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Rainwater Catchment

Graham Bell has lived in Scotland since 1988, having previously spent ten years in London.  His work has taken him around Europe, Africa, the Middle East and the USA. He has a Master's Degree in Old English & Linguistics from Oxford University.  It took him many years to actually make the connection that most of what he does is dependant on the use of language, and to revalue that educational start point. Graham teaches sustainable design, and has written two books on the subject The Permaculture Way and The Permaculture Garden.  He frequently contributes articles to local, national and international media. He is actively involved in the cultural scene of Scotland, including Scottish Traditional music, song, art and woodwork.  He enjoys his garden, which supports a historic collection of Scottish apple cultivars and a wonderful range of bird life.   "Family is the most sustaining thing in my life.  After that comes the valuable network of people that I draw on for creative progress, both for myself and the people I work with.  Home is where the heart is." For more about Graham and his work visit www.grahambell.org.

Questions and Answers

Q: Is any treatment required in a roof rainwater collection system to ensure that it is safe to drink?

A: (Penny Livingston-Stark) Traditionally if you keep your roof clean and it is made of non-toxic materials no filter is needed. Now a days with air pollution acid rain etc. It might be good to filter it.Make sure your tank is opaque and light can’t get into it. That will keep algae etc from growing in it. Clean your eaves trough before the rains and design a system that you can shunt the first rains into the garden until the roof is clean (Or you can just go up and clean the roof.) You can get a simple water test kit for koi ponds to test the ph. If it is too acidic a simple limestone filter or suspending a sack of limestone in the water will do the trick.

Re: pollution. If the pollution is so bad it affects your drinking water and you are breathing the air 24 hours a day 7 days a week, your water quality is the least of your problems. A simple mesh filter can take out most of the particulates. Ultra-violet light will take out bacteria. Ozone filters take out particles as well. Reverse osmosis take out almost everything except chlorine (which isn’t a problem in your case). I don’t use any filter and have never gotten sick. In fact I’m still looking for a case of anyone on the planet getting sick from drinking rainwater from their roof. (A thatch roof with critters living in it might be a bit sketchy).

C: You need to filter finer than one micron absolute in order to filter out bacteria; 99.9% of all carbon, even carbon block, filters have pore sizes large enough for bacteria to pass! I

Q: My family and I live in an old farmhouse which has a rather unusual plumbing system. We have a well, but our hot water supply comes from a cistern that catches rain from our roof. Because of this, we use rainwater for bathing, washing clothes and washing dishes. We've been living here for about 3 years, but have always wondered if this system represents a health hazard.

A: (Penny Livingston-Stark) I think rain water and clean spring water are the healthiest and most sustainable water source you can have, unless you have a thatched roof with a bunch of rats living in it. The roof water that I know of that has been tested is cleaner than city water for drinking, even if it comes off of an old asphalt shingle roof. So no need to worry about it for showering etc. I’m still looking for documented cases on people becoming ill due to drinking rain water harvested from any roof. Compare that to how many people get sick from ground water contamination. Well water can contain toxic minerals like arsenic, selenium and magnesium. That said, if you are doubtful, just have your water tested so you can relax. You could also have your well water tested for comparison. It would be interesting to know the difference.

Q: Hi Penny, we are planning on building a log home soon and I would like to know more about water conservation. I would like to somehow use the rain water that we get around here and use it for showers, cooking, cleaning etc. what would be the best storage systems and filtering systems?

A: (Penny Livingston-Stark) Roof catchment is very easy. To make a rough calculation here is the formula: 1” of rainfall creates 55 gallons of water per 100 sq. ft of the roof footprint. There are 7.48 gallons per cu. ft.

If you put water into an opaque tank made of food grade polyethylene commonly sold at a tank supply place or a ferro cement tank no light can get in, thus you won’t have an algae problem. Drain pipes from the eaves trough needs to be sealed and not leak. This allows you to locate your tank at a distance from the house if you want. The drain pipe can go down and then up to the tank as long as the input to the tank is below the level of the roof. You need to put a tee with a valve in the drainpipe at the lowest point before it enters the tank so you can drain any standing water in the pipes after the rainy season. I recommend you keep it open during the dry season and not close it during the first rains to allow the first rains to clean the roof and not go into the tank until the roof is cleaned. When you close the valve, then the clean water will go into the tank. If you live in the NE US you may need to put some limestone into the tank to deal with the acidity. You can also put the water through a simple charcoal filter after the tank if you want but that’s not always necessary. I don’t know of any documented cases of someone getting sick from drinking roof water.

