How to Build a Rainwater Catchment Cistern

More on how we get, use, and treat water at Dancing Rabbit.

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 a lot 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 and none of them seem to totally satisfy concerns for sustainability and environmental impact. Another issue to think about is tank location. Above ground? Below ground? With no solution that really appealed to our ecological sensibilities, my wife, Alline Anderson and I elected to go with an underground, reinforced concrete cistern that was integrated with our cottage’s foundation serving as a portion of the north and west sides. The main disadvantage of Portland cement is the huge amount of energy required in its manufacture. Advantages include the durability and added thermal mass, plus the fact that two walls of the cistern perform double duty as a portion of our cottage’s foundation.

Construction Begins With The Excavation

So how did we do it? What worked well and what would we have done differently? 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 that the bottom measured 8 x 12 as well. It wasn’t until after the excavator was gone that I realized that 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 1000 gallons in decreased capacity.

Next Was the Cistern Floor
The cistern was constructed in a series of 3 pours, the first of which was the floor.

I formed up the floor of the cistern much as you would 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 at intervals of about 2 feet. 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 but 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. (Maybe someone could share a technique that would allow this form wood to be salvaged.) I cut and laid down ordinary concrete reinforcing mesh and made sure on the day of the pour that it was pulled up into the middle zone of the 4 inch pad.

The next time I pour a cistern, I will provide for a key way to help seal the cistern floor with the walls. This involves cutting “V” shaped wooden strips ahead of time so they are ready to press into the wet concrete. As it was, providing for the key way didn’t occur to me until I was in the middle of the pour and as a result, it just didn’t happen. Instead, I constructed one afterward. Perhaps I’ll have an update in a future issue as to how effective this turned out to be.

And Then the Cistern Walls Were Formed
The cistern walls were constructed by first building the outside forms and then installing #6 rebar wired together on an approximate 1 foot grid. This grid was set into holes bored into the concrete floor of the cistern with a hammer drill. The grid was propped into place, the holes were marked, the grid was temporarily set aside so the holes could be drilled and then dropped into place. Care was taken not to bore deeper than the approximate 4 inch thickness of the concrete slab of the floor. After the grids were set into place, they were wired together at each corner with rebar cut from the same stock and bent into right angles. With the reinforcing grid in place it was time to build the inside forms.

Having never built a cistern before and being unable to find any literature to speak of describing building techniques (until after our pour), I did a lot of optimistic but careful improvising. I built the wall forms with the 2×4 supports situated flat. In an effort to use a minimum amount of material and also to use reclaimed material in alignment with our 5th covenant, I used what materials were available here at DR from past demolition projects. This included 1/2″ plywood. In retrospect I realize that 1/2″ plywood is simply not strong enough. The 2×4 and plywood forms were heavily supported with diagonal bracing and ultimately kept the forms from imploding, 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. What I would do next time is build a 2×4 framework much as you would a conventional stud wall on 16 inch centers and use sheeting a minimum of 3/4 inch thick.

The day the walls were poured was a nightmare of deflecting forms, creaking supports and repeated trips down into the hole for additional bracing (see photo 1). 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 given 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 Forms Are Stripped
Since there was really no rush to strip the forms I left them in place for 11 days while I worked on forming up the cottage floor and foundation. When the time came, the inside forms came out easily but the outside ones, having been jammed against the earthen walls of the excavation by the force of the wet concrete, became our second sacrificial wooden casualty. Part of the inside forms were re-used to form the lid of the cistern during the cottage floor and foundation pour.

The Lid and Hatches Are Formed Up
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. The same would hold true if either side developed a leak. Repairs could easily be made after pumping water to the opposite side.

What I forgot to do however, was form up a cross-over pipe between the two chambers. This was not too difficult to remedy with a hammer drill and masonry bit. After drilling multiple holes in an arc describing a half circle and knocking out the area with a hammer and chisel, I laid down a bed of mortar and placed a section of PVC pipe in the trough like hole of the chamber divider. I finished up using a pointing trowel around and on top of the PVC pipe.

The foundation/floor forms are really another story so I’ll just mention here that they consisted of 2-2 x 6s screwed to 2×4 stakes with 2 inch extruded polystyrene. The lid of the cistern, of course, was integral to the floor of the cottage, so was formed up at t
he same time. A “floor” of 2 x 6s was constructed much like you would put together floor or ceiling joists. These I supported with double 2 x 6s fastened together to form posts. After the frazzled nerves caused by all the bulging of the cistern walls, I was determined that these forms would be strong enough. The 2 x 6 joist systems were built in two sections per chamber and held up by 8 doubled 2×6 posts. In two of these 4 sections a hatch was formed up to allow access to the chambers, one hatch per chamber. 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. So finally after leveling out and screeding the gravel that would form the base of our concrete floor, it was time to call the ready mix folks once again and order the concrete.

THE POUR!
My extra vigilance in supporting the cistern lid (as well as the supports surrounding the foundation and floor) paid off and we had a perfect pour. Not a squeak, not a bulge. I was so ecstatic to have the foundation of our little cottage complete that I could hardly contain myself.

To seal the inside of the cistern we chose a Portland-based product with a latex additive called Damtite, available at your local building supply store, or directly from the manufacturer at:

Thomas Waterproof Coatings Company
627 Whitehall Street, S.W.
Atlanta, GA 30303
404-523-0493

Thank You!
I’d like to offer a special thanks to Trish Newport (debutante) for her indispensable and tireless help in forming, shoveling, and gravel screeding, not to mention her tireless energy on the day of the pour. And all this merely one day before the end of her internship and departure for home in Thunder Bay Ontario. My gratitude also goes out to all the volunteers who helped on pour day and before, who’s names were permanently scribed into the wet concrete of our cottage floor:

Alline Anderson, my charming wife
Amy (the) Best
Jeffrey ‘Jeffoir’ Harris
Miss Lindsey Jones
Rachel Katz, agent to the stars
Kate ‘Lizard’
Craig (Lizard’s friend)
Jess ‘Jazz’ Mund
Karin ‘KP’ Peterson
Kalen (building his own house) Prince
Cecil ‘Mad Dog’ Scheib
Steady On Tony Sirna
Jacob ‘The Wild Celt’ Stevens

If I missed your name I am none the less appreciative but do apologize. My gratitude also goes out to all those who provided background support to those who helped, thus freeing up their time so their assistance was available.

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!