Rainwater Collection Concerns

Great info, via MtnForge:

Here is I hope some handy info for consideration on calculating cistern size and water use.
I obtained this info from a number of sources and employed selectively it in developing such a system.

As a matter of oft neglected or unknown importance in the conversation pertaining to water collection and storage from roof rain water sources, it is advisable to employ a “washer” and filter system, as a roof naturally collects many types of organic substances and debris, from bird droppings and leaves, to soot and ash from a solid fuel house heating system.
A “washer” is basically a roof water sub system designed to divert a volume of roof water in the beginning of a period of precipitation, allowing debris to literally be washed from a roof surface, and not collected in a cistern/tank. (As untouched rain water is a great solvent, it is pure water being it has essentially been distilled through natural evaporation, it is it’s own built in scrubber, the idea is to have a system which takes advantage of this attribute). The traditionally accepted amount used in washing is open to interpretation due to a number of variables, frequency of rain, type and use of house heating, seasons, etc, but 1/3 of total annual rainfall should be considered “wash” and other lost water. A filter is a traditionally a 2 stage device, which employs a series of course screens/perforated metal, set at an angle prior to the entrance of input flow into the filter system, this aspect diverts large debris, such as leaves, twigs, dead animals etc, out of the filter system. Below this debris diverter there is a compartment/tank section, with a valved outflow at the lowest point, this valve and section allows for draining of filter to avoid freeze damage and permit a portion of roof water to escape during the roof washing operation, then it is closed, where continued collected roof water can flow into an upwards flow filter bed. There are actually two filter beds, the first is upwards flow, the second stage being a downwards flow, which is fed by the upwards flow filter, allowing for both filter beds to drain dry avoiding freeze damage. Both filter beds use perforated metal plates, on each end of the filter mediums, which consist of large stone, from roofing pea stone to golf ball size, which hold layers of finer gravel, sand, and charcoal from washing out during filtering. It is a very simple design, can be built out of wood, metal or concrete. The use of 304 stainless is highly recommended for all or and metal components in the filter/washer system, because of it food safe and non corrosive alloy characteristics. Also it is recommended to employ diverter valves or gates in the conductor pipes coming from gutters and scuppers.

