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A Gravity-Fed Rainwater System, by Tractorguy

(Standard Disclaimer: Use the information given below AT YOUR OWN RISK. The author accepts no responsibility for any negative outcomes of the application of the following information.)

We are all familiar with the axiom that we can live thirty days without food, but only three without water. We are indeed fortunate that, in most locations the Good Lord rains it down on us once or twice a week — all we have to do is catch it in something. Much has been written and many ideas forwarded about rain water collection. However, most of these fall short after the water has been collected, when we need to move it to where we use it, especially in a grid-down or off-grid situation where there is no electrical power.

The typical rain catchment system is a barrel or cistern which stores the water at or below ground level. Water is heavy, and requires a great deal of energy to lift and transport it to a higher level to a sink, toilet, or shower where it can be used. Since rainwater originates many hundreds of feet above the earth, it behooves us to take advantage of the ‘head’ or natural pressure that it is delivered in and not perform our collection efforts on the ground where the ‘head’ is zero.

My Catchment System

When I designed my rainwater catchment system (see main photo), I located my storage tank on a stand, approximately five feet tall. My 150 gallon storage tank is approximately three feet tall, and the bottom of my eaves are about 8 ½ feet off the ground. I directed the rainwater from the gutters though a screen, into 1 ½” PVC piping. I use 4” – 1 ½” PVC adapters to catch the water coming out of the gutter outputs, with the gutter output draining into the 4” input, and I cut out 4” circles of standard window screen and placed inside the adapters for the first filter after the gutters. I built first-flush diverters, one for each gutter front and back of the house, out of a 1 ½” tee and an 8’ section of 1 ½” pipe down to the ground, capping the lower end with a cap with a 1/16” hole drilled into it to make the diverter self-draining.

I ran my distribution piping between the front and back of the house at about 18” off the ground, which was about the level of the top of the concrete foundation, and the point at which I would run the water line into the house. I ran a 1 ½” line up to the tank, with a quarter-turn PVC ball valve shutoff at the tank so I could shut the water off easily if I had to, without losing what was in the tank. I ran a ¾” PVC line into the house, with a shutoff, drain, and an inline stainless steel screen of around fifty mesh size.

Now I have a supply of rainwater with about 5-8’ of ‘head’, or positive pressure on it, sufficient to deliver itself to the sinks and toilet by gravity with no other energy required. The water of course does not flow with nearly the same strength as a pumped system, but it is quite sufficient.

Overflow Diversion

Gravity-Fed Rainwater SystemIn order for the system to be automatic, there needs to be an overflow fitting in the tank to divert the incoming water elsewhere when the tank is full. Fluids tend to find their own level, and you will probably have to experiment to find the correct overflow point in the tank, which should be slightly below the lower edge of the piping that is feeding your first-flush diverter(s). Otherwise, your first-flush diverter drains will become your overflow. The overflow in my system is piped to my fruit trees, where they benefit from the excess water after the house tank is full. The overflow on my tank is illustrated in Fig. 2 – it is the ¾” fitting and line coming out near the top of the tank.

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[JWR Adds: I’d recommend using a larger diameter overflow pipe, to be better prepared for the event of a huge downpour.]

Polyethylene stock tanks of varying sizes are commonly found at agricultural supply stores and are relatively inexpensive. Their main shortcoming is lack of a top cover. A piece of treated plywood cut to size will keep debris out and keep mosquitoes from breeding in there. Most of the common sizes have a 1 ¼” NPT outlet with a threaded fitting, nut, and two pieces of sheet rubber to act as a gasket. Most are not very water-tight from the factory, and I normally make it a point to disassemble the connection, coat everything with silicone caulk, and reassemble before I attempt to use the tank.

