Roof Water Catchment and Tanks

Written by Doug Crouch

The General Pattern

As roofs are impervious surfaces that facilitate large amounts of water to runoff, it is advantageous to catch some of this water and store it for later use and/or direct it to water harvesting earthworks.  Roofs can be sources of pollution from this abundant runoff or these problems can be turned into solutions through interrupting its flow.  Earthworks are often employed to do this in Permaculture systems but also tanks, cisterns and rain barrels can temper this flow.  These systems are quite simple but must be implemented for longevity as each piece of the system will have its embedded energy.  This is another reason to leverage this resource as embedded energy from industrial manufacturing should be cycled more completely through making elements more multifunctional. Furthermore, different roof materials will lead to different qualities of rain catchment and the amount is easily procured through simple mathematical calculations.   This allows you to size tanks appropriately which will be climate context specific as prolonged dry seasons call for bigger tanks if this is your primary water source for domestic use or irrigation.

Picture credit Brad Lancaster

Components

So in conjunction with the picture above, we will examine the numbers on the picture that correspond to parts of the system.  Contextual tips will be provided along that in the following bullet points that again correspond to the picture.

1. Catchment surface: Roofs can be used to harness a directed flow into tanks.  For domestic and high quality irrigation use, metal roofs are best.  They shed the most water and are the most inert of all roofs.
2. Guttering system: This picks up the flow from the roof and directs it towards the catchment container (tanks, barrels, or cisterns).  While common place in some parts of the world, they seem to be fairly absent in the Mediterranean climate context and  most tropical ones as well.
3. Catchment Guard: This feature collects all of the bigger items that may have collected on the roof like leaves that you want to avoid flowing into the tank.  This feature helps to ensure good water quality and reduces maintenance of the tank.
4. First Flush: This part of the system helps to maintain an even higher level of water quality by reducing the finer particulates and pollutants that build up on roof surfaces.  The system is capped at the end so that water backs up and when full all new water then flows directly into the tank.  It requires opening the cap to let this water out after each rainfall event to make sure this feature performs.  This water can directed to basins or goes just straight onto the ground below it.
5. Catchment Container: in general I have and will be referring to this as a tank.  A cistern is a below ground tank and is used in cold situations where the tank would freeze if it was left above ground.  Tanks can also be simple rain barrels which hold a significantly less amount of water and can be used in areas that incur hard freezes in seasons where this risk is no longer present.  Tanks can be made of plastic, concrete, ferrocement, wood and all have their embedded energy and environmental repercussions.  Plastic tanks should always be shaded by plants or shade cloth to reduce leaching and increase longevity.  Tanks for drinking, and you could also do this for irrigation containers, should have a bag of diverse rocks to mineralize the water as rainwater is nearly distilled in its nature.  Prolonged drinking could have health effects but a range of rocks will remineralize.
6. Air Escape: To help the outflow of water, an air escape is employed to give the tank a correct pressure.  This is analogous to poking a hole in milk carton so its contents can flow out more quickly.
7. Tank Overflow: The tank when full needs a planned escape route just like a dam or swale.  This can be directed to a basin or any other earthwork to ensure completion of the hydrological cycle.
8. Water outflow: When the tank has enough water it is time to use and a spicket and hose or watering cans is employed.  It’s not located right at the bottom because sediments can build up which are to be avoided by having the outlet located above the bottom of the tank.
9. Distribution of tank water: this can be a simple hose system, watering cans or more elaborate systems of piping.  The tank in the picture is built on a slightly raised platform but cisterns and other tank situations may also need a pump for this distribution with the appropriate pressure.
10. Optional Pump: again if a cistern is employed a pump will need to be in place unless buckets of water are pulled out of the buried tank via hand and/or a crank. Electricity input is needed for such a design feature but is inherently needed along with pump maintenance.

As a review here is another rain catchment system with the descriptions along side of it above.  This system does employ a pump and is more sophisticated as it is an area of Australia where rain water harvesting is becoming mandated.  As legal authorities in drylands areas see this inherent need, these systems will become more and more commonplace.

Tanks:

Open tank in coastal Portugal

These concrete tanks are very common place in Portugal and are either fed from diversions of streams, road runoff, or roofs.  This one is covered in duckweed to reduce evaporation and mosquito invasion.  This is very common to see there and help bring much-needed water storage to these particularly dry areas.  These tanks then have piping for its distribution which is often done with gravity rather than pumps.

  Here in this photo of a small rain barrel of 60 gallons or 240 Liters, the top mesh is multifunctional that it filters the water to a certain degree and keeps mosquitos out.  With a tar and gravel roof in place, which is common is these hard freeze areas of Ohio, USA (zone 6), this feature really helps to keep the water quality higher.  With our abundant all year precipitation, these tanks help to supplement garden watering while not holding a huge volume.  They

Combined Sewer System schematic credit http://www2.cincinnati.com/blogs/gardening/2011/11/28/rain/

do reduce the amount of water flowing into storm water drains which any pressure taken off is more than worthy.  These tanks can be old plastic barrels retrofitted or purchased like this.  This particular barrel was part of a state research with the Ohio EPA to examine local measures in nearly all houses in an urban (but suburban context) neighborhood at reducing the runoff that is degrading to the cities crumbling infrastructure.  Along with rain gardens, depressions with plants and mulch where roof runoff is directed, these combined measures drastically increased water quality in the neighborhoods stream and kept much-needed water out of the sewers and advantageously in the landscape or stored for later use.  It’s simply not rocket science and with the foresight of grants and mandates governments could have saved millions as retrofitting piping is drastically more expensive than homeowners installing these or mandating that all new suburban developments design in communal runoff collecting features combined with individual household designing and implementing these measures. It shows the lack of pattern based thinking that is the bridge to future of harnessing abundant natural resources.

rain garden Cincinnati

Also in this cities context, Cincinnati, Ohio, USA, we use 250 gallon or 1000 liter tanks that we get from Coca-Cola as they use it in shipping their high fructose corn syrup.  While not perfect these tanks at least have never had really crazy chemicals in them and are a great reuse of something that is normally thrown out.  Coke is happy to give them away rather than paying a tipping fee and are easily retrofitted to contain water directed from roofs and gutters towards this container.  Painting them black helps as the clearness of them builds algae but a little never hurt any plants that you are watering.

stacked 250 gallon tote tanks

Cisterns are often unnoticed as they are combined with concrete pads that make a patio space.  Below is pictures of cisterns from the house and bar at the family land project in Kentucky, USA- Treasure Lake.  To augment the awkward placement of the cistern in relation to the gutters at the bar, an initial tank was installed to hold runoff and direct it into the cistern with a garden hose.  While not a perfect system, a bigger initial holding tank is desired to handle the heavy rains we often get and also be extra storage for when the cistern does fill. The cistern at the house comes from gutters and pipes that catch all the roof water and direct it easily there.  Many in the community are on cisterns and elect not to harvest rainwater.  We bring our drinking water in from trips to the city so domestic and irrigation use it is perfect.  We hope to never have to buy water again from the truck delivery man and increase the potential of the bar catchment system by adding a 250 Gallon tank to better deal with the flows.

Bar patio roof leading to an elevated tank with a hose leading to the cistern

house cistern with gutters and piping in background

bar catchment system with hose filling cistern

hose leading to bar cistern

kentucky style rain catchment system- but hey it works

Written by Doug Crouch

Header Art by Anita Tirone