Project guide
One inch of rain on a barn roof is more water than you think.
A thousand square feet of roof and an inch of rain yields about 623 gallons before losses — more than two totes' worth. Many people size a rainwater system against their optimism rather than their roof, end up with one tank that overflows every storm and runs dry in every dry spell, and conclude the idea does not work.
The short answer
One inch of rain on 1,000 sq ft ≈ 623 gallons before losses. A 275-gallon tote fills from roughly half an inch on that roof after typical losses. Fit a leaf screen and a first-flush diverter, manifold multiple tanks at the base, and wrap them opaque to keep algae down. Drain down before hard freezes.
Step one
Size it against the roof.
Catchment is roof footprint, not roof surface area — a steep roof and a flat roof of the same footprint catch the same rain.
| Roof footprint | 1" of rain | At ~85% efficiency | 275s to catch one inch | Sensible system |
|---|---|---|---|---|
| 500 sq ft | 312 gal | 265 gal | 1 | 1 – 2 × 275 |
| 1,000 sq ft | 623 gal | 530 gal | 2 | 2 – 3 × 275 |
| 1,500 sq ft | 935 gal | 795 gal | 3 | 3 – 4 × 275 |
| 2,000 sq ft | 1,246 gal | 1,059 gal | 4 | 4 – 6 × 275 |
| 3,000 sq ft | 1,869 gal | 1,589 gal | 6 | 6 – 8 × 275 |
| 5,000 sq ft | 3,115 gal | 2,648 gal | 10 | 8 – 12 × 275 |
The arithmetic: roof sq ft × 0.623 = gallons per inch of rain. Multiply by an efficiency factor (0.85 is a common planning assumption) for real-world collection.
Step two
Build it in this order.
- 01
Screen and divert
A leaf screen at the gutter outlet and a first-flush diverter that dumps the opening flow — the part carrying droppings, grit and pollen. Skipping the diverter is a common reason home systems smell.
- 02
Set the tanks level and high
On a level, load-bearing base. A full 275 holds about 2,300 lb of water alone, so use blocks or a slab, never bare soil. Every foot of elevation adds about 0.43 psi of gravity pressure at the outlet.
- 03
Manifold at the base
Connect tanks bottom-to-bottom with a manifold so they fill and draw evenly. With series plumbing, the last tank may never fill and the first may never empty.
- 04
Overflow, wrap and drain
Route overflow away from foundations. Wrap the tanks opaque. Fit a drain-down valve at the lowest point for winter, and use it.
The first-flush diverter
This is the component many people skip and one that largely determines whether your stored water smells. The opening flow off a roof carries what has accumulated since the last rain — bird droppings, airborne particulate, pollen, grit, roof granules and gutter debris.
A diverter is a vertical pipe that fills first, captures that opening volume, and only then lets flow continue into the tank. A ball float seals it as it fills. It drains slowly through a small orifice between storms and resets itself.
Sizing rules of thumb vary with roof type and local conditions, so follow the diverter maker's guidance for your roof area. It can be built cheaply from standard pipe and is one of the highest-value components in the system.
Pressure and pumps
Gravity gives you about 0.43 psi per foot of height. A tote on four-foot blocks delivers only a couple of psi at the outlet — adequate for a soaker hose or flood irrigation, not enough for most sprinklers or spray nozzles.
For pressure, a small on-demand pump with a pressure switch — the kind sold for RV and marine use — is a common choice. Fit an inlet screen; the pump is the component grit will find.
Algae, mosquitoes and freezing
Algae
Block the light. Opaque wrap, exterior paint or a shaded location. Removing the cause beats any chemical treatment, and natural HDPE passes enough light to support growth.
Mosquitoes
Any opening larger than a fine mesh is an invitation. Screen the inlet, screen the overflow, and make sure the cap seals. A sealed, screened, opaque tank is a non-event; an open one is a nuisance and may break local codes.
Freezing
Ice expansion can deform bottles and stress the cage. In hard-freeze areas, drain the tote through a valve at the lowest point before the first hard freeze, or insulate and heat-trace it if you need the capacity year-round.
Which bottle to start from
For irrigation of ornamentals, a used tote with known, compatible prior contents is often fine. For anything a person, an animal or an edible crop contacts directly, use a new bottle or one with documented food-grade history — ask the supplier for documentation and check current regulations.
Request a free, no-obligation quote and we will match you with a partner supplier in our network, or see parts and accessories.
Legality
Check the rules before you build.
General information, not legal advice. Rainwater rules are set by states and municipalities and they change.
| What to check | Why it matters |
|---|---|
| State water law | A few western states limit volume, require registration or restrict use |
| Incentives | Some states and cities offer rebates or tax incentives for collection |
| Local plumbing code | Any indoor or potable connection usually triggers code requirements |
| Backflow prevention | Often required where harvested water connects to a building's plumbing |
| Mosquito and nuisance rules | Open or unscreened tanks may break local codes |
| HOA or zoning rules | Some neighbourhoods restrict visible tanks |
Colorado is a well-known example of a state that limits residential collection. Check current regulations for your state and municipality before building. Outdoor, irrigation-only systems are generally the simplest case.
