Build a cistern out of
|Issue #146 • March/April, 2014|
Using a corrugated road culvert as a cistern is an economical way to add to your water storage. Here is the finished tank with PVC pipe directing water from a gutter into the tank inlet.
No matter where you live, the most important resource that a prepared homestead can have is access to stored water. It may be abundant or scarce, but you still have to be able to collect and store it somewhere. That’s where tanks and cisterns come in. Wells are fantastic, but if you have no holding capacity, your access to water is 100% dependent on the pump and the electricity to run it. Whether you’re in a wet or dry environment, being able to collect rain or just hold a reserve of water is essential.
We happen to live in the Arizona desert and water is a huge issue with us. We have no well on our property, so the storage of our summer and winter rains is crucial. Unfortunately though, storing water isn’t as cheap as it once was. It used to be that you could get a plastic tank or some used IBC (intermediate bulk container) totes for cheap or build an inexpensive concrete cistern. Large-capacity poly tanks have become very popular as well as pricey, and building a concrete cistern isn’t exactly cheap either. Not to mention very labor- and maintenance-intensive.
Frame was set and leveled with PVC pre-plumbed, rebar and remesh tied, and filler rocks added to save on concrete.
Author using a boom hoist to lower the culvert into the “soupy” concrete mix
Detail view of water spigot and valve. Note that the assembly is threaded to aid in cleaning in the event of a blockage
Inside of the tank was coated with Ames’ Blue Max Liquid Rubber along the culvert seams and tank floor to prevent leakage
Recently, my wife and I were considering the option of adding more capacity to our rain system. We currently have three tanks totalling about 10,500 gallons but we wanted to up our capacity just a bit due to the overflow loss we sometimes had at the end of our monsoon season when our tanks were full. I had wanted another polytank but she wasn’t keen on another piece of plastic sitting next to the house or spending another $1,200+ on a 2,500-gallon poly tank, so I suggested that we build a cistern out of road culvert. I had seen a couple around the Tucson area and after showing her pictures and giving her estimates in the $300-400 range, she was interested. We could get a 1,100-gallon cistern out of an 8×5-foot diameter road culvert. And if we wanted to go to an even larger diameter, the cistern capacity would exponentially increase versus the smaller increase in price.
In my research, I found some general plan diagrams out of the Texas and Arizona Rainwater harvesting manuals through each state’s extension service. They gave me a good starting point for the general layouts and dimensions. Then I went ahead and sketched out my own rendition (see diagram). Once I was confident in my plans, we started the groundwork. After selecting the location, I leveled the ground, compacted it well with a hand tamper, and built a 6×6-foot frame out of scrap pieces of 2×6 lumber. I fitted some 45 degree angles in the corners to save on concrete and give it a better look as well. To get the proper thickness out of our undersized 2x6s, we had to dig the ground inside the framed area to ensure that we were adding an additional 2½- to 3-inch slab thickness overall. It is hard to see in the picture, but almost all of the interior area is at 8-inch thickness from the ground to the top level of the slab frame.
After ensuring the frame was level and anchored, I placed the PVC overflow and hose spigot pipes prior to adding rebar and concrete remesh. The PVC overflow pipe runs through a hole in the frame so that water exits out the side of the slab but still above ground level and the hose spigot makes a “U” shape and will be located on the top of the finished slab for easy access. When adding the rebar/remesh, I made sure that spacing was even near the edges as well as the top and bottom of the slab area. And while most of the rebar grid was to be 3/8 inch, the main grid that would hold the weight of the culvert was to be ¾-inch rebar. This bar needed to be dead level, as that would be a huge factor in the finished cistern being level. Note: In addition to the commercial concrete spacer blocks, I added washed riprap rock to the slab area to help hold up the rebar and to help save on concrete (see picture).
Once the groundwork was done, it was time for the pouring. We used a small concrete mixer and mixed the bags to just slightly “soupy” so as to allow the culvert to sink freely into the mix and onto the rebar grid. Just make sure it isn’t too “soupy” as it can result in a weaker final slab if the mix is too moist. Once the slab was poured, leveled, and worked a bit, we set the culvert into the mix. Luckily, my father-in-law has access to a boom truck that we used to lower the culvert into the mix. But if you don’t have access to a hoisting device, then two galvanized fence pipes strapped to the sides with four adults on the corners will work just as well. The lowering of the culvert required a bit of jiggling to sink it down to the rebar but after a couple minutes it was set. After making sure that the tank was vertically level on two sides, we added a couple braces to ensure no shifting would occur before the final setup. One aspect to remember when placing the culvert in the mix is to make sure that you have at least 4-5 inches of space between the edge of the culvert and the edge of the slab to ensure adequate strength in the thinner areas of the slab.
After edging and working the concrete for the next couple hours (times will vary based on temps, humidity, and concrete mix) we covered the exposed areas with plastic and let it sit for 24 hours to cure. This is crucial to keep cracking down to a minimum as a slow cure is usually better than a fast one. Once your concrete has cured, you can remove the plastic and the forms, doing any touch-up work as needed. The next step will be to let the slab totally dry (we waited two weeks) before applying Ames’ Blue Max liquid rubber sealant. We applied two coats to the bottom of the tank and along the interior seams. This product was expensive but is 100% waterproof, non-toxic, and we only required about ¼ of the gallon to finish the tank.
Once this is done, you should attach a valve to your spigot stub and add a section of pipe to your interior overflow pipe. Remember, this overflow pipe should be a couple inches below the highest level of your tank (see diagram). After these are done it will be time to add a tank lid or covering. We opted to have a local HVAC guy fabricate one for us for about $100, but I’ve seen people use scrap lumber, tarps, etc., to make their own tank lids.
All in all, the project costs were $275 for the culvert, $88 for the concrete and rebar, $15 for the PVC, and $54 for the Ames’ Blue Max. In total, you could do this project with all new materials for $432 (add a $100 for the fancy galvanized lid). But, if you are resourceful like most BHM readers are, you can possibly get a culvert section for much less off Craigslist or from a local contractor, further lowering your costs. This project turned out great for us and I hope it gives you at least an idea of another option when it comes to storing water for your homestead.