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May 152012

This map shows the variation between expected average rainfall (clear white) and actual rainfall for the first four months of 2012. 0% is dark brown, 200% is dark green. Clearly there's a huge divergence between average and actual rainfall.

We started off an earlier article about rainwater (see ‘An Overlooked Source of Water‘) by suggesting that a few 55 gallon drums of water taken from your downspouts would be a great source of emergency water in a Level 1 type, at-home, emergency.

If you’re planning for a short duration Level 1 situation, then you don’t need a lot of water, and even a single 55 gallon drum of rainwater would probably be enough – albeit on an uncomfortable hardship type basis – for a week or so, and larger quantities will allow you to enjoy successively more ‘creature comforts’ (such as flushing toilets).

But what about Level 2/3 situations – possibly a year or longer, when you need to be self-contained in everything you eat or drink?  A 55 gallon drum of water doesn’t go very far in that sort of case, does it!

Unless you’re in an area where it rains reliably every day, you’ll need to have some storage to give you water on the days when it doesn’t rain.  We talk about how much storage you might need in our article How Much Rain Water Can You/Should You Store.  This article concerns it more with appreciating how much water you can actually get from your roof collection system and matching it to your consumption level.

This and the other articles about rainwater storage might seem a bit complicated, and we deliberately go into quite a lot of detail.  But – what is more important than water in your life?  Sure, you don’t want to run out of food either, and shelter is important too, but if you don’t have a reliable supply of water, you’ve chosen the wrong place to shelter, and you’ll die of thirst long before you die of starvation.

Water is also a great comfort item.  Whether it be for enjoying a long soak in a tub of water, or singing in the shower, or just washing our clothing more regularly than arguably essential, a positive supply of water translates to a much improved quality of life.

So please do bear with us, and if you end up needing further advice or assistance, by all means contact us.

Water Consumption Rate

The first issue is the rate at which you will use water.  This is a very ‘elastic’ number, because in the worst case scenario, you can live on just a quart or two of water a day for some time.  But in a best case scenario, living a normal life such as most of us do at present, you’ll be going through as much as 100 or more gallons a day (true – look at your water bills the next few times you receive them and do the math yourself).

So somewhere between less than one and more than one hundred gallons of water a day is a number you can settle on as an appropriate compromise between cost and convenience.  Deciding on the exact number is outside the scope of this article – you need to decide what the number is, and then we’ll work forward from that.

For the sake of this example, let’s say you want 10 gallons of water for each of three people, and another 20 gallons of water for household things in general – 50 gallons a day for all of you, combined.

Agricultural Water

There’s another thing to consider as well.  While in a Level 2 situation you are basically surviving on your stored food supplies, in a Level 3 situation, you’re needing to grow your own food into the future.

And if you thought your personal and household needs for water were high, you ain’t seen nothing yet.  Here’s an interesting table of water requirements in terms of cubic meters of water per metric tonne of food yielded.

The numbers are very imprecise (even though they seem exact) because it is hard to know how much of the needed water can be supplied from the moisture in the ground to start with and how much needs to be added.  But just to seize some numbers off the site and convert them to US measures, you’re looking at almost 2,000 gallons of water per pound of steak you end up with after raising cattle, 700 gallons for a pound of pork meat, and ‘only’ 200 gallons per pound of wheat.

Our discussion that follows is primarily to do with domestic water for your residence.  But in choosing your retreat location, have an eye for what you’ll be growing there and what the water needs for that will be, too.

In most cases, it will be close to essential to have access to a free-flowing spring or a gravity fed water source to support your farming needs.  We can’t stress this too strongly.  Everything in an ongoing Level 3 situation revolves around convenient access to water.

Equating Consumption to Rainfall

You know – sort of – how many gallons of water you need per day.  But what does this translate to in terms of inches of rain?

The first thing you need to know is the collection area of your roof.  Don’t ask a roofer to do this for you, because he will probably tell you the number of ‘squares’ of tiles needed to lay your roof.  Because your roof is on an angle, the surface area of the roof is appreciably larger than the square feet of floor plan it covers.

So measure the exterior of your house, and add extra space for any roof extensions out over the sides of your house, and from these dimensions calculate the footprint on the ground of your roof.

You also should adjust for any areas of roof which don’t feed the rain into gutters.  Clearly if the water just runs off the side of the roof, it does you no good unless you add extra guttering.  If you have a separate garage and roof and plan to collect the water from that, include that in your calculation, too.

Let’s say you have 1400 sq ft of roof area, and the two sides which the roof slopes down to are each 40 ft long.  Maybe you have a couple of 5′ gaps where the water just runs off the side, but the other 70 ft in total are guttered.  So 70/80 * 1400 means you have 1225 net sq ft of collection area.

One inch of rain falling on 1000 sq ft of roof will deposit 623 gallons of water.  So each inch of rain on your roof will have 763 gallons of water going down your downspouts.

