Rainwater Collection System Parts: The Components That Decide If It Works

Most rainwater setups don't fail at the tank. They fail at a 5 dollar screen, a missing overflow, or a roof that was never a good catchment surface in the first place. The tank is the part everybody shops for. The rainwater collection system parts that decide whether the water is usable are the small ones upstream and downstream of it.

One thing worth saying up front. No federal agency publishes usable guidance on these specific component questions. Not EPA, not CDC, not USGS. Every number here comes from a university extension service or a state water agency, and they occasionally disagree. Where they do, I'll give you both numbers instead of picking one and pretending it's settled.

Here is each part, what it actually does, and the number that tells you whether yours is sized right.

What are the parts of a rainwater collection system?

There are 7 parts, and they sit in a fixed order from the roof down. Oregon State University Extension lists the chain as the catchment surface (usually a roof), gutters and downspouts, leaf screens, a first flush diverter and primary filtration, the storage tank or cistern, the delivery system (gravity fed or pumped), and optional treatment.

Virginia Tech Biological Systems Engineering describes the same hardware but groups it into 6 named subsystems, which is a better way to think about it when you're buying parts:

• Catchment surface. The roof, and the single biggest decision in the whole system.

• Gutters and downspouts. The conveyance.

• Screens, first flush diverters and roof washers. The cleanup stage.

• Cisterns. Storage.

• Delivery system. How the water gets to where you use it.

• Treatment system. Only if you're going beyond irrigation.

NC State Cooperative Extension strips the tank end down to the 4 parts that carry load: the cistern, the gutter system, the overflow pipe, and the outlet pipe, which draws water from the bottom of the cistern, sometimes with a pump. Which materials a DIY rainwater tank can safely be built from narrows that choice fast.

That detail matters. The outlet comes off the bottom, not higher up where people assume the water is cleaner. That low draw is where your head pressure is, and keeping it clean is the whole job of the screens and the diverter upstream.

How much rainwater can you actually collect from your roof?

Multiply inches of rain by square feet of roof by 0.623. That's the whole formula. Texas A and M AgriLife Extension writes it out as [inches of rain] x [square feet of roof] x [0.623], and gives the example everybody should memorize: 1 inch of rain on 1,000 square feet of roof is 623 gallons.

That number is the ceiling, not the catch. Oregon State uses the rounded 0.62 gallons per square foot per inch and then applies a collection efficiency factor of about 75 percent "due to roof shape." The Texas Water Development Board treats 0.62 as the theoretical yield and says installers "assume an efficiency of 75 percent to 90 percent," assigning roughly a 10 percent loss each to clay or concrete tile and to composite shingle, because rough surfaces are less efficient than smooth metal.

So run it both ways. That 1,000 square foot roof in a 1 inch rain gives you 623 gallons on paper, about 467 gallons at 75 percent, and about 561 gallons at 90 percent. The spread between a tile roof and a clean metal roof is most of that gap.

One more efficiency number matters if the water is going to a garden. Oregon State rates sprinkler irrigation at 75 percent efficient and drip or soaker at 85 to 90 percent. Stack that against a 75 percent catch and a sprinkler puts roughly half of what fell on the roof into the soil. Drip gets closer to 65 percent. The cheapest way to make a small tank bigger is to stop spraying water into the air.

One gap worth naming: none of these sources publishes gutter or downspout sizing, meaning inches of gutter per square foot of roof. That is a roofing and local code question, not something I can hand you a sourced figure for.

What does a first flush diverter do, and how big should it be?

It throws away the dirtiest water of the storm, the part that rinses bird droppings, dust and grit off the roof before anything reaches the tank. Oregon State's rule of thumb is to divert a minimum of 10 gallons per 1,000 square feet of collection surface for every 1 inch of rainfall.

Virginia Tech publishes a wider band for the same job: "The suggested volume ranges from 10 to 50 gallons for every 1,000 ft2 of catchment surface." Virginia Tech also adds the detail most people get wrong, which is that each downspout needs its own diverter, sized to the roof area feeding that downspout. One diverter on one downspout does nothing for the other 3.

