Knowledge · G-03
Rainwater
harvesting.
Short answer
- Food-grade donor if the water touches edible crops
- Mosquitoes find the overflow, not just the inlet
- 1.6 psi on two blocks — fine for drip, useless for a nozzle
- Above the 35th parallel, plan for freezing
Order a food-grade tote
Sizing
Start from the roof, not the tank
Work out what your roof actually yields before deciding how many totes you need. Most people over-tank a small roof and then wonder why the tanks never fill.
| Roof area | 0.5 in rain | 1 in rain | 2 in rain | Annual at 48 in (Savannah) |
|---|---|---|---|---|
| 250 sq ft (small shed) | 78 gal | 156 gal | 311 gal | ~6,350 gal |
| 500 sq ft (large shed / carport) | 156 gal | 311 gal | 623 gal | ~12,700 gal |
| 1,000 sq ft (half a house) | 311 gal | 623 gal | 1,246 gal | ~25,400 gal |
| 1,500 sq ft (typical house) | 467 gal | 934 gal | 1,869 gal | ~38,100 gal |
| 2,500 sq ft (barn) | 778 gal | 1,557 gal | 3,113 gal | ~63,500 gal |
Short answer
- Yield (gal) = roof sq ft × inches of rain × 0.623
- Subtract ~15% for splash, wetting and first-flush
- Size tanks for the typical storm, not the annual total
- Two 275s on a 500 sq ft roof is well matched
The build
Seven steps
- 01
Choose the donor grade honestly
Edible crops or any chance of drinking: food-grade tote with a documented history, sanitised before first use. Ornamentals, lawns, washing down equipment: a rinsed unit is genuinely fine and costs half as much. Never an ex-chemical tote for anything you will eat.
- 02
Build the stand first
Two concrete blocks under each of the four pallet corners — never under the centre, which puts a point load through the middle of the base. A full 275 is 2,440 lb, so this is a structural decision. Check it is level in both directions: an out-of-level tote drains unevenly and stresses one bottom corner.
- 03
Fit a first-flush diverter
The first few gallons off a roof carry bird droppings, grit, pollen and whatever the roof has accumulated. A diverter sends that to waste before the flow reaches the tank. Without one the tank fouls within a season and nothing downstream fixes the smell. This is the step that separates a working system from a green soup.
- 04
Screen the inlet and the overflow
Fine insect mesh on both. People always screen the inlet and forget the overflow, and mosquitoes find the overflow within about a week. A 275-gallon tank is a highly effective breeding pond, so this is not a cosmetic detail.
- 05
Plumb a proper overflow
Sized at least as large as the inlet, directed well away from the stand and any foundation. An overflow that dumps at the base of the stand will undermine it, and a tote that falls off a block stand is a serious incident rather than an inconvenience.
- 06
Shade it, wrap it or paint it
Translucent HDPE in full sun grows algae and UV-crazes within five to eight years. Shade cloth, a purpose-made wrap, or dark exterior paint all work. Dark paint raises water temperature less than people assume, and the algae suppression is worth far more than the couple of degrees.
- 07
Fit the outlet you actually need
S60 to 3/4-inch garden hose adapter for drip irrigation and watering cans. A full-port ball valve if you want throttling. A camlock if you will be connecting a pump. Brass rather than nylon if the fitting sits in direct sun — UV embrittles nylon in about two seasons.
Pressure
What gravity will and will not do for you
Head pressure depends on the height of water above the outlet, not on the volume in the tank. A full 275 on blocks gives you about 1.6 psi, and that number decides your whole irrigation design.
| Outlet height | psi, full tank | What it runs |
|---|---|---|
| On the ground (6 in) | ~1.3 psi | Bucket filling, slow soaker hose |
| Two blocks (20 in) | ~1.6 psi | Drip irrigation, gravity soaker lines |
| Four blocks (36 in) | ~2.2 psi | Longer drip runs, low-flow emitters |
| Welded stand (60 in) | ~3.1 psi | Multiple drip zones |
| Elevated platform (120 in) | ~5.2 psi | Short hose runs, still no nozzle |
| Any height | — | A hose nozzle needs ~20 psi. Use a pump. |
Note that the pressure drops as the tank empties — the last 50 gallons deliver noticeably less than the first. Drip systems tolerate this well; anything with pressure-compensating emitters tolerates it badly. Full head-pressure maths →
Five ways these fail
- 01No first-flush diverter — tank fouls in one season
- 02Unscreened overflow — mosquito breeding within a week
- 03Clear tote in full sun — algae bloom and UV crazing
- 04Stand loaded at the centre — base cracks under 2,440 lb
- 05Left full through a freeze — bottle splits at the bottom corner
Parts list, one tote
- 1 × 275 gal tote (food grade or rinsed)
- 8 × concrete blocks
- 1 × first-flush diverter
- Insect mesh, inlet and overflow
- 1 × S60 to 3/4 in GHT adapter, brass
- Overflow pipe and fittings
- Shade wrap, cloth or exterior paint
- New lid gasket (solid lid, not vented)
Linking totes
Three ways to build a bank, and only one is good
| Method | How it behaves | Verdict |
|---|---|---|
| Outlet manifold | All tanks share a common header at the outlets and sit at the same level. Isolation valve per tank lets you clean one without draining the bank. | Best. Do this. |
| Overflow cascade | Tank one overflows into tank two, and so on. Fills sequentially, so tank three is empty until one and two are full. | Acceptable if tanks are at different heights, otherwise wasteful. |
| Top-fill splitter | Inlet pipe split to fill all tanks at once from above. Needs careful balancing and almost never balances. | Avoid. One tank always fills first. |
A three-tote outlet-manifolded bank gives you 825 gallons with one discharge point and the ability to take any tank offline. We build these to order if you would rather not plumb it yourself — custom fabrication.