Q: I am building a southwest style house with 14" poured pumice walls. I will have a slight pitch on the roof with parapets and canales. I wanted to install roof drains in the roof, right before the canale exits. This is to collect rain water. From the roof drains I wanted to go down PVC pipes in the walls to exit below ground level and end up in a buried cistern. The canales will take care of overflow from the roof. I need help with overflow in the cistern. Also I have searched for information ( is it safe? how does the plumbing go? etc.) on such a system with no luck. Can you help?

A: (Penny Livingston-Stark) I’m not sure exactly what your asking, but if your asking if the pvc pipe is safe for a downspout, my answer would be: as long as it doesn’t get heated up from the sun it shouldn't’t’outgas. PVC outgases carcinogenic molecules and endocrine disrupters when exposed to heat. Also the manufacture of it is really really toxic. The best plastic to use so far is Polyethylene. I would avoid anything with a “v” in it. Like vinyl or visquene.

Q: I am planning to use the roof of a vaulted earthbag house as a water catchment system. Any suggestions?

A: (Kelly) Doing water catchment off of a vaulted roof as you describe could be a bit tricky in terms of the materials being used. Whatever material the roof is made of must not impart pollutants into the water, if it is to be potable. Clay-earth would have to be fully stabilized to avoid erosion. Using papercrete, like I did over the earthbags, would not work for water catchment, because the papercrete absorbs almost all of the water. A cement stucco might work, but it is prone to leaks.

Q: Are there any commercial water catchment systems for showers/baths that you know of?

A: (Maya Madrigal) It is not clear whether you want to catch rainwater for showering, or whether you want to catch the gray water for later use. In either case, I know of no commercial system designed specifically for these functions. Basically, what you would need is a tank, or cistern, to catch the water, with the associated plumbing. All of these materials can be purchased independently. There are some good books listed here that describe how to go about this. Also some specific products are available here.

Q: If I have a water cistern how do I obtain increased water pressure if the unit is below my roof?

A (Kelly): The simple answer to your question is to add a booster pump to increase the pressure. Often this is done in conjunction with a tank that has a bladder in it to keep the pump from cycling on and off all the time. The bladder keeps the water in the tank from absorbing the air that gets pressurized.

Q: I am setting up a rainwater catchment system. I have a 3000 gal galvanized steel water tank that has some rust spots on the inside. I would like to cover/seal these spots to keep the rust from discoloring the water and to keep the tank from further damage. Can you tell me the most applicable sealant for this use or where I could find this information?

A (Kelly): It is a tricky business to repair a galvanized tank. Probably the best thing to do would be to apply an appropriate paint to those spots, after sanding them down to bare metal.  Where the tank is galvanized, it takes a special kind of paint or preparation to make it stick, so this is where it gets tricky. A good paint store should be able to help you make the best choice.

Q: I represent Alpha Communities Sichuan - an NGO that works with poor villages in Aba Tibetan prefecture. We are helping some villages research on concrete water cisterns which they can build cheaply outside their houses, as there is a need for a means to collect water for irrigation as their area is pone to drought. I have gathered some information from another NGO who is doing something similar in another part of China. This is what they described to me:

"The soil is clay, which means it keeps its shape very well. So, we dug into the ground, but left the shape of the top half of the cistern. The clay soil acts as a mold. We cover the soil mold with a layer of plastic. Then we placed a 7 cm layer of concrete on top of this mold, leaving a small hole at the top for the opening. After this concrete has dried, we dig out the solid inside the concrete, until we have dug out the shape of the bottom half of the cistern. Then we place a layer of concrete to make the bottom half of the cistern. When it is finished, the cistern is covered up. Of course, there should be a pipe that directs the water from the roof into the cistern. We have the water from the roof first go to a sediment tank and then to the rainwater cistern. If you think there is a danger of the cistern getting completely filled, you can also put an additional overflow hole in the cistern with a temporary stop. We have found that a cistern with a diameter of about 3-3.5 meters is adequate for a typical family's household needs."

As you can tell, the description is very vague and we need some expert advice on how to build a cheap but effective home water cistern.

A:  (Paul Sarnstrom)  If these people are doing concrete I hope they are using rebar or some other type of reinforcement. No mention is made of how they are joining the two halves [?] of the tank together. Cold joints can be notoriously unreliable. I think FC may be a better option than RC. It may take more reinforcement, mostly wire mesh, but will use less Portland cement. It also can be shaped into virtually any form you want and shouldn't require any digging out or mounding up of earth for a form.