One method for determining the capacity of the cistern required is to multiply the square foot roof area used to collect water by inches of rainfall, and divide by 1.6
Then determine the daily gallons of water requirements per person per day times 365 days.
Using this equation, a family of 5, @ 5 gals per day, times 365, figures out to an annual requirement of 9,125 gallons of water to be collected and stored. Any figure works, as you might only want potable water for cooking, canning, other food processing, water for livestock, and intimate washing of body and cookery, and your needs could be met with smaller, or larger demands.
An important point to keep in mind, you don’t receive all this water normally in one shot, and you have to figure in for dry spells and low precipitation years, so a minimum storage capacity is a prerequisite in determining cistern capacity.
There are different schools of thought on how large a cistern, based on use verses potential precipitation, but 1/3rd of total yearly requirements is considered a bare minimum, usually half a years needs is considered prudent. You can not have too much clean safe water.
In my AO, we receive on average 47 inches a year, figure 1/3 as wash/waste, on our roof we have the potential for 53,000 gals of usable water. A 6’x6’x12′ inside dimension block and motor cistern, set on a concrete footer, =’s 2,537 gals capacity. We have more water in normal weather years than we require, @ a rate of use of 10 gals per day for 2 people. You must take into consideration your climactic conditions in all instances, as for example you may receive all your rain in only one season, so it is critical to have cistern capacity to carry through the dry seasons, or any of the myriad of different weathers seen across the continent.
Another aspect of cisterns, which Mike touched upon in his previous comment above involves gravity feed, or what is also known as water column. Water column is a function of height only, not volume, a 1 inch pipe 10 foot tall has the same pressure at the bottom of this column of water as a tank 10 foot tall by 100 foot wide.
Remember here we are talking about sustainable resources and all it entails to employ, use, and run them. Labor is valuable, so is time, in this sense, it makes common sense to have a tall as possible cistern, use the water column to benefit, either by placing your cistern strategically where gravity can provide direct flow to your plumping, or assist what type of pump system you have incorporated, saving muscle power or off grid power. It is that whole holistic thing again, where you try to incorporate as many sustainable and beneficial aspects of your labor and resources into as many functions and dual uses/multi tasks as can be figured for. When you have to hand pump your water, or use precious fuel or off grid power to run an electric pump, you end up using less water by rational standards. So that can end up being a safety margin in usage. Things to think about.
We have what is called a rotary vane pump, or a SIGMA Double Acting Semi Rotary Hand Wing Pump. It is plumbed into our domestic water system, in parallel to a 12 volt service pump which runs off an off grid battery bank. With the use of a couple of ball valves and check valves, a pressure bladder tank, we have the option of running either pump, and have a pressurized domestic water system. The rotary hand pump can produce a max 37psi, 20 ft of lift, and 25ft discharge. We have to run down to the basement and pump our system to full pressure a couple times or three a day if our battery bank is getting low like on a run of cloudy days or little wind, as we have solar panels and wind turbines to charge our battery set. (It is a big honking 850lb 12 VDC 1200 amp hr fork truck battery).
The block cistern as built incorporates a troweled on mortar liner and concrete filled blocks to fully reinforce and waterproof it. It has a sump formed in the concrete footer to collect any sediment, and to fully drain the cistern for annual clean-out when precipitation is adequate to permit such maintenance. A conductor pipe from the outlet of the roof washer/filter, terminates about 6 inches from the footer, enclosed in a “tinkers damn”, or baffle, made of block sitting on the footer so when flow is present, turbulence is minimized to limit stirring up any accumulated sediment. Before we filled it for the first time, we washed the interior surfaces with a strong solution of water and baking soda, let it set for a day, gave it a good rinsing with a hose, and let her fill up with rain. The baking soda being highly alkaline, neutralized the high mineral content of the mortar and concrete, improving the quality of the collected water. I hear some folks let theirs fill up, leave it to set for awhile, drain it and refill to use. Probably either method works equally well as it basically fully cures the concrete if I understand the science correctly.


To determine possible cistern volume, gallon capacity can be determined within a high degree of accuracy using the two following trusty pipe fitters formula’s *:

Round tank capacity in gallons, measurements in inches:
Capacity = diameter x diameter x .7854 x length divided by 231

Rectangular tank capacity in gallons, measurements in feet:
Capacity = length x width x height x 7.48

*(Regardless of type of cistern design, be sure to calculate using inside measurements of your type of tank to obtain accurate capacity)

Source for rotary hand pumps:

Rintoul’s above now carries a wider range of hand pumps, the Sigma pump is been renamed “Excelsior E2”

There are a variety of online papers, from university and state/county agricultural extension offices on the theory and construction of cistern systems. Though like most agrarian and rural related self determining/self sufficiency resources which in the past where promoted and sanctioned by state entities, they are disappearing rapidly.

Just to reiterate some important aspects which can get lost in the forest for the trees, the cleaner your water which is first collected and stored in your cistern, the longer it will remain viable and sanitary. After all the idea is to have a source of safe water to begin with. I know that sounds simple, but just collecting as much water as possible is but one object of the scope of such a system. All sorts of organic contaminants can possibly be collected and build up, to pollute your water source without suitable washing and filtering of roof water, from dead mice and birds, frogs, toads, insects, and animal droppings, vegetable matter and tannins from tree leaves, pollen, etc. Then there is soot, ash, creosote and other byproducts of combustion to consider that can deposit on your roof if you heat your building with a solid fuel stove or furnace.
The imperatives of a roof washer/filter bed are something which only you can determine, but if potable water storage is the prerequisite, their importance can not be overstated.

Water is a cornerstone of building a sustainable community, and off the grid it’s tougher than it looks.


