Direct answer: A working rainwater system has five parts in order: a clean roof surface, gutters sized to the roof area, a first-flush diverter that throws away the dirtiest water, a screened inlet into storage, and an overflow that moves excess water away from the foundation. Add filtration at the point of use, not at the tank.
Parts of the system
- Catchment. Metal, tile, or asphalt shingle roof. Avoid lead flashing, treated wood shingles, and anything with moss killer.
- Gutters and downspouts. Size for peak rain, not average. A 1,000 sq ft roof needs roughly 5-inch gutters and 3x4-inch downspouts.
- First-flush diverter. Diverts the first 10–20 gallons per 1,000 sq ft, which carries most of the bird droppings, dust, and pollen.
- Storage. Barrels, IBC totes, or a cistern. Covered, screened, and on a stable base.
- Distribution. Gravity hose for the garden, pump and pressure tank for household use.
Gutters and the first-flush diverter
Gutter slope should be about 1/16 inch per foot toward the downspout. Hangers every 24–30 inches in snow country, every 36 inches elsewhere. Keep gutters clean: a single clogged section can send water behind the fascia and into the wall.
A commercial first-flush diverter costs $40–$120 and installs in an afternoon. The DIY version is a vertical standpipe capped at the bottom: water fills the pipe, the cleanest water flows over the top into the tank, and you drain the pipe after each rain. The rule of thumb is 10 gallons of flush volume per 1,000 square feet of roof.
Storage options and sizing
Size storage around your longest dry period, not annual rainfall. If your garden needs 200 gallons a week in July and you get three weeks without rain, you need at least 600 gallons — more if you also want household backup.
| Storage | Capacity | Cost (approx.) | Notes |
|---|---|---|---|
| Rain barrel | 50–65 gal | $60–$150 | Easy, stackable, good for a small garden |
| IBC tote | 275 gal | $80–$200 used | Best value per gallon, needs a solid base and light blockout |
| Poly cistern | 500–2,500 gal | $700–$3,000 | Buried or above ground, proper installation required |
Rules and practical limits
Rainwater collection is regulated in many US states and Canadian provinces, and the rules range from unrestricted to permit-required with volume caps. Some municipalities also restrict connections to household plumbing. Check with your local building or water authority before plumbing a tank into a house — garden use is usually simpler than potable use.
Two practical limits matter more than the law. First, roof area: most suburban lots have enough roof for garden water but not for a household. Second, storage: without a tank sized for your dry spell, most of what you collect goes down the overflow in the first storm.
Filtration basics
Filter in stages: a coarse screen at the inlet, a 100-mesh filter before the pump, and a 5-micron cartridge plus carbon block at the tap for household use. Roof water is not potable without testing and treatment — assume it is garden water until proven otherwise.
For drinking, the safe sequence is sediment filtration, then disinfection (UV, boiling, or approved chemical treatment), then testing. Local health authorities set the rules; check them before plumbing rainwater into a house.
Maintenance
- Monthly: clear gutter screens, check the first-flush drain, inspect the tank inlet for mosquito mesh.
- Seasonally: flush the tank, clean the sediment layer, check the overflow.
- Before winter: drain exposed pipes, empty barrels that will freeze, and disconnect hoses.
- Annually: inspect the roof for damaged flashing and replace filter cartridges.
Frequently asked questions
Is rainwater safe to drink?
Not straight off the roof. It can carry bird droppings, dust, and bacteria. With first-flush diversion, filtration, disinfection, and a lab test it can be used for drinking where local rules allow — otherwise keep it for irrigation.
How big a tank do I need?
Roughly your weekly garden demand multiplied by the number of weeks in your longest dry spell, with a minimum of 275 gallons for a serious vegetable garden.
Do I need a pump?
Only if you need pressure or want to move water uphill. A barrel raised 2 feet gives about 1 psi, enough for a soaker hose but not a sprinkler.
Will one barrel fill from one storm?
Easily. A 1,000 sq ft roof sheds about 600 gallons per inch of rain, so a 55-gallon barrel overflows within minutes. Plan the overflow before the first storm.
Water is one of the nine chapters in The Self-Sufficient Backyard. Next: calculate your roof's real yield and choose between barrels, IBC totes and cisterns.
Frequently asked questions
Is rainwater safe to drink?
Not straight off the roof. It can carry bird droppings, dust and bacteria. With first-flush diversion, filtration, disinfection and a lab test it can be used where local rules allow — otherwise keep it for irrigation.
How big a tank do I need?
Roughly your weekly garden demand multiplied by weeks in your longest dry spell, with a 275-gallon minimum for a serious vegetable garden.
Do I need a pump?
Only if you need pressure or want to move water uphill. A barrel raised two feet gives about 1 psi.
Will one barrel fill from one storm?
Easily. A 1,000 sq ft roof sheds about 600 gallons per inch of rain, so a 55-gallon barrel overflows within minutes.