Question 1

Head Pressure

Accepted Answer

is a critical physics concept in rainwater systems. Because rain barrels are typically gravity-fed, the water pressure at the spigot is determined entirely by the elevation of the water surface relative to the hose outlet. The standard constant is that every 1 foot of vertical elevation generates 0.433 Pounds per Square Inch (PSI) of water pressure.

Question 2

The Standard 55-Gallon Drum:

Accepted Answer

The most common residential method is the single 55-gallon food-grade plastic barrel. These are inexpensive, widely available, and easy to install. They are ideal for small urban lots or casual container gardening. However, as the calculations prove, a 55-gallon barrel is drastically undersized for almost any roof. They require robust overflow hoses to redirect the massive excess water away from the foundation.

Question 3

Daisy-Chained Systems:

Accepted Answer

To increase capacity without buying massive industrial tanks, homeowners often "daisy-chain" multiple 55-gallon barrels together. This is done by connecting the barrels with PVC piping or flexible hoses. They can be connected at the top (where Barrel A fills completely before overflowing into Barrel B) or at the bottom (where all barrels fill simultaneously, maintaining an equal water level across the system). Bottom-linking is mathematically superior for maximizing head pressure across the entire system, but top-linking is safer against catastrophic leaks.

Question 4

Intermediate Bulk Containers (IBC Totes):

Accepted Answer

For those requiring more serious volume, the 275-gallon or 330-gallon IBC tote is the next step up. These are square, plastic tanks encased in a galvanized steel cage, originally used for shipping industrial liquids. Because a standard 1,000 sq ft roof shedding 1 inch of rain generates roughly 530 gallons, a pair of IBC totes is the most mathematically appropriate storage solution for a standard residential home looking to capture a full storm event.

Question 5

Underground Cisterns:

Accepted Answer

For ultimate capacity, underground cisterns ranging from 1,000 to 10,000+ gallons are utilized. These systems require complex excavation and rely on electric pumps rather than gravity. These are typically used in "Whole-House" rainwater harvesting systems, where the water is pumped back into the home, filtered, sterilized with UV light, and used for flushing toilets, washing clothes, and sometimes even drinking (where legally permitted).

Question 6

Dry vs. Wet Systems:

Accepted Answer

The routing of the pipes also presents two distinct methods. A "Dry System" uses pipes that slope continuously downward from the gutter to the top of the tank; the pipes empty completely after the rain stops. A "Wet System" routes the downspout down into the ground, horizontally across the yard, and then vertically up into the tank. The water in the underground pipe remains trapped between rain events. Wet systems allow tanks to be placed far away from the house, but the calculations must account for the static water left in the pipes, and they must be drained before winter in freezing climates.

Question 7

Scenario 1: The Suburban Gardener in Seattle, Washington

Accepted Answer

Seattle is famous for its rain, but it actually experiences frequent, light, drizzly rain rather than massive downpours. A gardener in Seattle has a 500 sq ft roof section feeding a downspout and an asphalt roof (0.85 coefficient). They receive about 38 inches of rain annually. 
Annual Yield: 500 × 38 × 0.623 × 0.85 = 10,061 gallons per year.
Because the rain is frequent and spread out, this gardener does not need massive storage. A simple setup of three daisy-chained 55-gallon barrels (165 gallons total capacity) is sufficient. The barrels will fill up, the gardener will use the water during brief dry spells, and the barrels will quickly refill during the next drizzle. The continuous cycle of use and replenishment means small storage handles a large annual yield.

Question 8

Scenario 2: The Off-Grid Cabin in Tucson, Arizona

Accepted Answer

Tucson is a desert climate that receives only about 11 inches of rain per year. However, this rain does not fall evenly; it arrives in violent, massive monsoon downpours during the late summer, dropping 2 or 3 inches in a single afternoon, followed by months of absolute drought. The cabin has a 1,200 sq ft metal roof (0.95 coefficient).
Single Storm Yield (3 inches): 1,200 × 3 × 0.623 × 0.95 = 2,130 gallons in one afternoon.
If this homeowner uses 55-gallon barrels, they will capture almost nothing and lose 2,000 gallons to overflow. Because they face months of drought, they must capture the entire storm event when it happens. This scenario mathematically demands a large 2,500-gallon cistern. The calculation proves that in arid climates, massive storage capacity is non-negotiable.

Question 9

Scenario 3: The Urban Market Farm

Accepted Answer

An urban farmer operates a 2,000 sq ft greenhouse in Ohio. They need to water their crops, requiring roughly 1 inch of water per week over a 1,000 sq ft vegetable bed. 
Water Demand: 1,000 sq ft × 1 inch × 0.623 = 623 gallons required per week.
By calculating their weekly demand, the farmer knows exactly how much storage they need to survive a three-week summer drought: 623 × 3 = 1,869 gallons. They can then size their collection tanks accordingly to ensure their business survives dry spells without paying for expensive municipal water.