Comment (Kelly): I once wrote an article (around 2000) for the local paper in Crestone, CO (in the San Luis Valley) about water conservation and mentioned rainwater harvesting as one way to do this. Little did I know at the time that I was advocating an illegal activity. I nearly got into a lot of trouble when a local resident reported me to both the local water conservation district and the state water agency. It turns out that in certain places in Colorado (it depends on how scarce the water is in the various jurisdictions) rainwater does not belong to the person who owns the land on which it falls...it belongs to the person who holds the historic water rights for that particular watershed. This of course is confusing and inappropriate for true conservation, but is none-the-less the law. At the time that this was happening, the matter was being considered by the State Attorney General's office to try to clarify it, and in general the water authorities were not going around looking for infractions to prosecute. But I was admonished for advocating this illegal activity! I did some further research into this, and found that in some other jurisdiction in Colorado, the owner of an earthship that was set up to harvest rainwater (who happened to be an attorney) argued before the court that his usage was "deminimis," meaning that it was so insignificant that it didn't really matter. This may have been what the Attorney General was evaluating. I don't know if this has been resolved yet.

Q: What kind of roofing material is compatible to rainwater catchment? What do you think of glass roof as rainwater catchment?

A: Any as long as it is: 1) Impervious; 2) not toxic, which would rule out (for example corrugated asbestos sheet - don't think anyone is still installing this but there's plenty of it around on
old warehouses etc).

Yup glass would be perfect.

There are also various designs around for self-cleaning traps to ensure that
debris is kept out if the water if it's for human use.

Q: I am designing a building combining bamboo and glass. Is it ok to use bamboo roof in some parts but the main catchment will be the glass roof? What do you think of this combination?

A: Don't see why not. The only difficulty I would foresee is rendering bamboo/glass overall watertight. That partly depends how windy the site is I guess....Don't neglect the value of bamboo (in larger bore) split longitudinally in half as guttering, and whole as down pipes.

Q: I read your related comments about safe versus unsafe roofing materials for rainwater catchments. Where we are in the US (the Pacific Northwest, near Seattle) untreated wood shake roofs combined with copper ridge caps and flashing are common. Would the copper render the rainwater unsuitable for drinking and/or use in the vegetable garden?

A: In my view, whilst Copper in any quantity would be an unwelcome additive to plant water, the run off is likely to have very minor concentrations and be perfectly safe.  I have no scientific proof of this.  As was pointed out to me recently, roof water tends to concentrate nitrogen (bird droppings etc) so can be very beneficial.

Q: I have a water catchment system using gutters and would like to have your recommendation on what sealant to use to seal where the pieces of the gutters connect together. As we drink the water (we do have a filtration system) we, of course, would like to avoid any toxic sealant.

A: Materials need to be appropriate for whatever the gutters are made of and for the temperature range they will have to support.  It is of course preferable to use an inert substance.  Silicon mastic should be Ok in most circumstances.  Tar or pitch based substances may give great flexibility.  In the case of serious leakage, fibreglass resin may work, but you need to be careful no spare fibres are floating about.  Any plumbing store nearby should be able to advise.

Q: I have two questions about a new to-be constructed residence on the Big Island of Hawaii. I will have a rain catchment system (including for drinking) and solar power. Are standard roof mounted solar panels consistent with a rain catchment system? Would a fully weathered solid copper roof (Zappone shingles) be consistent with a rain catchment system?

A: Don't see any problem with solar voltaics on a water harvesting roof.  As ever you need mechanisms to ensure the stored water is actually clean.  If you're going to drink it that's doubly important.  Copper roof should be OK as weathering is most likely copper oxide which is not easily soluble.  HOWEVER in this climate people install copper ridge strips to stop moss building up on tiled roofs.  That would indicate there is some effect from the copper long term.  As with all metals small amounts are a normal part of life.  It's concentrations which are problematic.  My guess would be that there will be acceptable drinking water, but I'd avoid using it with babies/young children and probably seek advice from a professional chemist.

Copper is a natural anti-bacterial and that's why Hawaiians use it for gutters, so I'm told. Your answer matches up pretty well with my research. As to quantities, copper water supply lines have been standard for a very, very long time, so I think I'm OK there. I'll be careful and test, though.

Q: I am living in the Dominican Republic where I plan to build a small round dome house on a hillside. My idea is to construct a support ring for the roof from concrete (over vertical walls of local rock), then build a catenary dome ferrocement shell roof. This will serve as the dome interior surface and will be the form for a rammed earth cap with a waterproof liner on top of the rammed earth, and a soil and grass cover. I am looking at a 30 foot diameter structure. If I build an under drain for the soil and grass cover and collect percolated water, is that water likely to be as potable as rainwater from an impervious roof? I mean, if the under drain is sand, will that constitute sand-filtering?

A: The usual route would be to grade the substrate. The fine on top to coarse below. This gives airspace for any bacterial action needed to breakdown pathogens. High lime content is a good choice for cleansing. You also want reasonably fast throughflow. If in doubt boil the water.

 


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