The Texas Water Development Board cites two more ranges that bracket the same idea: 1 to 2 gallons per 100 square feet, and an Australian study range of 13 to 49 gallons per 1,000 square feet. Four sources, four slightly different numbers, all landing in the same neighborhood. Take 10 gallons per 1,000 square feet as your floor and go up from there if your roof sits under trees.

Now the part that turns a number into hardware. Most first flush diverters are a vertical standpipe that fills before water passes on to the tank, so the volume you want becomes a length of pipe. The Texas Water Development Board publishes the fill rates:

• 3 inch pipe: about 1 gallon per 33 inches of pipe.

• 4 inch pipe: about 1 gallon per 18 inches of pipe.

• 6 inch pipe: about 1 gallon per 8 inches of pipe.

Run the 10 gallon target through those and the design problem shows up immediately. Ten gallons in 3 inch pipe is 330 inches, which is 27 feet of standpipe. In 4 inch pipe it's 180 inches, or 15 feet. In 6 inch pipe it's 80 inches, under 7 feet. That is the entire reason you see fat 6 inch diverter tubes on real installations instead of tidy 3 inch ones. If your diverter is a 3 foot length of 3 inch pipe, it's holding roughly 1 gallon, and it is not doing the job the guides describe.

What size screen keeps mosquitoes out of a rainwater tank?

One millimeter or smaller, on every opening. NC State puts it plainly: "any open pathways to the captured water should be covered by 1-millimeter or smaller mesh screen." Every pathway. Inlet, lid gap, vent, overflow outlet, the gap where a hose enters.

Here's the mix-up that costs people a tank full of mosquito larvae. A leaf screen is not a mosquito screen. Oregon State describes leaf screens as 1/4 inch square wire mesh. A quarter inch is about 6 millimeters, roughly 6 times the opening that keeps mosquitoes out. The leaf screen does exactly the job it was built for. It just isn't the one that controls insects.

You need both, and they're different parts. Coarse 1/4 inch mesh at the gutter for leaves and twigs, fine 1 millimeter mesh at every opening into the water. The fine screen is the cheapest part here and the one most often missing, the gap behind most algae, mosquito and overflow problems.

Why is the overflow pipe as important as the tank?

Because every tank overflows, and the overflow decides where that water goes. NC State states the requirement directly: "The cistern overflow must handle the same flow as the gutter system." The sizing isn't guesswork either. It's computed from the runoff of a 100-year, 1-hour storm, with the overflow diameter picked from a table so the maximum roof area that diameter can handle exceeds the actual contributing roof area.

Two placement details come out of the same guidance. The overflow sits near the top of the tank, leaving several inches of storage above it for intense storms. And it has to go somewhere deliberate. Oregon State says to direct overflow discharge away from the house foundation and away from septic drainfields. For a retrofit rain barrel, Alabama Extension at Auburn University gives the specific minimums: an overflow of "at least 1 1/4 inch" discharging "at least 10 feet away from your home."

One note on how to size it. You'll see pipe-diameter shortcuts repeated in forums, and I can't source any of them, so I'm not passing them along. The sourced standard is a flow standard: the overflow has to move as much water as the gutters bring in, checked against a 100-year, 1-hour storm. That's a tougher test than matching pipe sizes, and it's the one that keeps storm water off your foundation.

Which roofing materials work as a catchment surface?

Smooth metal, and it isn't close. The Texas Water Development Board's recommendation for potable catchment is that "a plain galvanized roof or a metal roof with epoxy or latex paint is recommended," while "composite or asphalt shingles are not advisable."

Two more material notes from the same manual that people miss because they're about trim, not roofing. Lead "cannot be used as gutter solder" on a potable system, because slightly acidic rain can dissolve it. And copper flashings can discolor porcelain fixtures. If you're plumbing collected water indoors, the solder and flashing choices matter as much as the panels.

For non-potable use the list opens up, with limits:

• Composite and asphalt shingle: acceptable for gardening, but Oregon State is blunt that "They leach residues, so do not use for potable water systems," and notes they lose about 10 percent of collected water compared with metal.

• Wood shingles, tar, gravel and concrete: Oregon State lists these as suitable only for non-vegetable irrigation.

