CH – Rainwater Harvesting System: Catchment

Rainwater Harvesting System

Basic Elements of the Rainwater Harvesting System

These days, rainwater harvesting is not anymore an example of extravaganza; it’s rather a clear “statement” expressing our eco-responsibility and commitment to Mother Nature. If it is still not convincing, let’s also mention that thanks to a lack of chemical treatment (chlorine), rainwater is usually healthier than any municipal water. And the best is – it is free (at least so far)!

Functional and effective Rainwater Harvesting System should include at least the following elements:

⦁ Water Catchment Area (the subject of this chapter)
⦁ Pre-filtration System
⦁ Conveyance System
⦁ First-Flush Diverter
⦁ Storage Tanks
⦁ Distribution System
⦁ Overflow System
⦁ Protection from insects & vermin

And, if needed:

⦁ Water Pump
⦁ Bladder or Gravitational System.

Assuming that you are located far from heavily-polluted industrial and/or agricultural zones, such a system if properly designed and maintained, should be able to provide the bathing-quality water. (See the chapter: CH -Fundamentals of Rainwater Harvesting).

For obvious reasons, to achieve the quality of drinking water, the system will need extra treatment stages including suitable Filtration and Disinfection. This process is regulated by law and will be subject of a separate chapter.

Keep water where it falls (Michal Mobbs)

Basic elements of the Rainwater Harvesting System (in red components being subject of this article). Source: Your Home (Australia’s guide to environmentally sustainable homes). 

Water Catchment Surface

Usually, the house’s roof is the best candidate for the water catchment. Situated well above the ground, exposed to a dusting effect of wind, roofs represent the cleanest surface for rainwater harvesting. And in the majority of cases, it’s an area readily available (one could say – for free). Note that ground-level surfaces like patio or driveway can also be used for harvesting rainwater water, however in general they are much more prone to contamination than roofs. As a result, the water collected this way can be used only for watering gardens and lawns.
Knowing the square footage of the roof you have to answer probably the most important question when it comes to the water harvesting system:

Will your roof (as it is) provide enough water for the needs of your household?

The answer is not an easy one, because it largely depends on statistical data based on the past: Average Annual and Monthly Precipitations for a given location. Such data represents a so-called moving average based on measurements taken over years of measurements. It represents the “probability”, with no guarantee (so it’s sort of ”take it or leave” data). What makes it worse is that our climate changes as you read this text and statistical values based on the past may represent not very reliable estimations. Regardless of what you think about it and how much you can trust the past, it is still the ONLY available data for all of us.

30-years Average Annual Precipitations map (measured in inches/sq. ft.). Source: PRISM (

How much of rainwater falls on your roof?

The math is simple: 1” of rain falling over the surface of 1 square feet represents 0.623 gallons of water. To calculate the amount of water you can collect from the roof (in gallons) multiply the Roof Surface [sq.ft] x Local Average Annual Precipitation [Inches/sq.ft] x 0.623 [Gallons/inch].

The result represents the estimation of the total amount of water that will fall on your roof over a period of 1 year. In wet years it will be more, in dry times less! When the climate is stable, the calculated result will offer a pretty accurate estimation of reality. While in our time climate changes almost follow our swings of mood, this is still the best estimation we may have and a starting point to Rainwater Harvesting Project.

Note that the weather has a cyclical pattern (Year-long), that’s why this calculation must be done based on the average Annual Precipitations.

Example of 1 x TEU Container

Flat roof of 1 TEU (20ft-Equivalent Unit) shipping container has a footprint of 8ft x 20ft = 160 square feet. Assuming location in Williamsburg (Virginia) with its average annual precipitations of 47 inches, you can expect to collect 160 x 47 x 0.623 = 4,685 gallons of water per year. Let’s call it Annual Rain-Water Supply (ARWS).
Now, you have to compare the expected amount of harvested water with the household’s needs. If the whole project is a result of eco-consciousness and you still have access to the main water system (or other sources like well, river, lake…), then you can consider your project as an act of eco-friendliness. However, if that’s all you have, then the next questions are:

⦁ How much water do you need?
⦁ How much of roofing you have to add to meet your household’s needs?

In both cases you will have to make calculations over some reasonable periods of time, keeping in minds that rain falls only sometimes, while water is used on a daily basis. For example in summer, most likely you will use more water than in winter (more frequent showers, irrigation etc…)…..
How much water do you need?

Once again – a good starting point is statistics. Obviously, daily water consumption heavily depends on climate zone, personal habits, water-efficiency of your appliances as well as outdoor activities (garden, pool, Jacuzzi etc….).