That is the theoretical best case outcome for this scenario.  We’ll fine tune it for the real world, but let’s first do a quick reality check before going any further.

In our example, you want 50 gallons of water a day, and you’ll collect 763 gallons of water from every inch of rain.  This means that in a year, you’ll consume 18,250 gallons of water, which will require 24 inches of rain (best case scenario).

Have a quick look at the average rainfall data for your region (see links below).  Just using the very basic annual rainfall average number, how much rainfall does your region get?  If the answer is less than 24″, you know that you’re either going to have to cut back on your water use, or you’re going to have to find additional sources of water.

Is Rainwater a Practical Partial or Complete Solution

Now for one further quick thought.  Maybe your region will give you 15″ or maybe even 30″ of water.  15″ is good – you could get 63% of your water needs from rainwater.  That’s great.  And obviously 30″ is brilliant.

But what say you can only expect 6″ of rain a year?  And what say none of that rain falls in June, July, August or September, and less than you’ll consume in May and October?  That means you’ll need to stockpile perhaps 7600 gallons of water in May to see you through to some time in October.  That’s a lot of storage, and a lot of cost.

Being as how you have already determined that you’ll need a well or some other water source, if you find a reasonable well (anything over a 2 gallon/hour well will be perfectly adequate – most wells will give you more than 2 gallons/minute!) is it really worth while also investing into a rainwater collection system?

Our recommendation would probably be to continue with your rainwater system, unless you were drawing water from a free-flowing spring or taking it via a gravity feed from a reliable clean year-round river.

There’s one interesting thing about rainwater.  It typically falls in months where solar and wind energy is the lowest – the winter months.  So maybe you use a solar/wind powered pump to draw up water from a well in the summer months, when you have plenty of energy but not much rainwater, and in the winter months, you use rainwater at a time when you have plenty of rain but not much energy.  The two sources balance each other out nicely.

In a Level 3 situation, you’ll be on your own for the foreseeable future with only the resources you have at hand.  Energy will be terribly scarce, as will spare parts for water pumps, and anything you can do to use as much low tech/energy free resources to  help extend the useful life you can get from energy powered and higher tech solutions is to be considered as compelling.  After all, once your high-tech gadgets are gone, they’re probably gone for good.

Furthermore, you know you can trust the water you collect from your roof. Well water is probably okay, as long as you know what else is happening to the water table, but river water depends on what is upstream of you – something you mightn’t be able to control.  That herd of deer that likes to go down to the river to drink?  Guess what else they do at the same time?  The camp set up by the less well prepared survivors of the city a mile up-river?  What do you think they do to their sewage?  Yup – you’re drinking it.

For all reasons, we urge you to keep a month or more’s emergency supply of water on hand – what say your well’s pump breaks, or the river dries up or ices over for the first time in 50 years?

So if you have some tankerage already in place for a reserve supply of water, why not use it to collect the rainwater you get as a supplementary source of water whenever possible, to save on your water pump and the energy needed to drive it.

Real World Imperfections in Water Collection

Remember back to our theoretical collection of 623 gallons of water for every 1000 sq ft of collection area (ie roof)?  Well, now let’s start considering some of the imperfections that reduced the true net water you actually get in your tank from the rain that falls on your roof.

What say you have a very light drizzle on a warm windy day?  What say your roof is made of wood shakes?  Maybe the first bit of rain will soak into the wood, and maybe the warm wind will evaporate most of the light drizzle before it forms into sufficiently large droplets on the roof to start tumbling down and into the gutters.

Maybe you have some dirt, leaves, moss, or debris of any other sort on your roof (and in your gutters) that soaks up some of the water too.

Whatever the circumstance, it should be obvious that a very light drizzle, while possibly adding up to a measurable amount of rain over some hours, might actually be entirely uncollectable.

The steeper the pitch of your roof, the less rain you need to get the water started running down and into the gutters.  If you have a smooth impervious substance like tile or metal, the water will run more readily than if you have a textured or slightly absorbent material such as artificial or natural wood shakes.

For months when the amount of rain that falls is probably less than the minimum you need, these issues become relevant.  You need to analyze how the rain falls, on a daily basis (or even hourly but this is probably not readily available) to determine how much of the rain will end up being collected and how much will be lost.

Our own experimenting has suggested that the first 0.01″ (one hundredth of an inch) of rain is lost, and if the rain is falling very slowly, there will be appreciable ongoing losses.  A 1″ downpour, all within an hour, will give you close to 100% water recovery, but a fine drizzle totaling 1″ over two days might see you only collect half the water (this is a WAG on our part!).

Storage Losses

You know that water boils at 212°, and if you think about it for a moment, you also know that water evaporates, without boiling, at lower temperatures too.  Even a lump of ice loses some of its mass each day due to evaporation.