21 thoughts on “Rainwater Collection Concerns

  1. Crosby

    As I recall, each foot of elevation gives 0.6 pounds per square inch of pressure. So a tank 40 feet off the ground gives 24 psi. City water pressure is around 60 psi with a usual max of 80 psi.

    Liked by 1 person

    1. It’s possible to incorporate a stant tank in your system to boost static pressure, a low wattage 12vdc transfer pump actuated on a float switch, would be fairly easy to construct. Use a 55 gal food safe open top type plastic drum up in the attic. Lot of buildings in places like NYC use those wooden cooperage type stant tanks.

      Liked by 3 people

      1. danielkday

        This post is highly appreciated and I will print this out.
        Mtnforge, I looked for “stant tank” and found Amazon listings for things made by a company named “Stant”. Are you sure your spelling is correct. Thank you in advance!

        Liked by 2 people

      2. What about using a windmill to pump water from a well to an elevated tank?
        If you put tank higher than any sink,toilet,or shower in a home,you would have enough pressure to have running water throughout a house. wouldn’t get city water pressure-but at least you would have running water.
        Could get hot water on sunny days by pumping water into lines on roof-
        houses in Florida pump swimming pool water onto roofs at night to cool down pool water temp.
        Fairly sure it would work in reverse-use the roof to heat water in daytime for showers and laundry.
        Maybe powered by 12v pump run via batteries charged by a few solar panels?

        Liked by 2 people

  2. Pingback: Three From Brushbeater | Western Rifle Shooters Association

    1. Wind powered pumps where very common before electrification of rural America. You still see them out in the prairie lands used to supply stock tanks to water livestock from wells. Another method of pumping which was lost in the advent of electrification is the Ram pump. Invented late 1890’s, it never quite caught on as it probably would have without cheap AC mains power coming online. (do you see a pattern here? As corporate and industrial interests began to monopolize and marginalize peoples self sufficiency and determination, under the guise of these products and services being cheap and “labor saving”, we began to loose the essential character of ourselves, our country, or very way of life and traditions, we lost or past. That is how you rule souls, how you create and impose tyranny, through economic destruction).
      You can get what is termed solar slow pumps too, they run direct from a solar panel, basically a piston pump, with a motor and controller that function in a wide range of varying available wattage from the solar cell. FYI, there is a company that has freezers and refrigerators which run direct from a single solar panel.
      A wind turbine, to run a pump, if you are in a decent wind zone is advantageous, as you have an electrical source, unlike a mechanical pump wind mill, which the surplus electrical energy can be directed to other uses.

      I’ve looked at those pool roof heaters, betting they can crank out some serious heat. I understand they don’t even require direct sun, cause of the thermal radiation that still gets through cloud cover. Neat thing about passive solar water, you can run them on a convection circulation circuit, no pump required. Been seriously considering augmenting my domestic hot water with a passive solar convection system. Do the pipes in a box, everything painted flat black thing. I’m heating water now by incorporating my dump load into a 1st stage insulated water tank, as a hot water pre heater, the addition of a passive hot water solar booster would by all accounts provide a considerable boost and reduction of electrical usage. 220vac is a real power hog. The dump load pre heater I built uses 12dc water heater elements, so when my battery bank is at full charge, the battery regulator dumps the excess from the panels and wind turbine directly to the pre heat 12vdc elements. Another FYI, with a wind turbine, unless you are using the types with a digital speed controller, you always must have a load on the permanent magnet alternator, or it free wheels and can overspeed in high winds and self destruct. But why stop making power? Find ways to use all your making. In that light, you can run a dump load to a water pump, up into your gravity pressure tank, so that way your using excess power effeciently.
      I must say, because it is not only a great discovery for me personally, but becomes a philosophy and a way of living, along the lines of agrarian thinking, this whole development of self sufficiency/sustainable resources, and self determining methods and techniques of living, so much of it is how you think, and especially as you take this path, of changing your thinking.