Questions
Rainwater questions
How much rain can I collect on my roof?
One inch of rain on 1,000 square feet of roof yields about 623 gallons, before losses. Allow roughly 15% for splash, wetting and first-flush diversion, so call it 530 usable gallons — which fills a 275-gallon tote twice. A 500 sq ft shed roof fills one tote in a single one-inch storm.
Do I need a food-grade tote for rainwater?
For irrigating ornamentals, no — a rinsed unit is fine. For edible crops where the water contacts produce, and for anything you might drink, yes: a food-grade donor with a documented history, sanitised before first use. Roof runoff is not potable regardless of the tank, but the tank should not be the problem.
How high should I raise the tote?
Every foot of liquid above the outlet gives 0.433 psi. On two concrete blocks the outlet sits about 20 inches up, giving roughly 1.6 psi when full — plenty for drip irrigation, nowhere near enough for a hose nozzle. Support the pallet at its four corners, never at the centre; a full 275 is 2,440 lb.
How do I stop algae and mosquitoes?
Algae needs light: shade the tote, wrap it, or paint it a dark colour. Mosquitoes need access: fine insect mesh on the inlet AND the overflow — they find the overflow within a week. Both failures are preventable and both are extremely common.
Can I link several totes together?
Yes. The simplest reliable method is a common manifold at the outlets with a shared discharge, which keeps all tanks at the same level and lets you isolate one for cleaning. Linking at the top via overflows works but fills them sequentially rather than evenly.
Parts and cost
A complete single-tote build, itemised
So you can price it before deciding whether to build or buy. These are realistic 2026 figures for a food-grade harvester.
| Item | Quantity | Typical cost | Notes |
|---|---|---|---|
| Food-grade 275 gal tote | 1 | $185 – $340 | Required if the water touches edible crops |
| Concrete blocks | 8 | $20 – $32 | Two per pallet corner — never under the centre |
| First-flush diverter | 1 | $45 – $95 | The component that decides whether it works |
| Insect mesh, inlet and overflow | 2 | $8 – $16 | Both openings, not just the inlet |
| Overflow pipe and fittings | 1 set | $18 – $35 | Direct it well away from the stand |
| S60 to 3/4 in GHT adapter, brass | 1 | $12 – $19 | Brass rather than nylon if it sits in sun |
| Solid 6 in lid with new gasket | 1 | $14 – $27 | Solid, not vented, for storage |
| Shade wrap or exterior paint | 1 | $25 – $60 | Suppresses algae and slows UV crazing |
| Total | $327 – $624 | Against $340 – $620 for our built kit |
The build and the kit come out close, which is honest rather than a sales argument — the kit saves you sourcing time and a trip, and building it yourself means you understand every joint when something needs attention in year four.
Water quality
What is actually in roof runoff
Harvested rainwater is not potable and the reasons have little to do with the container. Knowing what is in it tells you what treatment, if any, your use needs.
| Contaminant | Source | Addressed by | Matters for |
|---|---|---|---|
| Grit, leaf litter, pollen | Roof surface | First-flush diverter, inlet mesh | Everything — it fouls the tank |
| Bird and rodent droppings | Roof and gutters | First flush; not fully removed | Edible crops, livestock |
| Bacterial load | Droppings, organic matter | Filtration and disinfection | Any contact with people or produce |
| Metals (zinc, copper) | Galvanised or copper roofing and gutters | Roof material choice | Edible crops, aquaponics |
| Mosquito larvae | Any unscreened opening | Mesh on inlet AND overflow | Everything; also a nuisance obligation |
| Algae | Light plus nutrients | Shading or opaque tank | Everything — it clogs emitters |
Short answer
- Harvested rainwater is not potable regardless of the tank
- A first-flush diverter addresses more than any filter downstream
- Mesh the overflow — mosquitoes find it within a week
- Metal roofing matters for aquaponics and edible crops
From the Yard Journal
Longer pieces from the people who run the wash line, the test bay and the trading desk.
28 August 2025 · 4 min
Rain barrels, food grade, and the conversation we have every spring
Somebody wants one tote for a vegetable garden. The honest answer takes four questions and occasionally costs us the sale.
Marcus Vreeland · Founder
Read27 February 2024 · 4 min
Why food-grade supply collapses every August
It is not demand. Documented food-contact stock depends on affidavits, and affidavits depend on somebody at a beverage plant having time to sign one.
Nina Caldwell · Trading desk
Read8 May 2025 · 4 min
Reading prior-contents affidavits like an auditor
Most affidavits are honest and useless. The difference between a document that protects you and one that does not comes down to four specific fields.
Nina Caldwell · Trading desk
Read
Next move
One tote, collected from the yard, is a normal order.
There is no minimum here. Tell us whether the water touches food and we will pick the grade, then stage it for collection — Saturday mornings included.