Figure 8-3: Photo of a downspout. Downspouts funnel water
from gutters along a vertical drop.
In most water harvesting systems, there is at least one place in the conveyance system with a sharp vertical drop. From pitched roofs, this is typically a downspout. From flat roofs, this is usually the arc of projected precipitation off of the edge of a canale. Harvesting water that free-falls from a canale imposes a need to efficiently direct the runoff. This manual offers three methods to deal with the vertical-drop issues associated with canales: funnel drains, downspout-canale connections and in-wall drainpipes.
Most roofs have more than one vertical-drop point. If, however, you can limit the number of these points in your design, you often can save money in the installation process. This of course assumes that your roof can handle the limited number of vertical-drop points during large storm events.
Downspouts are the pipes that direct water vertically from gutters toward cisterns. It is recommended that your downspouts be made of the same material as your gutters and that you affix them to your structure, as in Figure 8-3. Connections between gutters and downspouts should be monitored periodically for leaks and blockages.
Downspouts are highly efficient at conveying water along a vertical drop. They are also very precise at directing runoff, so the water they convey is less expensive to control than the water associated with canale drains (see next section). Aluminum, galvanized steel and vinyl downspouts normally come in two-inch by three-inch or threeinch by four-inch rectilinear sizes, but downspouts can also be cylindrical pipes made of polyvinyl chloride (PVC), steel and corrugated metal.
For the do-it-yourselfer, vinyl is a good downspout choice since it comes in manageable lengths with an assortment of parts that can be fit together quickly and easily. Aluminum gutters and downspouts are economical and relatively easy to install. Wood gutters are used primarily in special applications, such as historic preservation, due to their extra expense and fire-hazard potential.
A funnel drain catches water that falls from a canale. It does this by directing roof water through a rubber or vinyl liner (such as EPDM pond liner, which is a highly flexible synthetic rubber material that is commonly used to line decorative ponds) into a below-grade box drain. A top dressing of gravel or river rock covers the liner to protect it from degrading in the intense New Mexico sun and hides the drain from view. Most importantly, the gravel prevents mulch, soil and other particulate from being conveyed toward your cistern. This gravel becomes the first step in the filtration of the captured water.
Figure 8-4. An above ground demonstration of how a funnel drain works. Materials, shapes and proportions of funnel
drains can vary.
When determining the location for a funnel drain, take into account the fact that roof water can arc far away from a structure (especially during intense storm events). Other times—at the beginning and the end of storm events, during light storms and when snow melts off of a roof—significant quantities of roof water drip straight down from the canale along what is called the drip line. In storm events that are associated with gusty winds, the collected roof water may be blown against your building, away from your building, or sideways, out of range of your funnel drain.
To combat the variable nature of the trajectory of the water, funnel drains are located in the ground at least four inches below grade and about 12 inches out from the drip line of the canale. The edge of the liner should start against the house and protrude at least three feet from the drip line, the point directly below a canale where it literally drips tiny amounts of water. Different liner materials come in different widths; standard five-foot-wide liners are typically sufficient for one-story structures, while a minimum of a six-foot-wide funnel should be used for two-story structures. Make sure that your funnel drain is pitched in such a way that water does not settle against your building, particularly in the case of adobe structures.