Introduction: Earthbag Water Tanks

About: Owen Geiger is the former director of Builders Without Borders, a Mother Earth News Green Home Adviser, The Last Straw Journal Correspondent and the director of the Geiger Research Institute of Sustainable Bui…

If you’re new to earthbag building, please read the introductory Step-by-Step Earthbag Building Instructable first. This water tank project is similar, except it uses tubes instead of bags. Also, my new Earthbag Building Guide and Earthbag Building DVD are now available. Stay up-to-date on all the latest earthbag news by following our Natural Building Blog. Our flagship website archives all of the best information about building with bags, including photos and videos of the best projects, where to buy resources and much more. Just about everything you need to know is free.

Water scarcity is a very serious problem in many parts of the world. The lack of clean water kills around 3.4 million people every year. Capturing roofwater and storing it in water tanks is one of the lowest cost, easiest ways of obtaining safe drinking water and water for cooking, bathing, laundry, pets and chores around the house.

This Instructable explains each step of construction for building an earthbag water tank using polypropylene tubing filled with crushed limestone. Moist, crushed limestone when compacted will turn into a stone like material, and get harder and harder over the years.

This water tank can be built in 2-3 days by four workers with minimal skill, although plastering skill is very helpful. You will need at least one helper to fill the tubes and help plaster.

1,056 gallon (4,000 liter) plastic water tanks retail for around $700 - $1,300, so at $200 this design costs far less than the main competing product. Unlike plastic, cement tanks won’t blow away in a hurricane, break down from UV rays or taint the water with plastic chemicals. Our water tank includes a domed ferrocement top that’s exceptionally strong. Tanks like this should last decades with only minimal maintenance.

Cost: Total cost of materials is about $200, although prices will vary somewhat.

Basic project information: 1,056 gallon capacity, 72” interior diameter, 94” exterior diameter, 60” interior height, 65” exterior height, 13 courses of 5” tubes (if filled to capacity).

Tools: Tamper, round nose shovel, knife, hammer, 3’ level, tape measure, lineman pliers, wire cutters, hacksaw, trowel, hawk, cement mixing hoe or flat bottom shovel, garden hose with spray nozzle, small stepladder or straw bales to step on, plastic bucket for filling the tubes (cut the bottom out, cut it vertically, adjust to fit inside the tubing and then duct tape together to make a funnel).

Supplies: 275’ of polypropylene tubing (you may have to buy a whole roll), crushed limestone (small pickup load), stringline, center stake, clean sharp builders sand, 8 bags of cement, 70’ of ½” rebar, 30’ of 1/4″ rebar.

The following instructions assume you have cleared and leveled the site, removed topsoil and positioned crushed limestone next to the building site to minimize work. You might need to add some gravel to raise the site if you have to remove a lot of topsoil.

Step 1: Earthbag Water Tank Plans

Let’s briefly take a look at the plans to get a better understanding of the building process. The drawing above shows all of the parts.

Our polypropylene tubing was purchased from Bundaberg Bag Company, Australia. Specs: 14-1/4” (36cm) wide when measured flat and empty, wall width before plaster 11-3/4” (30cm) x 4-5” (10-13cm) high. Flatten the wall surfaces somewhat to save on plaster and the wall width is then closer to 11”. Try to find a local supplier, because shipping costs can be quite high. Note: 18” wide bags and tubing are best for most earthbag houses and other large structures, but excessively wide for all but very large water tanks. Tubes are much faster and more efficient than typical sand bags because they’re narrower, use fewer materials and create fewer gaps.

For large tanks like the one in Mele Village, I would use rebar down through the center of the tubes or one strand of barbed wire between courses. Use both on really large tanks, because water creates a lot of outward force against the sides of the tank.

There’s no pump on this design, although you could add one of course if you wanted to pump the water into the house. On this one, water flows into the tank from the gutters. Then you open the tap near the base to draw water out. It’s all gravity fed.

Step 2: Earthbag Tubes

Mark the radius: Use a stringline on the center stake to mark the radius of the tank (72” in this case). Scratch a circle in the ground as you walk around 360 degrees. The tubes will go on the outside of this circle.

The tubing sits directly on stable mineral soil or gravel fill. The crushed limestone needs to be slightly moist – just enough to pack densely in the tubes. It will not compact correctly if too dry. Mist lightly with a garden hose as needed until it clumps in your hand.

Fill the earthbag tubes: For this tank you’ll need tubing cut to 21’. Tie an overhand knot on the end you start with. The other end will be folded under itself when the circle is complete. Slide the tubing onto the plastic bucket funnel then bunch up the tubing onto the funnel until you’re near the end. Now you’re ready to start filling the tubes. Have one person hold the funnel with their hands on the sides as another person shovels. You can shovel directly into the funnel or fill the tubing with another bucket. Using a bucket is easier as the wall gets higher. 2-gallon buckets are easy to work with. Align the tubing with the circle you made on the ground.