How much water will you lose out of your tanks due to evaporative losses?  This depends on the ratio of the surface area of the tank to the volume of water within it, the amount of open space above the water, the temperature of the water and of the air (the two might be very different), the ambient humidity, and if there are any winds blowing over the surface of the water.  It even depends on the altitude you are at.

That is enough variables to make it very difficult to offer up for sure always accurate rules of thumb for calculating evaporative water losses.  Suffice it to say that the cooler you keep your water, and the more enclosed you can keep the top of it, and the smaller the surface area as a proportion to water stored, the lower your water losses.  You can’t really control the outside temperature or humidity, but you can stop winds from blowing over your water (and over any openings) and you can perhaps insulate the tank some and maybe even bury some or all of it to take advantage of the natural tendency of the ground to be cooler in summer (and warmer in winter, too – you don’t want your water and pipes freezing up come winter-time).

If you’d like to see an excellent worked through formula, you can go to this webpage which concludes that a typical cup of water would take 44 days to evaporate in a still room at 72°.  In other words, it is sort of evaporating at a rate of 2.3% a day, at least for the first day.

Let’s use the same formula and basic data, but instead say we had the cup of water outside at 85°, and the humidity was 40% rather than 60%, and there was perhaps a light 3 mph breeze running across the top of the water.  This would give us a massively different result – the water would be gone in 11 1/4 days!  You’re losing almost 10% of the water every day.

Clearly, the rate of evaporative loss can be a huge factor and has to be carefully optimized.  Let’s say you can limit your evaporative losses to 1% a day with careful design.  But this does mean that if you need 1000 gallons at the end of 30 days, you’ll need to start off with 1300 gallons at the beginning of the month.  That’s a significant impact.  And if you’re storing 10,000 gallons to last you six months, if you’re experiencing even ‘only’ a 0.5% loss a day, that comes to 9,000 gallons.  Yup – you’ll lose 9,000 of your 10,000 gallons just to evaporation.

Okay, we are slightly simplifying things here, but you see the issue.  The most important thing to obsess over in designing your water storage system is the evaporative loss of your stored water.

Water Collection Rate

So you need (in the case of our worked example) 50 gallons of water a day, right.  Let’s run the risk of stating the obvious, and make two assumptions.

First, if you are getting more than 50 gallons coming down your downspouts each day, you’re free of problems.

Second, it doesn’t rain every day.  Even if you average 50 gallons a day of water collected, you’ll probably be getting less in the summer and more in the winter, and you’ll have occasional unseasonably dry spells that you’ll need to plan for.  Sure, you might get some unseasonably wet spells too, but they are not so relevant.

You need a buffer of stored water to carry you over the dry days with no rain or insufficient rain.  How big a buffer should that be?

It all depends on the rainfall pattern in your local area.  You’ll need to go off and do some research to get not just monthly average rainfall numbers, but hopefully daily rainfall data and averages for many years.

How do you do this?  Well, we’re glad you asked that question!  Please now visit our article How Much Rainwater Can You/Should You Store for a detailed working through of how this can be analyzed and calculated.


David Spero[suffusion-the-author display='description']

  9 Responses to “How Much Rainwater Can You Collect From Your Roof”

Comments (4) Pingbacks (5)
  1. It seems absurd to mention, but there are areas in The United States that have declared that rainwater does not belong to the owner of the property that the rain falls on. Readers might be wise to check local laws before setting up a rainwater collection system.

    • Hi, Eric

      It is indeed absurd that some states have deemed that one does not own the water that falls on one’s own property. There are many reasons why this is beyond ridiculous and beyond unfair, but it is what it is and definitely is something to be considered when choosing a retreat location.

      In addition to possibly not being able to collect your own rainwater from your own roof, there are additional rules and regulations, potentially at city, county, state and federal levels to do with other water sources that might be on or run through your property as well.

      I can better understand how a river running through your property isn’t all yours, because if that was the case, what about the guy upstream of you (and downstream too!). But water off your roof? If I recall correctly, a guy was locked up in OR last year for having a rainwater collection system.

  2. Sorry to reply to such an old article, but it was a popular one…

    A good rain water collection system will have a way to catch the first X gallons of water, then send the rest to storage. Those first few gallons will contain dust, leaves, bird poop, etc. I don’t have it handy, but there is a certain number of gallons per 1000 sq ft of roof which you want to keep out of your storage. You’ll also need to subtract that amount from your collection calculations.

    • Hi, Stew

      All comments are always welcome, no matter when they’re made.

      I’m familiar with the concept of junking the first part of any water collection. But it is not a concept I’ve ever employed, myself. I’ve several times lived in places where all our water comes from the roof, and while if we looked inside our tanks, there was a huge amount of ‘stuff’ in them, we drank the water and used it for everything else, and never had a problem, bird poop, dust, leaves and all.

      I think we are way too cautious about such things these days. They never hurt our parents/grandparents/etc, and indeed, it never seemed to have hurt me either!

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