Roof Catchment
System
This system is
so old that it's probably impossible to know who
discovered it. It works, and that's what's important. Rain
falls on your roof and runs off. You put a gutter on the edge
to make the water flow to one end, and a barrel catches the
water.
What's new is the kind
of roof used, and modern materials are not necessarily
better than the old ones. Asphalt shingles - used my many
millions of people now - are relatively new,
historically (a product of oil), but they are not inert
when it comes to rainwater collection. Better materials exist,
however, such as sheet steel which has baked-on enamel, clay
tiles, concrete, polycarbonate, and fiberglass.
The Roof Catchment
system, in it's modern incarnation, is only slightly
more complicated than those used for millenia. Three
additions can improve the quality of the water being stored: a
screen to keep out leaves and other debris, a flush system, to
direct the water which cleans the catchment away from the tank,
and a drop filter, to catch sand and stones on their way to the
tank.
The screen can be
anywhere in the system between the roof and the tank,
but two common places are in the gutter and in the pipe coming
from the roof, or both. I've seen entire gutters screened over,
from one end to the other.
Here's a clever screen
design, installed in the vertical pipe coming from the
roof in a city park in Prescott, Arizona:

The roof water falls
vertically onto a sloping metal screen, like window
screen. The water passes through to the funnel-like bottom of
this thing, while the leaves and debris hit the screen and
bounce off, that way they don't block the water. A close-up of
this device:

This water
system is installed on the building containing a men's
and women's rest room. The entire system looks like
this:

The display
box on the wall shows the system in a
diagram:

Notice number 4 bottom
left - the drop filter. A drop filter is simply a
vertical pipe somewhere in the path of the water from catchment
to tank where the water goes over a 'drop' - in this case, a
vertical pipe, plugged at the bottom, allowing heavy debris to
fall into it. The drop filter has been removed, probably
because they are not so useful on roof systems, where most
debris is light. On surface
(ground) systems, they perform well, removing sand and
stones from the water. However, sand and stones don't affect
water quality much; they do contribute to tank sludge
though.
Here's a
friend's roof catchment system in Arizona, this
one over a studio or workshop and its roof extensions, front
and back.

The 600 + sq. ft. roof
is sheet steel with baked-on enamel paint. It produces
very clean water with no taste. The gutter is on the short,
back side, with the downspout at far right.

The pipe leading to
the 600-gallon tank is stepped with elbows instead of
angling the pipe. This allows for the inclusion of a vertical
drop filter, just visible to the upper left of the
tank.

Note the screw-in plug, here
used as a cleanout.
Another way to divert
the first water (dirty) away from the tank is a simple
flush valve. It requires that someone operate it when the
roof has been washed by the first minutes of hard rain. I
installed this on the house at the top of this page.

Water from the roof
comes from the left. The first ball valve is closed,
the second 'through' valve is open, allowing water to
flow to the tank. To operate the flush feature, the left valve
is opened and the right one closed, diverting the dirty roof
water out of the system. When the roof has been
cleaned, the 'through' valve is opened and the flush valve is
closed. If the roof is
cleaned at the beginning of a heavy rainy season, it may not
need to be cleaned again.
A variation on this,
combined with a drop filter is the one removed from
the park system above. The bottom of the drop filter has a
valve, to allow water to seep out at a gallon per hour. When
the rain washes the roof, the first water fills the vertical
pipe. When the pipe is full, the rest of the water flows
straight on to the tank. After the rain stops, the slow seep
valve allows the dirty water to drain, preparing for the next
rain.
There are drawbacks to
this system. The first 'dirt' to wash off a roof is
usually leaves and bird droppings, both lighter than water.
When the drop filter/flush pipe fills with this dirty water,
the debris floats up and is then washed into the tank.
Oops.
The dirty water slowly
seeping out of the pipe at the bottom can plug up the
valve with debris, creating a stagnant mess, some of which
will wash into the tank in the next rain. Also, light,
drizzling rains, which only produce a gallon per hour, will be
lost completely through the seep valve.
Trying to 'automate'
catchment cleaning doesn't always give the results
intended. It might be easier to just wash your catchment
surface with a hose before the first seasonal rains. In that
case, the flush valve is convenient and effective.
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