditions are leading to a reversal of previous
policies, and many cities, including San Francisco, are now encouraging
homeowners to disconnect their downspouts - and reconnect them to rain
barrels or larger storage tanks that retain water for landscaping use.
These "rain catchment" systems reduce sewer loads, conserve water and can
provide emergency water if an earthquake disrupts the public water supply.
(Stored rainwater is not safe to drink, but small quantities can be
treated with water purification tablets during emergencies.)

Although rain barrels, if widely used, will help relieve sewer overloads
and are relatively inexpensive and easy to install, they don't store
enough water to last for long in the Bay Area's dry season. Storage tank
systems, which can be installed either by homeowners or by professionals,
can serve as a hedge against future water shortages and add value to your property.

Storage tanks are not filled just once during the rainy season then
emptied in the summer. They are filled during spring and fall rainstorms,
used for watering in the intervals between storms then refilled during the
next storm. Thus the amount of water that can be applied to your
landscaping far exceeds your tank's nominal capacity.

Ideally, your storage tank would be big enough to meet all your
non-potable-water needs for the entire dry season, but such a tank might
be too expensive and too large for your yard. Also, it's difficult to
quantify landscaping needs because of the many variables involved. It's
probably better, then, to start by installing a moderate-size tank, say in
the 500- to 1,000-gallon range for a "typical" quarter-acre lot, and plan
to add a second tank if needed. (There will be plenty of water to fill the
tank(s). A 2,000-square-foot roof in the Bay Area can shed as much as
24,000 gallons of water per year.)

Tanks can be built on-site or purchased. City lot-size tanks are cheap
enough - $400 to $700 for a 750-gallon tank, for example - so that it's
probably more practical to buy a good-quality fiberglass, polyethylene or
metal tank than build one of wood, concrete or masonry. If appearance is
an issue, you can disguise the tank behind a fence or arbor. (Tanks should
be dark to prevent algal growth, so they should be opaque or covered.)
Tanks are best placed aboveground: Underground-rated tanks cost about
twice as much as surface tanks, excavation adds more expense and
maintenance is difficult. Also, underground tanks need electric pumps to
deliver their water, while most surface tanks can be positioned to take
advantage of gravity.

Seismic considerations are important in tank design and placement. Water
is heavy - 1,000 gallons weighs more than 4 tons - and tall, narrow tanks
are less stable than squat tanks. Decks need reinforcement to support
water tanks, and all ground-level tanks should sit on reinforced concrete
pads and be fitted with earthquake bracing.

Tank water should be as clean as possible, so water from the rainy
season's first good storm, which will flush a whole summer's worth of dirt
off the roof, should not be stored but should be returned either to the
ground or to a storm sewer - whichever your city requires.

More dirt will settle on your roof between storms, so subsequent rainwater
should cleaned in a "first-flush washer" that separates the initial surge
of water from the water destined for storage (see drawings). First-flush
washers, filters and other components are available from numerous vendors
(try an Internet search for "rainwater catchment"), and a lively
discussion about the advantages and disadvantages of the various products
can be found on the Web.

In most cities, you'll need a permit to install a rainwater catchment
system - there are seismic and health issues and plumbing and building
codes involved. Check with your building department before designing your system.

You'll probably need to consult with a structural engineer and might need
a carpenter or plumber to help you. But if you are handy and willing do
some research and planning, there's no reason you can't design and build a
rainwater catchment system for your ecological house.


-- "The online rainwater harvesting community": www.harvesth2o.com.

-- "Rainwater Collection for the Mechanically Challenged," Suzy Banks and
Richard Heinichen, Tank Town Publishing, 2006.

-- "Water Storage," Art Ludwig, Oasis Design, 2005.

Philip S. Wenz is the founder and former director of the Ecological Design
Program at the San Francisco Institute of Architecture and teaches classes
at the Building Education Center in Berkeley. The author of "Adding to a
House" (Taunton), he lives in Corvallis, Ore. For information, go to


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