• Tar and gravel, asbestos, treated cedar shake: University of Wisconsin Extension says rain barrels are "not recommended for homes with tar and gravel roofs," and advises against barrels on asbestos roofs or treated cedar shake shingles, because chemicals and microorganisms can leach from roofing materials.

Copper roofing I'll leave alone. None of these sources addresses whether copper is or isn't a recommended catchment surface, so I don't have an answer for you.

What does testing show about roof runoff water quality?

Enough to explain why every one of these guides treats untreated roof water as irrigation water. NC State Cooperative Extension compiled the measurements, and the numbers are specific:

• Zinc: 11.788 mg/L from galvanized iron, with a maximum of 14.7 mg/L from galvanized metal, against 0.139 mg/L in rainwater before it touched a roof. Wood shingle roofs came in at 16.317 mg/L.

• Lead: 65 percent of the compiled studies reported lead above the EPA drinking water standard of 0.015 mg/L.

• Copper: exceeded the NC freshwater standard of 0.007 mg/L regardless of roof type.

• pH: runoff ranged from 4.1 (old metal with aluminum paint) to 6.69 (composition shingle), against an EPA drinking water range of 6.5 to 8.5.

Notice the tension. Galvanized metal is the recommended potable catchment surface and also the source of the highest zinc readings. Both are true, because they measure different stages. The water quality work samples raw runoff before any diverter, filter or treatment. The roofing recommendation is about smoothness and leached residues over the life of the system. The summary: your roof changes what's in the water, and no roof makes untreated runoff drinking water.

What parts does the delivery and treatment side need?

It depends on where the water is going, and the gap between irrigation and indoor use is large. Oregon State splits delivery into gravity fed or pumped. Gravity costs nothing and needs elevation. A pump costs money and brings more components with it.

If you pump, Virginia Tech specifies the part people leave off: a pump and pressure tank need "a one-way check valve between the pump and cistern to prevent backflow."

Filtration, as these sources describe it, is modest. Oregon State names exactly 2 micron sizes: a 30 micron roof washer filter, and a 3 to 5 micron filter ahead of pump-fed emitters. That second one is drip protection, because emitters clog on tank water. Those are the only in-line filtration figures in these documents, and I will not invent a cartridge spec beyond them. What each filtration stage actually removes is its own subject.

For indoor potable use, Texas A and M AgriLife Extension gives the equipment list without hedging: "The system must include pre-filters, pump, pressure tank, filters and a sanitizing device such as an ultraviolet light." That is 5 components past the tank, and it is the honest price of drinking what falls on your roof.

Then there's the plumbing side, which is where rules get strict:

• Keep the networks separate. NC State recommends entirely separate plumbing networks for harvested rainwater, and labeling every faucet or fixture supplied by the system with its source. It also notes that plumbing codes may restrict which fixtures can use harvested water.

• Protect the municipal supply. The Texas Water Development Board describes Portland, Oregon's requirement that a cistern which can be filled with either harvested rainwater or municipal water have "a reduced pressure backflow prevention device and an air gap protecting the municipal supply from cross-connection."

• Watch for pressure drops. The same manual notes that conjunctive use "would require an appropriate backflow prevention device to keep rainwater from entering the public water supply" when utility pressure drops. Portland's code also limits rainwater to nonpotable uses.

Your city is not Portland, and your rules will differ. The point is that the backflow preventer and the labels are components, same as the tank, and in a lot of jurisdictions they are the ones an inspector looks for.

The part most rainwater guides skip

The honest answer is the overflow, and the reason is simple. It's the one component that only matters on the worst day, so it's the one that gets left for later and never gets built. A tank with no overflow doesn't fail gracefully. It dumps the top of a storm at the base of your foundation, which is the exact thing Oregon State tells you to direct water away from.

The second skipped part is labeling. NC State's advice to label every fixture fed by the system reads like paperwork until you picture a guest, a plumber or a kid at a tap that isn't drinking water. A label is a 50 cent component with a real job.

The third thing is the shape of the guidance itself. First flush sizing alone comes out as 10 gallons per 1,000 square feet from Oregon State, 10 to 50 from Virginia Tech, and 1 to 2 gallons per 100 square feet or 13 to 49 per 1,000 from Texas. That isn't sloppiness. It's 4 regions with different rain, roofs and dust, and no federal number to settle it. Build to the top of the range your trees and your climate justify and you'll be inside all 4.