Average per capita household water use in the United States in 2014 (results are shown for selected cities in gallons/day). Source: Statista

Based on presented statistical data, the numbers are 61 -to- 111 gallons/capita/day for single-family and correspondingly 56 -to- 83 gallons/capita/day for multifamily. While these numbers may not reflect the reality of your individual household, they give an idea of what to expect.

Fortunately, this enormous amount of consumed water represents our “lavishness” rather than necessity. Similar statistics for off-grid households bring these numbers to a more reasonable range 15 -to-30 gallons/person/day.


Water use in typical Off-grid House in Sidney (numbers in black color are in US gallons). Source: Sustainable House (Australia) by Michael Mobbs.

The message is clear – we are massively overusing fresh (potable) water. The good news is that we do not have to give up much on our lifestyle lo lower the consumption of water. We just have to change our wasteful habits (which often is the most difficult task to accomplish).

While it is not the subject of this article, let me encourage our readers by giving some good examples. As we mentioned earlier, toilets are real sinkholes in our water budget. However, collecting the water from a shower, washing machines and kitchen sinks in a Gray Water Tank can offer more than needed for flushing toilets. It’s a good example of recycling, and frankly, it is in our vital interest to get used to it.


Recycling water in an Off-grid House. Source: Sustainable House (Australia) by Michael Mobbs.

But recycling is only one part of the “equation”. Other steps in the right direction are: replacement of old flushing systems by low-flow, dual-flushing ones, unrestrained shower heads by water-saving models, wasteful irrigation by drip-based one, and so on…

The final math test is easy: Multiply the (Daily Water Use per Person) x (Number of Family Members) x (365 days). Just as an example: assuming the daily use of 30 gallons/person and family of 3, you will need correspondingly 30 x 3 x 365 = 32,850 gallons of rainwater per year.

Roof’s Requirements

Traditional cargo shipping containers are designed for the transport of goods. In-line with transport requirements their corrugated CorTen steel roofs are flat. Such structures offer protection from weather elements and allow for stacking several modules on top of each other. The corrugated surface offers mechanical strength and helps to channel rainwater towards edges, while Corten steel provides some protection against corrosion. Do not expect more, that’s all!

Container roof

The shipping container roof is designed according to the needs of the transport industry (and nothing more). Source: Multiboxx Ltd.

However, what is good for the transport industry may not be good enough for the housing industry and certainly, is not optimal for rainwater harvesting. Depending on plans for the end-use of collected rainwater, you may decide to take advantage of the original flat roof or install a more suitable sloped one.

The latter may also serve other functions, however, for the purpose of this chapter, we will focus here discussion only on the impact of the roof’s structure on the efficiency of rainwater harvesting and the quality of collected water.

Earlier we analyzed the amount of available rainwater based on the local average precipitation pattern and square footage of the catchment area (roof). The calculated value (Annual Rain-Water Supply) is an estimation based on the statistical past. But it’s also an optimistic estimation that does not take into account particular characteristics of the catchment (roof) as well as how much of collected water will meet relevant quality requirements (potable, utility or irrigation only…).

It should not be surprising that in reality, the amount of collected water (CW) of required quality (Q) is lower than the available supply (ARWS). The reduction factor known as Roof Coefficient (RC) is lower than 1 (typically in the range from 0.6 to 0.9). It means that out of 100 gallons of available rainwater, you will get 90 gallons from the roof with CR=0.9 and only 60 gallons from the roof with CR = 0.6). Mathematically, the RC = Collected Water in Tank / Available Rainwater Supply from the Roof and should be calculated as an average over longer period (preferably 1 year to include the impact of the cyclical nature of weather pattern). As you will find below, the value of RC is determined not only by the roof itself, unfortunately it also depends on several “exterior” factors.

Factors affecting RC

Roofing Materials

The catchment surface must be impermeable to prevent the retention of the water. The bottom line is – whatever falls on it, should run down (hopefully to gutters and then via a network of downspouts to the tank). Fortunately, container-based houses overwhelmingly meet this criterion.

Original shipping containers roofs are made from Corten. Theoretically, this weathering steel is corrosion resistant. In reality, however, in an aggressive ambiance (salty humidity), Corten is almost as prone to corrosion as the regular steel. That’s why most shipping containers are coated by weather-resistant paints. Also, most custom roofs built over container-based houses are made from coated metal sheets or composite materials (PVC etc…).

Just to give an idea about what happens if the roof can retain a certain amount of water. A good example here is a flat, gravel-covered roof so popular in municipal architecture. It will capture a noticeable amount of water at the initial phase of rain. During rainstorms, it probably wouldn’t matter, but short-lasting rain showers will be first absorbed by the gravel and will rather evaporate than drain into the tank. Similarly (although to lower extent) act roof from clay-tiles. They are porous and easily absorb rainwater.