Question 10

Rain Barrels vs. Municipal Tap Water:

Accepted Answer

The default alternative is simply paying for municipal water. Municipal water provides infinite volume and 60 PSI of guaranteed pressure at the turn of a handle. It requires zero maintenance, zero mathematical calculation, and zero physical footprint on the property. However, it costs money (which compounds over time), it contains plant-inhibiting chemicals like chloramine, and relying on it does nothing to prevent destructive stormwater runoff from the roof. Rain barrels require upfront capital, physical space, and maintenance, but provide free, chemically superior water and protect local watersheds.

Question 11

Rain Barrels vs. Greywater Systems:

Accepted Answer

Greywater harvesting involves capturing the gently used water from a home's bathroom sinks, showers, and washing machines, and routing it into the garden. A greywater system provides a highly predictable, year-round yield that is completely independent of the weather; a family of four will generate roughly the same amount of shower water in a drought as they do in a monsoon. However, greywater systems are heavily regulated, require complex plumbing modifications inside the house, and the water cannot be stored for more than 24 hours (as the organic matter in it will turn septic and smell). Rainwater requires no indoor plumbing, can be stored for months if properly sealed, but is entirely at the mercy of the weather.

Question 12

Rain Barrels vs. Passive Rainwater Harvesting (Earthworks):

Accepted Answer

Active harvesting uses barrels and tanks to store water. Passive harvesting uses landscaping—such as swales, rain gardens, and French drains—to direct roof runoff directly into the soil where it is stored in the earth itself. If a calculator reveals that a roof will shed 2,000 gallons in a storm, buying 2,000 gallons of plastic tanks is incredibly expensive. Alternatively, digging a shallow, planted basin (a rain garden) at the end of the downspout can absorb those 2,000 gallons into the aquifer for the cost of a few hours of labor with a shovel. Passive harvesting cannot provide pressurized water for a hose later in the week, but it is vastly cheaper and handles massive volumes of water better than barrels.

Question 13

Can I drink the rainwater I collect from my roof?

Accepted Answer

Absolutely not, unless it undergoes extensive, professional-grade purification. Roofs are highly contaminated surfaces covered in bird droppings, animal urine, dead insects, pollen, dust, and chemical residues from roofing materials. Untreated rainwater collected in a barrel is strictly non-potable and should only be used for watering plants, washing cars, or flushing outdoor debris. To make it potable, it must be filtered to the sub-micron level and sterilized using ultraviolet (UV) light, chlorine, or ozone treatment.

Question 14

How do I increase the water pressure from my rain barrel?

Accepted Answer

Water pressure in a gravity system is dictated entirely by elevation. You gain 0.433 PSI for every foot the water is raised above the outlet. To increase pressure, you must physically elevate the barrel on a sturdy, level platform made of cinder blocks or pressure-treated wood. Raising the barrel 3 feet will provide enough pressure for a low-flow soaker hose. If you need enough pressure to run a standard oscillating lawn sprinkler, gravity will not suffice; you must purchase and install an electric transfer pump to force the water through the hose.

Question 15

How do I stop mosquitoes from breeding in my rain barrel?

Accepted Answer

Mosquitoes require stagnant water to lay their eggs, and a rain barrel is an ideal environment. You must ensure that every single entry and exit point on the barrel is sealed with fine-mesh screen (standard window screen works perfectly). This includes the top intake hole and the overflow pipe. If mosquitoes do manage to get inside, you can drop a "Mosquito Dunk" into the water. These are small, organic, donut-shaped tablets containing Bacillus thuringiensis israelensis (Bti), a naturally occurring bacterium that kills mosquito larvae but is completely harmless to plants, pets, and humans.

Question 16

What happens to the system when the rain barrel is completely full?

Accepted Answer

When the barrel reaches its maximum capacity, the excess water must be safely diverted away from the tank to prevent it from spilling over the top and flooding the foundation of your house. Every properly designed rain barrel must have an overflow port near the top rim. This port must be connected to a hose or pipe that is equal to or larger than the intake pipe, directing the excess water downhill, at least 6 to 10 feet away from the building's foundation, ideally into a rain garden or existing drainage swale.

Question 17

Does a steeper roof catch more rain than a flat roof?

Accepted Answer

No. The pitch or angle of the roof has zero effect on the total volume of rain caught. Rain falls vertically from the sky. Therefore, the amount of rain intercepted is based entirely on the horizontal footprint of the building. A perfectly flat roof covering a 1,000 square foot footprint and a steeply pitched A-frame roof covering that exact same 1,000 square foot footprint will catch the exact same amount of water during a storm. This is why calculators only ask for the length and width of the catchment area.

Question 18

How long can I safely store rainwater in a barrel?

Accepted Answer

If the barrel is completely opaque (blocking all sunlight to prevent algae), properly screened (to prevent mosquitoes and debris), and the water passed through a first-flush diverter (to remove initial organics), rainwater can be stored safely for several months. For gardening purposes, storing water for 3 to 6 months is generally safe. However, the longer it sits, the more likely anaerobic bacteria will develop, which can cause a sulfurous "rotten egg" smell. It is best practice to use the water regularly and let the barrel flush out completely with fresh rain whenever possible.

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