Frequently asked questions

What are the main parts of a rainwater collection system?

Seven, in order: the catchment surface (usually a roof), gutters and downspouts, leaf screens, a first flush diverter with primary filtration, the storage tank or cistern, the delivery system (gravity fed or pumped), and optional treatment. That's Oregon State University Extension's chain. Virginia Tech groups the same hardware into 6 subsystems, and NC State reduces the tank end to 4 load-bearing parts: the cistern, the gutter system, the overflow pipe, and the outlet pipe, which draws from the bottom of the cistern.

How many gallons of rainwater can I collect per inch of rain?

About 0.623 gallons per square foot of roof per inch of rain. Texas A and M AgriLife Extension writes the formula as inches of rain x square feet of roof x 0.623, so 1 inch on a 1,000 square foot roof is 623 gallons. Plan on capturing less than that. Oregon State applies about 75 percent efficiency due to roof shape, and the Texas Water Development Board says installers assume 75 percent to 90 percent, with roughly 10 percent losses for tile and for composite shingle.

How big should a first flush diverter be?

At least 10 gallons per 1,000 square feet of roof for every 1 inch of rainfall, per Oregon State University Extension. Virginia Tech suggests a wider 10 to 50 gallons per 1,000 square feet and notes each downspout needs its own diverter sized to the roof area feeding it. In standpipe terms, the Texas Water Development Board gives about 1 gallon per 33 inches of 3 inch pipe, 1 gallon per 18 inches of 4 inch pipe, and 1 gallon per 8 inches of 6 inch pipe.

What size mesh keeps mosquitoes out of a rain barrel?

One millimeter or smaller. NC State Cooperative Extension says any open pathways to the captured water should be covered by 1 millimeter or smaller mesh screen. A leaf screen is not a substitute. Oregon State describes leaf screens as 1/4 inch square wire mesh, which is roughly 6 millimeters, so it stops leaves and twigs while leaving insects a wide open door.

Can you collect rainwater from an asphalt shingle roof?

For gardening, yes. For drinking, no. Oregon State says composite and asphalt shingles are acceptable for gardening but "They leach residues, so do not use for potable water systems," and that they lose about 10 percent of collected water compared with metal. The Texas Water Development Board calls composite or asphalt shingles "not advisable" for potable catchment and recommends plain galvanized metal or metal with epoxy or latex paint instead.

What equipment do I need to make collected rainwater safe to drink?

Texas A and M AgriLife Extension is explicit for indoor potable use: "The system must include pre-filters, pump, pressure tank, filters and a sanitizing device such as an ultraviolet light." Alongside that, NC State recommends entirely separate plumbing networks and labeling every fixture fed by the system, and plumbing codes may restrict which fixtures can use harvested water. Check your local code before you plumb anything indoors.

You don't have to be extreme to be ready. Start with what you have. Walk out and look at your roof material, count your downspouts, and check whether anything covers the opening into your tank. Those 3 observations tell you more about your system than the size of the tank does.

If you're building from scratch, spend your money in this order: the catchment surface you can't change later, a real first flush diverter on every downspout, 1 millimeter mesh on every opening, and an overflow that carries the gutters' full flow somewhere 10 feet from the house. Treatment comes last, and only if the water is going indoors.

You never know, but you can always be ready.

Sources: Oregon State University Extension, EM 9101 Rainwater Harvesting | Virginia Tech Biological Systems Engineering, Rainwater Harvesting | NC State Cooperative Extension, Residential Rainwater Harvesting Guide (AGW-588-11) | NC State Cooperative Extension, Water Quality of Rooftop Runoff (AG-588-18W) | Texas Water Development Board, Texas Manual on Rainwater Harvesting, 3rd edition | Texas A and M AgriLife Extension, Build Your Own Rain Barrel | Texas A and M AgriLife Extension, Rainwater Harvesting in Texas | Alabama Extension, Auburn University, DIY Rain Barrel Construction (ANR-2794) | University of Wisconsin Extension, Pierce County, Rain Barrels