Tamp the tubes after the circle is complete: Tamp each course solid, starting with the high spots. Keep the tamper moving so you don’t create low spots. The first pass doesn’t require much force. After you’ve gone around once you can start tamping more vigorously. Final tamping is usually done with more force. You’ll hear a change in pitch as the earthbags become solid. Here are free plans to my favorite tamper.

Level and plumb each course: Check each course for plumb and level after tamping. Using the same technique throughout the process will help maintain level. Stand back and double check your work as you go. Make sure the courses don’t bulge in or out.

Insert pipe for the water tap and drain: Place pipe between courses of tubes as you go, leaving 4” inches sticking out on both sides. The floor drain will be at finished floor height. Since the floor isn’t made yet, add the drain pipe on top of the first course (4” – 5” above ground level). The water tap pipe can be placed one or two courses later, depending on how high you want it to be. Attach the water tap and drain after plastering is complete.

Continue filling and tamping the tubes until you reach the desired height -- in this case, about 65” high (13 courses if they’re filled as much as possible). At this point you can pound ½” rebar down through the center of the tubes if you want extra strength. This is only needed if you’re in a seismic area or if you’re building large tanks.

Step 3: Ferrocement Top

Ferrocement is made of layers of mesh tied to steel rebar and then plastered on both sides with cement. It is incredibly strong. You can even make boats and many other things out of ferrocement.

The slope of the top can vary, but don’t make it totally flat. There should be adequate slope so water drains off fast enough that mold doesn’t grow on top.

Cut a piece of 4″ diameter bamboo or plastic pipe to the desired height and set it in the center of the water tank. Hold it plumb as someone else measures the distance from the earthbags to the top. Add extra for the ends. One end is bent over and goes in the bamboo about 3″. The other end is pounded 8″ or so into the earthbag tubess. Our 1/2″ (#10) rebar was pre-cut at the building supply center at 5’ (1.5 meter) lengths so there was no waste. Bend the rebar with a piece of steel pipe as it’s secured in a bench vice, etc. Once you have one piece that fits correctly then make all the others to match. That way the shape is uniform. Now insert the rebar. We put ours about 16″ apart. Add tie wire around the top of the rebar. We poured a little cement in the top to help lock it all together, although that’s not necessary. Add two circular pieces of 1/4″ rebar spaced so they reduce the largest openings. (You don’t want the mesh to sag under the weight of the plaster.) Tie this small rebar on top of the ½” rebar. Then tie chicken wire mesh (with small holes) on top. Then add a layer of 1/4″ galvanized mesh hardware cloth. Add plenty of galvanized tie wire until all the mesh is close together. It helps to pre-cut U-shaped pieces of tie wire and work with a partner to tie the mesh = one person inside pushes the tie wire through as the other person twists it tight.

Be sure to plan for an access hole so someone can climb inside and clean the tank once a year. Also plan how the downspout from the roof will enter the tank. Now you can remove the bamboo or plastic pipe center pole.

Step 4: Plastering

Plaster mesh is not needed if you apply the first coat in the recesses between courses. You can throw the plaster into the recesses or work it in with a trowel. It’s important to add just the right amount of water so the plaster is not too wet or too dry. You’ll probably have to experiment a bit. Wait till it sets up. The second coat covers the entire tank. The plaster that falls down inside will naturally make the floor with minimal effort.

Plastering the ferrocement top: A mix of 1:2 cement to builder’s sand is common. Don’t add too much water. Have one person on the inside holding a hawk as a second person trowels on the plaster from the outside. Push against each other as the cement is troweled on. The first coat should be thin. Apply more coats as the plaster sets up. Be sure to completely cover all the rebar and mesh on both sides with plaster or otherwise it will rust.

The last coat, the finish coat, is cement ‘paint’ that’s made with cement and water. Brush it on with a paint brush to smooth and seal both sides. On the exterior, you can add iron oxide pigment to the finish coat to make the water tank any color you want.

Miscellaneous water tank information

Desirable water tank features: ‘first flush’ pre-filter to flush the first water after a rain to rid dust and bird poop, removable strainer to screen out leaves and other debris, lid for strainer (covers strainer between rains), 2 lockable brass taps: one raised a ways from the bottom and one clean out drain on the bottom, overflow pipe. Some people build a recessed area below the water tap for ease of filling buckets.

Periodic cleaning: A brush and some bleach in a bucket of water works good. Rinse with a garden hose and the drain valve open.

There are some very helpful calculators on the Internet for determining volume of cylinders, circumference, converting units and so on that will help you design custom sized water tanks.