A very desirable feature is a sight level gauge, so you can see how much water you have, since the tanks are opaque. A 1/8” NPT tapped hole near the bottom of the tank, with an elbow, hose barb to ¼” hose, and a length of clear ¼” polyethylene tubing running to the top of the tank, gives an excellent sight gauge to check level. I placed mine near the bedroom window so I can easily look out and see the water level. (See the inset photo. Double-click to see it full size.)

Toilet Trials

Modern toilets have a multi-stage fill valve that requires some pressure in the incoming water supply to open reliably. After connecting the system to the toilet, I found I could manually pull up the fill valve plunger with my fingers and the toilet would fill and shut off, but the valve would not always open again when the toilet was flushed and the float went back down. My solution was to add a large-diameter but soft spring between the plastic housing and the bottom of the rubber diaphragm that acted as the valve mechanism. I found a suitable spring at the local hardware store spring selection, Handi-Pack part #88228, 13/16” by 3 ¼” by .050.

The spacing of the turns is such that cutting a piece of the spring with about five turns on it is about 1” long, and is just right to keep pressure against the bottom of the valve to make sure it opens when the toilet tank float drops when water is required to fill the tank. Make sure the cut end of the spring is placed down against the plastic valve housing, so the looped or smoother original end of the spring is against the soft rubber diaphragm so as to not damage it by punching a hole through it. This resulted in a reliably-flushing toilet. Note that it does take longer for the tank to fill – about five to ten minutes – so you may want to warn family members to wait until the tank has finished filling completely before flushing again.

Filtration, Of Course

Of course, rainwater must be purified before it can be used for drinking. I have a Doulton Rio 2000 water filter with six silver-impregnated candles. This is fed with a standard hardware store grade water filter with a five micron element to remove the larger dirt before it reaches the Doulton and extends the life of the Doulton filter. The flow rate through the two filters is a steady stream, sufficient to fill a drinking glass in around twenty seconds.

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My off-grid location is in the woods, and tree debris is a challenge to rainwater collection systems. I first used the standard ¼” hole gutter screens. These did NOT remove the majority of the debris. A product that just became available in our home-improvement stores are blocks of rigid sponge foam called Future Foam. They are four feet long, with a triangular cross-section to match the inside of the gutters. Placed end-to-end the entire length of the gutters, this keeps all but the smallest debris out, and they can be easily cleaned by taking a small hand-held broom and brushing off the tops of the foam blocks several times per year.

Rainwater catchment systems require regular maintenance. The input screens at the output of the gutters require cleaning around once a month. The drain holes at the bottom of the first-flush diverters should be eyeballed regularly after a rainfall to make sure there is water running out of them – if not, an opened paperclip poked through them will immediately open them back up.

Drain It For Winter

If your location freezes in the winter, as most do, you will have to close down and drain your system before the first hard freeze. I attach the caps on the bottom of my first-flush diverters with 1 ½” rubber sleeves for easy removal for cleaning. When freezing weather comes, I remove the caps at the bottom of the first-flush diverters, and attach an 18” section of 1 ½” PVC pipe with an elbow and another ten-foot length of pipe on it, to divert the water out away from the house. Since the rainwater enters the first-flush diverter first before it goes anywhere else, the majority of the water goes down the diverters and out of the additional ten foot sections of pipe.

There is a tee with a drain plug in the main distribution manifold, and I also have a section of pipe with a female adapter on the end that screws into the tee after the drain plug is removed, to carry away any water that accumulates in the main distribution line after draining. Remove the screens at the output of the gutter outlets when winterizing, to make sure they don’t get clogged with ice.

Another potential benefit of this system exists even if you have an existing well and pump. Even in case of extended drought, if the system runs dry, well water can be pumped into this storage tank, and gravity fed water will continue to flow after the well and pump is stopped. I can fill my tank with the generator and well pump in about ten minutes, and then have a week’s worth of flowing water without running the generator again.


My system has worked well for me for the last year. It is a wonderful feeling to trip the flush handle on the toilet and know that it will function, regardless of the state of the outside world or the power grid!


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