Components
A bill of materials for a three-tank system.
A typical setup for a medium-sized roof. Quantities and sizes depend on your roof, rainfall and use.
| Item | Spec | Qty | Why |
|---|---|---|---|
| New or food-grade-history 275 gal tote | Opaque wrapped | 3 | Any water a person or crop contacts |
| Leaf screen at the gutter outlet | Stainless mesh | 1 | Bulk debris before anything else |
| First-flush diverter | Pipe chamber, ball float, drain orifice | 1 | Dumps the dirtiest opening flow |
| Base manifold | With unions | 1 | Tanks fill and draw as one vessel |
| Isolation valves | Ball valves | 3 | Service one tank without draining three |
| Overflow assembly | Routed away from the foundation | 1 | Tanks overflow in heavy storms |
| Drain-down valve | At the lowest point | 1 | Winter drain-down |
| Inlet and overflow screens | Fine mesh | 2 | Mosquitoes; often a code requirement |
| Opaque UV sleeve or wrap | Per tank | 3 | Algae need very little light |
| Blocks or slab | Rated for a full tank's weight | — | Never on bare soil |
| On-demand pump, optional | 12V or 120V, pressure switch | 1 | Gravity gives about 0.43 psi per foot |
| Inlet filter for the pump | Washable | 1 | Grit finds the pump first |
Yield by region
Size storage for the dry spell, not the year.
Annual rainfall tells you how much you could collect. The longest dry spell in your growing season tells you how much you need to store.
- Look up average monthly rainfall for your area from a weather service.
- Multiply roof footprint × 0.623 × monthly inches × your efficiency factor to estimate monthly yield.
- Estimate your monthly use for the same period.
- Size storage to cover the gap in the driest stretch you need to carry.
In dry climates the limit is usually storage and dry-season length, not catchment area. In wet climates you may fill and overflow often, and the overflow route matters more.
Maintenance
A calendar for a system that lasts.
| When | Task | Skipping it causes |
|---|---|---|
| After heavy storms | Check the first-flush diverter drained and reset | It stays full and diverts nothing |
| Regularly | Clear the leaf screen | Overflow at the gutter, not the tank |
| Regularly, in summer | Check for light getting in at seams | Algae growth |
| Periodically | Inspect overflow route and screens | Water at the foundation; mosquitoes |
| Periodically | Flush the manifold and isolation valves | Sediment jams a valve |
| At least yearly | Inspect the bottle for cracks or crazing with a flashlight | Finding a crack by a leak instead |
| At least yearly | Check blocks and base level | Uneven loading stresses the base |
| Before hard freezes | Drain down | Deformed bottles, stressed cage |
| As needed | Replace cap gaskets and valve seals | Weeping and drips |
| As needed | Replace the UV sleeve or wrap | Sleeve fails, algae returns |
Water quality
What each use asks of harvested rainwater.
General guidance only. Irrigation needs little treatment; anything a person drinks needs a great deal, and may be regulated.
| Use | Filtration | Disinfection | Container |
|---|---|---|---|
| Ornamental irrigation | Leaf screen | Usually none | Any compatible |
| Edible crop irrigation, soil | Leaf screen, first flush | Usually none | New or food-grade history |
| Edible crop, foliar or hydroponic | Sediment filter | Often advisable | New or food-grade history |
| Livestock drinking | Sediment filter | Often advisable | New or food-grade history |
| Washing equipment | Sediment filter | Depends on use | New or food-grade history |
| Toilet flushing | Sediment filter | Check local code | Any compatible |
| Drinking | Multi-stage | Required, with testing | Check current regulations and your health department |
Rainwater harvesting questions
How much rain does it take to fill a 275 gallon tote?
One inch of rain on 1,000 square feet of roof yields about 623 gallons before losses. So a 275-gallon tote fills from about 0.44 inches on that roof in theory, or roughly half an inch once you allow for splash, evaporation and the first-flush diversion (assuming about 85% collection efficiency).
Is it legal to collect rainwater in the US?
In most states residential collection is allowed, and some encourage it with incentives. A few western states have rules on volume, registration or use — Colorado, for example, limits residential collection. Rules change, so check current regulations for your state and municipality before you build. This is general information, not legal advice.
Can I drink rainwater collected in an IBC tote?
Not without treatment, and not from a tote with industrial history. Roof runoff can carry bird droppings, airborne particulate, roofing material and gutter debris. Potable use needs a suitable bottle, proper filtration, disinfection and testing, and may be regulated where you live — check with your local health department. For irrigation, most of that is unnecessary.
Why does my rainwater tank go green?
Light. Natural HDPE is translucent, not opaque, and algae need very little light to grow. Wrap the tote, paint it or shade it. It is a cheap, effective fix and one of the most common things missed in DIY systems.
Will my tote survive winter?
An empty one usually does; a full one may not. Water expands as it freezes and can deform the bottle and stress the cage. In hard-freeze areas, fit a drain-down valve at the lowest point and empty the tote before the first hard freeze, or insulate and heat-trace it if you need year-round capacity.