At first, the finish of metal or composite roofs does not seem to matter. Unfortunately, it is not the case. The rough, grainy surfaces will accumulate much more pollution (dust, birds’ droppings, pollen etc…) than smooth, shiny surfaces. As a result, it will take more time (and rainwater) to wash it out. Usually, water harvesting systems are equipped with First-Flush devices, diverting initial, heavily contaminated rainwater (known as “first-flush”) from the tank. Longer it takes to flush the roof, more water you lose. The sad part of the story is that rainwater from short (sporadic) showers may never reach the tank.

Note, that with time, original uncoated Corten-steel roofs will develop a layer of oxide changing the texture of its surface from smooth and glossy to matt with some dose of roughness. It will be a perfect area for contamination to build-up.


The sloped roof has a larger surface than the flat one covering the same container (roof angled at 45 degrees will have 41% bigger surface compared to the flat roof with the same footprint). But do not get it wrong – it will not change its rain-capturing surface when the rain falls vertically.

All roofs are exposed to pollution, however, flat roofs have a tendency to accumulate more pollution than sloped ones. It’s because even gentle breezes with a help of gravitation can clean (at least to some extent) sloped, smooth roofs. For that to happen on flat roofs you may need much stronger winds. The bottom line – if the quality of water is important, flat-roofs will need longer First-Flush run to make sure that accumulated over time pollution is diverted from the water collecting reservoir (tank).

From that point of view it may look like higher the pitch, better it is. Well, not really – first of all, usually the pitch of custom roofs is determined by the optimum angle of solar panels rather than rain. But that’s not all – it turns out that steep roofs may actually decrease the efficiency of the rainwater harvesting process.

In such a theoretical situation (in fact quite unlikely) the effective catchment area will equal to zero. To make it clear – the effective catchment area is determined by the projection of the roof in the direction of the rain. For simplicity of calculations, we assume that rain falls vertically!
Note that it will not be a problem with gabble-type roofs (2-sided slop) because what will be lost on one side of the roof, will be compensated on the opposite one.


        Effective catchment area of the sloped roof. Source: Smart Solar Tech (South Africa)

⦁ The effective catchment area of the sloped roof is equal to its vertical projection (in other words to its footprint). This assumption, however, applies to the theoretical case when the rain falls perfectly vertically (perpendicularly to the catchment’s footprint). Small deviations from verticality will not alter the situation (especially for the flat roof).

However in windy conditions and high-pitched single-slop roof, one can imagine rain falling in a direction parallel to the surface of the roof. In such a theoretical situation (in fact quite unlikely) the effective catchment area will equal to zero. To make it clear – the effective catchment area is determined by the projection of the roof in the direction of the rain. For simplicity of calculations, we assume that rain falls vertically!

Note that it will not be a problem with gabble-type roofs (2-sided slop) because what will be lost on one side of the roof, will be compensated on the opposite one.


Effective catchment area of the sloped roof depends on the angle of impact of rain (here positive scenario). Source: Intech Open (Smart Rainwater Management: New Technologies and Innovation by Raseswari Pradhan and Jayaprakash Sahoo)

⦁ On steep roofs, water will flow down to the edge at higher velocity. In light rain conditions, it wouldn’t change much the ability of gutters to channel rainwater towards downspouts. However, during rainstorms, rushing down rainwater may partially overpass gutters. From this point of view, it may be beneficial to limit the roof’s angle to about 30 degrees.


⦁ Due to the fact that original and custom container roofs are made from metal, they are smooth and impermeable. As a result, their Roof Coefficient (RC) will be in the higher end of the range. Factors limiting the efficiency of the water harvesting process (catchment in this case) are rather related to imperfections of the system (leaks, overflows) and need to meet water quality goals.

⦁ Flat roofs require longer First-Flush period to wash-out pollution, so more rainwater will be diverted from the system. In general, this effectively lowers the value of RC for flat roofs compared to sloped ones. The impact may be not visible in climate zones with frequent rainstorms. However, it will be significant in climate zones with short, light rain showers separated by longer dry periods. It’s because the water captured by the roof may be mostly diverted out of the system!

⦁ The equivalent RC determines how much of available rainwater will end-up in the water tank. The tricky point is that equivalent RC of the roof not only depends on the roof itself (shape, finish, etc) but also on the weather patterns (rainstorms or short light showers), level of local pollution and end-use of the water (irrigation, utility or potable). In fact, the same roof at different geographical locations may have different water-harvesting efficiency and so a different Equivalent Roof Coefficient (ERC). In fact, the ERC represents the efficiency of the whole rainwater harvesting system.

⦁ The value of the ERC has a direct impact on the design of the Water Catchment subsystem. Knowing the Average Annual Precipitation (P [inch/sq.ft]) at your location and Annual Household Water Needs (AHWN [gallons]), you can calculate the required catchment surface (CS [ sq. ft]) as:

CS [sq. ft} = AHWN [galls] / (P x ERC x 0.623)

Where: CS is the footprint of the catchment.

