Just How Much Rainwater Can You Collect Off Your Roof?


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When we watch the evening news, the weatherperson will oftentimes say things like “X city received an inch of rain today”, which does not really sound like all that much. We imagine a single inch of rain in a small puddle somewhere it seems rather insignificant. But what if you knew that a single inch of rain could allow you to collect hundreds and hundreds of gallons of water, if not more? Now that one inch of rainfall seems like a lot more!

Let’s take a look at a hypothetical example to see just how much water we can collect from rainfall. If you have 1,000 square feet of roof on your house, and it rains just 1 inch, you can collect 600 gallons of water to be used in your garden, for washing your car, or just for drop irrigation around your property. That’s 600 less gallons you have to pay for and use from your town water supply! So how can you do the math for your own roof? Just multiple the square footage of roof space you have available X 0.6 gallons per square foot per inch of rain, and you can see how much water you can collect from each inch of rain that falls.

So if you have 2,500 square feet of roof available for water catchment, and a single inch of rain falls one day, we see that:

2,500 X 0.6 = 1,500 gallons of water can be harvested for future use…from only one inch of rainfall!

On average, Americans use about 69 gallons of water per person per day for bathing, cooking, cleaning and flushing toilets – and is just for indoor water use and does not account for any watering/car washing going on outside. That amounts to about 2,100 gallons a month for each person – or only slightly more than you might be able to collect off your roof in a single rainstorm. Sure, you might not want to use that water for your showers or drinking water, but it can be done…and many people are putting cisterns in their yard and systems in their house to be able to do so. But if you are not interested in doing that and just wanted to use it outside, using a rain barrel or two under your downspouts can make a big difference in your monthly water usage and bill. Let’s take a look at one more example to see how much of a difference collecting rainwater can make – If you live in Boston, Massachusetts, which gets an average of 42.53 inches of rain per year, and you live in a house with 1,000 square feet of roof space…

1,000 X 0.6 X 42.53 = 25,518 gallons of water collected each year in Boston, MA.

That’s a lot of water that you can save (and money!) by hooking up and using rainwater catchment systems at your house. It’s a big return for a very small investment, and I bet you will hear something different the next time the weatherperson says “X city received an inch of rain today”!

Photo by laffy4k

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  1. It depends on the area, and as nutty as it sounds, I do understand why some of those laws are in place. I do think they need to be looked at again though in those communities, as the laws were set in place a long long time ago for the farmers and their acequias….

  2. Yes, but is it legal to collect it in NM? Here in Colorado, it is actually illegal to do so! Can you believe that? Complicated water rights in this state dictate that unless you have water rights on your property, rain barrells are illegal. Isn’t that nuts?

  3. You are right, tons of people here have cisterns, and we are looking at 2 houses where that is the only source of water. But you cannot have one if you live in certain parts of town, as you are not allowed to collect rainwater. But most of the area is fine…

  4. I know many people in the Taos area of NM that have cisterns… Don’t know much about the legalities of it, though for many of them it is their only water source, so it must be legal for that reason. However, most such laws are dependent upon county and/or city zoning codes which can be petitioned and changed at whichever level they belong to. If it’s dependent upon water rights some counties allow a land owner to purchase such rights, again this usually depends upon zoning.

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