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:

Debris Screen

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:

Debris Screen close-up

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

Park Roof System

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

Park System 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.

Roof Catchment green metal

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.

Downspout pipe and tank

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.

Drop Filter

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.

Flush Valve

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.

 DVD Soon!

Watch this space for the coming DVD on how to install your own rainwater harvesting system.