Unfortunately, you will NOT find in books the exact value of the ERC for the system at your location. The goal of the above discussion was rather to make you aware of problems waiting ahead on the path to “independence” from the municipal grid(s).

But there must be also The Conclusion: So here it is:

To be on the safe side add an extra catchment area (sort of “buffer”). Depending on the type of your roof, local pollution, weather patterns and water end-use is should be in the range of 20% to 30%.

We could write it shortly a few pages ago, but at least now, you can make your educated decision instead of blindly following someone’s advice.


Air-pollution is the reality we live in….. Source: Pittsburgh Post-Gazette by Don Hopey (April 26, 2019)


Original containers’ roofs from Corten steel as well as eventual coatings (paints) are rather not suitable for harvesting potable water. Heavy metals (iron but also alloys) and potentially harmful chemicals used for industrial-quality coatings will contaminate water making it unusable for drinking. While such water can be still perfect for flushing toilets and watering lawns, it may be questionable for shower and veggie gardens and certainly forbidden for drinking. It’s true, that anyhow, rainwater has to be treated to assure its drinking quality, but it is also obvious that extra contamination will make the treatment process more complex and costly.

The bottom line – the minimum you should do is to coat the original roof with suitable paints or (when building an extra roof) – to make sure that its surface is certified for water harvesting.

The good news is that majority of modern roofing materials and coatings used for containers’ custom roofs (as well as solar panels themselves) are safe for rainwater harvesting. Note that even potentially harmful surfaces, when coated with non-toxic baked paints or enamel, may be suitable for water harvesting.

The list of materials to avoid includes: galvanized steel (zinc), copper, all lead-based paints… Roofing materials popular in traditional residential housing like asphalt/bitumen (shingles), treated wood, and asbestos are also harmful, but they are rather not used in container-based houses.

Note: You can find the actualized list of approved coatings for rainwater catchment systems on the site of National Sanitation Foundation (NSF)
Or just

Pros and Cons of the Original Flat Roof:

The original roof comes for free! If we put aside the fact that it is also easy to walk on it when needed, it’s pretty much its only advantage.

⦁ Rainwater will naturally flow towards both sides of the container (that’s the original design), so the gutters will have to be installed on both sides. It’s not about extra cost (certainly the new roof will cost more), but about potential extra overflows and leaks decreasing the effectiveness of the water harvesting system. It may matter if the water budget is tight.

⦁ Given the fact that shipping containers are built according to requirements relevant to the transport industry, their exterior finish may not be suitable for water harvesting.

⦁ Gutter leaves-screens are quite ineffective on flat roofs. Without gravitation-driven shedding effect (as on sloped roofs), leaves will have a tendency to obstruct screens, leading to overflows and loss of precious rainwater.

⦁ Probably the biggest problem of flat roofs is how to keep them clean. Organic matter (birds’ droppings, dead insects, leaves …) will have a tendency to stick to the surface. Without the help of gravitation (as on pitched roofs), a gentle breeze wouldn’t be able to clean it, so this may be your job. If the dirt is left on the roof for longer periods of time, it will take more time to wash it out with the next rain. As a result, you will have to divert larger quantities of rainwater from the storage tanks, losing this way the precious gift from the sky – the Water. This is the most important factor in reducing rainwater-harvesting efficiency on flat roofs.

Sloped roof

Even the best roofing materials will accumulate industrial and organic pollution, so periodic cleaning is the must if the roof is intended for water harvesting. Source: Sheffield Metals International

Pros and Cons of Sloped Roofs:

A new sloped roof will add to the overall cost of a container-based house, but that’s pretty much its only shortcoming. Note that when the custom roof is properly-designed it may also serve as a natural support for solar panels and/ or part of a cross-ventilation system. It may also add stylish architectural accents. The bottom line – designing your own roof gives you the advantage to do it right from the start.
⦁ Most roofs will be covered with metal sheets. Using durable non-toxic coatings will make them safe for rainwater harvesting. Fortunately, most of the metal-based roofing materials available on the market are finished with harmless coatings (baked paints, enamel…)

⦁ All sloped roofs (especially steeper ones) have sort of self-cleaning characteristics. When finished with smooth and shiny coating, even gentle breeze will act as a “duster”. Organic matter (birds’ droppings, dead insects, leaves) if not cleaned by wind, will quickly flow down with first drops of rain so the pollution can be effectively handled by First-Flush Diverters.

⦁ Gutter leaves-screens are more effective on sloped roofs (effect of shedding) although you may still need to clean them from time to time!

Next chapter: CH-Rainwater Harvesting: Pre-Filtering Systems

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