      There is a really splendid fellow, runs a blog called The Deliberate Agrarian, Herrick Kimball is his name. A great advocate of Jeffersonian Agrarian thinking and living. Highly recommend his site to all:

      It not about the devices or the technology, it is about using our minds, thinking freedom and prosperity. A lot of these “alternatives” are not, they are born out of ages long tried and true methods and traditions. It is the unfettered choices of how to live, the way YOU and I choose to live, unrestricted by the state and tyranny which is liberty. It is free economy and unrestricted self sufficiency that is such integral an inextricably large component of liberty. Wether it is growing a garden, building your own weapons, or collecting rain water, and everything else that comes with it, that makes us free men, makes us powerful and righteous people, people who can not be denied and will not be denied. It is holistic in entirety. This is what built America my friend. Go for it like NC Scout says. You have more to gain than you can imagine. Every little thing you do, one bit at a time, you gain freedom. The freedom gained become logarithmic.

      Liked by 2 people

  3. An old, low tech but very effective method for filtering and purifying water is the slow sand filter (SSF). The SSF is both a mechanical and biological filter; the 4′ thick filter develops a top layer of biological organisms and the resulting combination removes dissolved organic and particulate contaminants, as well as removing bacteria.

    It does have drawbacks; the SSF requires constant flow, (which can be provided by a circulating pump,) it takes a relatively large area per GPM filtered, and it does require some maintenance periodically, primarily scraping off the top layer of the Shmutzdecke from time to time to keep the filter from getting choked, but it was the primary water filtration system used here in these united States from after the Civil War until after World War 1. They are not used for large scale municipal systems much any more, having been replaced by rapid sand filters of various sorts, but they are a viable option for smaller water filtration systems. Look for older water treatment books; there is also some limited information online.

    Liked by 2 people

    1. I’ve seen filters along those lines too. There is one that is made from a hand poured concrete upright pipe with a closed bottom, a thin wall concrete vessel really, it also keeps the filtered water chilled through evaporation. Uses a stone charcoal aggregate filter bed, they are about 5 ft tall and about 2-3 foot in diameter.
      Essentially I think they all work like how a fast moving stream cleans itself over distance and contact with the geology, or how the filtering effects of water in the ground and drawn from a well is cleaned. There are company’s that sell aggregate back flush filters, which incorporate specify stone aggregate in specific grade size. They work very well for filtering out undesirable elements like ferric and ferris iron in well water. Some rely on using a tiny amount of potassium permganate to re-ionize the aggregate during the backwash cycle. (Handy stuff PP)

      Liked by 1 person

      1. Most of the filtering that slow sand filters provide is from the top layer of biological organisms. The mechanism of operation is different from most other filters.

        Here are some links that your readers may find of interest-


        http://oasisdesign.net/water/treatment/slowsandfilter.htm (this site is primarily geared towards water treatment for plant nursery installs but has good information)

        http://www.cdc.gov/safewater/sand-filtration.html (not as useful as it shows a small atypical filter and has bad information on cleaning the filter, but does have good links)

        http://www.slowsandfilter.org/ (this site has some excellent information on small slow sand filters with design and performance data posted)

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  4. Unfortunately this is now illegal in Washington state and several others.
    The logic of how they manage to lay claim to something that originated hundreds and thousands of miles away and falls from the sky indiscriminately has still eluded me.

    Liked by 1 person

    1. It is only illegal if we all go along with such diktat. Self sufficiency is nullification, is resistance, is winning. It is liberty my friend. It all begins with each of us, nothing is as legitimate. It is winning your heart and mind.

      If you don’t care, look at it from this perspective: the further the spirit advances in the thought and act of defiance to tyrants, the greater the increase of adversarial nature to eradicating that defiance and resistance becomes. It is an evolution of insurgency that is required, and that it comes in stages, at some point the equation changes, from one of being a subject to tyranny, to one of subjecting tyrants to the motive energy of being so free, freedom can not be resisted any longer. It is being human instead of slave, so you can be free, the human terrain is always on top, always upstream of tyranny.

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