The March Hare: Summer '00
Issue 25

Summer in Swing * Help Us! * What's for Supper * Earthen Plaster Workshops [on a separate page] * How to Build a Rainwater Catchment Cistern (Technical, detailed version) * The Big Commitment * Side Notes * Autobiography of a New Member: Jess [on a separate page]

How to Build a Rainwater Catchment Cistern

by Kurt Kessner

A more detailed version of this article, including diagrams and photos, is also available. It is highly recommended for those wanting the full scoop.

Environmental concerns

Although it may go without saying, at Dancing Rabbit we're always looking for ways to be more sustainable and self sufficient. One of the ways we've determined we can do this is by developing rainwater catchment systems. The water from these systems, if managed correctly, can be used for drinking, bathing and cooking as well as for garden irrigation. As with much of what we're doing, this causes a lot of dilemmas and often necessitates compromise. Rainwater catchment is no exception, especially when it comes to the storage tank itself. What should it be made of? Redwood is a proven building material for water tanks but is a diminishing resource and not local to our bioregion. Plastic? This, of course, is a petroleum product and carries with it certain health concerns when used for storing drinking water. Fiberglass? Pretty gnarly manufacturing processes. Metal? Concrete? Ferro-cement? The options are many but none seem to totally satisfy concerns for sustainability and environmental impact. Another issue to think about is tank location. Above or below ground?

With no solution that really appealed to our ecological sensibilities, my wife Alline and I elected to go with an underground, reinforced-concrete cistern. While Portland cement requires a huge amount of energy to manufacture, it is durable and adds needed thermal mass to our passive solar collection system. In addition, two walls of the cistern perform double duty as a portion of our cottage's foundation.

Construction begins with the excavation
The excavator we chose was highly skilled and did an excellent job of digging the cistern and the rest of our foundation. However, I should have jumped down into the 8 foot deep, 8 foot wide and 12 foot long hole to see if the bottom measured 8' x 12' as well. It wasn't until after the excavator was gone that I realized the hole was smaller at the bottom than at the top. Oops. This caused me untold aggravation building an oddly shaped form and cost us about 1,000 gallons in decreased capacity.

Next was the cistern floor
The cistern was constructed in a series of three pours, the first of which was the floor.

The floor of the cistern was formed much like a sidewalk, driveway or other flat work. A rectangular framework was built from 2 x 4s and secured with 2 x 4 stakes driven into the ground. When doing concrete flat work, the ideal technique is to fasten the stakes to the forms by nailing through the stake and into the form from the outside. In this instance it was impossible, due to the wall of earth formed by the excavation. As a result, I had to nail from the inside, the forms became "sacrificial" and after the pour were left in the ground. I cut and laid down ordinary concrete reinforcing mesh and made sure that, on the day of the pour, it was pulled up into the middle zone of the 4-inch pad.

And then the cistern walls were formed
Having never built a cistern before and being unable to find any literature describing building techniques (until after our pour), I did a lot of optimistic improvising.

The cistern walls were constructed by first building the outside forms and then installing #6 rebar wired together on an approximate one-foot grid. (see photo #2)

I chose to have a partition down the center of our cistern, essentially dividing it into two chambers. My reasoning was that when it came time for periodic cleaning of the cistern, I could pump water from one side to the other and clean the empty side while maintaining a constant supply of water for the cottage. So, to finish up the wall forms, a short section of 3-inch PVC was secured in the outermost form to serve as an overflow. Another short length was placed in the wall between the two cells as a cross-over pipe. Rain is initially collected in the first chamber and then overflows through this cross- over pipe and into the second chamber. When both chambers become full water overflows and is diverted to the outside.(See figure #1)

Although I wanted to use a minimum amount of our reclaimed lumber in alignment with Dancing Rabbit's fifth covenant, I realize in retrospect that the forms were underbuilt. Next time I would build a 2x4 framework much as you would a conventional stud wall-on 16-inch centers- and use sheeting a minimum of 3/4-inch thick. With deflecting forms, creaking supports and repeated trips down into the hole for additional bracing, the day the walls were poured was a bit stressful. The forms were heavily braced, which ultimately kept them from imploding, (see photo #3) but the bulging that occurred necessitated bringing in a second ready mix truck and using 50% more concrete than estimated. It also caused wavy cistern walls, (jokingly but lovingly referred to as freeform sculpture) and further reduced the cistern's capacity. We did, at least, have the presence of mind to pour the concrete as evenly as possible in circular patterns around the perimeter, so as not to stress the forms in any one spot. We also were blessed with a very patient and understanding ready mix truck operator. Thank you M&O Ready Mix of Memphis, Missouri!

The lid and hatches are formed up
As the lid of the cistern is actually a portion of the dining room floor, both were formed at the same time. A framework of 2 x 6s was constructed as one would assemble floor joists. These were supported with double 2 x 6s fastened together to form posts. A hatch was formed above each chamber to allow access. (see photo #4)After a layer of plywood and then a sheet of 6 mil plastic sheeting was laid down (to prevent the concrete from adhering to the plywood forms) these hatches were secured in place above the openings provided.

The final pour
My extra vigilance in supporting the cistern lid paid off and we had a perfect pour. Not a squeak, not a bulge. I was ecstatic to have the foundation of our little cottage complete. Well, that's it for now. Next time you're in Northeastern Missouri, stop by #3 Dancing Rabbit Lane for the official cistern tour and a glass of cool, delicious water!