How to Build a Rainwater Collection System That Does Not Overflow

A rain barrel under a downspout is not a rainwater system. It is one component of one, and on its own it overflows in the first serious storm, breeds mosquitoes by midsummer, and fills with the roof grit that the first ten minutes of rain washes off. The difference between that and a system is five parts, and none of them is expensive.

This guide is about building the system. If what you want to know is how much water a roof yields and what that water is safe for, our guide to rainwater harvesting answers that question instead.

Before anything else: check whether you may

Rainwater collection is legal in most of the United States, and in a few western states it is regulated, because water law there treats precipitation as already belonging to downstream users. Start here rather than after you have bought a tank.

Colorado is the strictest case and worth knowing in detail, because it is the example most often garbled online. Under Section 37-96.5-103 C.R.S., established by House Bill 16-1005, the rules are specific:

The state's Division of Water Resources adds that properties with a residential well, or which could qualify for one, may be allowed to collect more under separate statutes. Other western states have their own rules and some offer rebates rather than restrictions. Check your own state and your municipality before you build, and check your HOA covenant too, which is more often the real obstacle than state law.

How much water the roof actually yields

The arithmetic is simple and the number surprises people. One inch of rain on one square foot of roof produces 0.623 gallons. That is geometry rather than an estimate: a square foot is 144 square inches, an inch of rain on it is 144 cubic inches, and a US gallon is 231 cubic inches.

The formula is therefore: roof footprint in square feet × inches of rain × 0.623 × efficiency.

Those figures use an efficiency factor of 0.85, which allows for what the first flush diverts, what evaporates, and what splashes past the gutter. Use the roof's footprint, the area it covers on the ground, not the pitched surface area: a steeper roof catches the same rain as a flat one of the same footprint.

The important conclusion is in the second column. A 1,000 square foot roof in a single two-inch storm produces over a thousand gallons, which is ten times what two standard barrels hold. Your system will overflow on most storms, and the design question is where that overflow goes, not whether it happens.

The five parts of a system

1. The catchment and gutters

Roof material matters for what the water is fit for. Metal and tile are the cleanest; asphalt shingle sheds grit and some compounds; and anything with lead flashing, treated wood shakes or old paint is a reason to keep the water well away from edible planting. Keep gutters clear, and fit a leaf screen, because every leaf that reaches the barrel becomes sediment.

2. The first-flush diverter

This is the part most home systems leave out and the one that makes the largest difference to water quality. A first-flush diverter is a vertical pipe that fills before water can continue to the tank, capturing the dirtiest initial runoff carrying roof dust, droppings and pollen. It drains slowly through a small weep hole so it is empty and ready for the next storm.

A common sizing convention is roughly 10 gallons of diversion per 1,000 square feet of roof. It costs very little and it is the difference between stored water and stored sludge.

3. Storage, and the overflow

Containers must be opaque, because sunlight grows algae, and sealed, because open water breeds mosquitoes. That sealing requirement is not just good practice; it is written into Colorado's rules, and it is why a repurposed open drum is a poor choice.

Plan the overflow before you buy the tank. Given the numbers above, the outlet will run, and it must be piped somewhere that drains away from the foundation. An overflow discharging against the wall of a house is a worse problem than the one the barrel was solving. Barrels can be linked in series at the top to chain capacity, which is the cheapest way to expand.

Raise the tank. A spigot at the bottom of a barrel standing on the ground gives you almost no head and will not fill a watering can. On a stand of concrete blocks it will, and gravity is the only pump most systems need.

4. Filtration, matched to the use

Filtering for irrigation means keeping debris out of a hose and nothing more: a mesh screen at the inlet and a tap filter do it. Filtering to a standard where water is fit for washing or, where it is legal, for indoor use is an entirely different proposition involving sediment filtration, disinfection and testing, and it belongs with a professional rather than a weekend build.

5. Maintenance, which is the part that fails

Colorado requires barrels to be emptied at least monthly, which is a sensible habit everywhere: stored water that sits for months is water nobody wants to use.

What this water is and is not for

Treat harvested rainwater as non-potable. It has run across a roof and through a gutter, and it carries whatever was on them. In Colorado, drinking it is specifically not permitted. Elsewhere it is a matter of treatment and testing rather than permission, but the default assumption should be irrigation, not drinking.

That still makes it valuable in an emergency, because a great deal of household water use is not drinking: flushing, washing, cleaning and keeping a garden alive. It extends your stored drinking water by taking the other jobs off it. If you intend to drink it in a genuine emergency, it needs the same treatment as any water of uncertain quality, and our water filtration guide sets out what each method does and does not remove, including the fact that hollow-fibre filters do not remove viruses.

A realistic first build

Two linked 55-gallon barrels on a block stand, under the downspout that serves the largest roof section, with a leaf screen, a simple first-flush diverter and an overflow piped into a garden bed. That is an afternoon, it is legal almost everywhere including Colorado at exactly 110 gallons, and it will cover routine garden watering through most of the growing season.

Expand by adding barrels in series or by moving to a single larger tank once you know where the overflow goes and how fast the system actually fills. Nearly everyone who builds one underestimates both.

What we could not verify

We do not physically test the products linked on this page; the picks come from manufacturer specifications, retailer data and what buyers report over time.

The 0.623 gallons per square foot per inch figure is geometry and is exact. The 0.85 efficiency factor is a planning convention rather than a measured value, and real efficiency varies with roof material, pitch, gutter condition and how much of a storm the first flush takes; treat the yield table as an estimate and not a guarantee. The first-flush sizing convention of roughly 10 gallons per 1,000 square feet is likewise a rule of thumb rather than a standard.

The Colorado rules are from the state's Division of Water Resources, checked on 3 October 2026. We have not surveyed the other 49 states, and rules, rebates and HOA covenants change. Confirm your own jurisdiction before building; nothing here is legal advice.