Rainwater Tank Installation Planning Checklist
A tank is a fifteen-year construction project compressed into one afternoon. Every hour spent on the plan buys a decade of useful harvest without a surprise.
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Size the tank from the roof
The tank should fit the catchment and the demand, not the catalogue.
- CORERoof catchment area measured in square metres, split by downpipe if needed Amazon
- COREAverage annual rainfall in millimetres for the site, and the driest-month figure Amazon
- A harvest calculation: area times rainfall times a runoff efficiency factor Amazon
- A demand figure from household or garden use, compared to the harvest number Amazon
Choose the site
The best site balances three competing demands: gravity, access and maintenance.
- COREClose to the downpipe so long gutter runs do not lose water and cost more Amazon
- On or above the natural grade so gravity pressure feeds the house or garden taps Amazon
- Clear of a building's fire-rated wall and any septic field or drain line Amazon
- COREA maintenance access of at least 600 millimetres on every side of the tank Amazon
Build the foundation
A full tank weighs thousands of kilograms and does not forgive a soft base.
- COREA compacted hardcore or concrete pad sized and levelled to the manufacturer's spec Amazon
- A concrete slab with a steel-reinforced top for tanks above a certain volume Amazon
- Level within a few millimetres across the whole footprint; a slope creates a stress point Amazon
- Ring-lock or strap anchors fitted for a cyclone or high-wind zone Amazon
Plan delivery and placement
A full-size tank arrives by truck, crane or hiab and needs a clear working space.
- COREA delivery path with no low wire, no soft ground and a turning circle for the truck Amazon
- A crane, hiab or roller-and-cam approach chosen in advance for the site access Amazon
- A clear area around the pad three times the tank's footprint for a swing Amazon
- COREA second pair of hands on the day to guide the tank onto the anchor bolts Amazon
Inlet, first-flush and screens
A tank without a first-flush diverter stores the roof's dirt with the rain.
- COREA first-flush diverter sized to hold the first 1 millimetre of rain from the roof area Amazon
- A leaf screen or gutter guard at the downpipe inlet to keep debris out Amazon
- COREA mosquito-proof inlet screen on the tank itself, rated at less than 1 mm mesh Amazon
- A calmed inlet so inflowing water does not stir sediment off the tank floor Amazon
Overflow and drainage
Every full tank needs somewhere to put the next storm.
- COREAn overflow pipe sized for the roof's peak storm flow, not for a normal rain Amazon
- A mosquito screen fitted on the overflow outlet so it cannot back-flow Amazon
- COREA drainage path to a swale, garden or stormwater line, not to a building's footings Amazon
- A visible level gauge on the tank so overflow can be anticipated, not discovered Amazon
Pump, power and distribution
Getting the water out of the tank is another small project.
- COREPump choice between a pressure pump, a garden pump or gravity, based on distance and head Amazon
- Power to the pump: mains, solar, battery, or generator, decided in the plan not after Amazon
- A non-return valve on the pump suction so a prime is not lost every shut-off Amazon
- COREA distribution line planned to garden taps and house cold-water if used indoors Amazon
A rainwater tank is one of the most satisfying pieces of household infrastructure because it harvests for free from a source nobody else controls. It is also one of the most commonly under-planned. The tank arrives, the site is not quite ready, the overflow has nowhere sensible to go, and the garden becomes a construction project by day two.
The order matters. The size falls out of the roof and the rainfall, not the other way around. The site falls out of the size, the downpipe location, the delivery access and the maintenance reach. The foundation falls out of the site. Only then does the tank arrive, and then the plumbing, the first-flush, the overflow and the pump.
Every decision at the planning stage is cheap. Every decision revised after the concrete is poured is expensive. An afternoon with a tape measure and the sizing calculator prevents a weekend of breaking a wall to re-run an inlet.
This checklist runs the installation sequence in order: size and volume, site selection, foundation, delivery and positioning, inlet and first-flush, overflow and drainage, pump and power. For the ongoing care of the harvest see our rainwater harvesting maintenance checklist. Every buyable line links to current options on Amazon.com, and Spare Theory may earn a commission on those links. We never set or verify Amazon's prices.
At a glance
- Tank size is a calculation from roof area and annual rainfall, not a shelf decision from a supplier's catalogue.
- A foundation that is flat, compacted and load-rated is the installation. Everything else follows it.
- First-flush diverters and leaf screens decide whether the water in the tank is clean or a soup.
- A delivery truck needs a path, an unloading method and a clear space three times the tank's footprint.
The full list
The same list the builder above ticks. Use it for printing or when JavaScript is off.
Size the tank from the roof
The tank should fit the catchment and the demand, not the catalogue.
- CORERoof catchment area measured in square metres, split by downpipe if needed Amazon
- COREAverage annual rainfall in millimetres for the site, and the driest-month figure Amazon
- A harvest calculation: area times rainfall times a runoff efficiency factor Amazon
- A demand figure from household or garden use, compared to the harvest number Amazon
Choose the site
The best site balances three competing demands: gravity, access and maintenance.
- COREClose to the downpipe so long gutter runs do not lose water and cost more Amazon
- On or above the natural grade so gravity pressure feeds the house or garden taps Amazon
- Clear of a building's fire-rated wall and any septic field or drain line Amazon
- COREA maintenance access of at least 600 millimetres on every side of the tank Amazon
Build the foundation
A full tank weighs thousands of kilograms and does not forgive a soft base.
- COREA compacted hardcore or concrete pad sized and levelled to the manufacturer's spec Amazon
- A concrete slab with a steel-reinforced top for tanks above a certain volume Amazon
- Level within a few millimetres across the whole footprint; a slope creates a stress point Amazon
- Ring-lock or strap anchors fitted for a cyclone or high-wind zone Amazon
Plan delivery and placement
A full-size tank arrives by truck, crane or hiab and needs a clear working space.
- COREA delivery path with no low wire, no soft ground and a turning circle for the truck Amazon
- A crane, hiab or roller-and-cam approach chosen in advance for the site access Amazon
- A clear area around the pad three times the tank's footprint for a swing Amazon
- COREA second pair of hands on the day to guide the tank onto the anchor bolts Amazon
Inlet, first-flush and screens
A tank without a first-flush diverter stores the roof's dirt with the rain.
- COREA first-flush diverter sized to hold the first 1 millimetre of rain from the roof area Amazon
- A leaf screen or gutter guard at the downpipe inlet to keep debris out Amazon
- COREA mosquito-proof inlet screen on the tank itself, rated at less than 1 mm mesh Amazon
- A calmed inlet so inflowing water does not stir sediment off the tank floor Amazon
Overflow and drainage
Every full tank needs somewhere to put the next storm.
- COREAn overflow pipe sized for the roof's peak storm flow, not for a normal rain Amazon
- A mosquito screen fitted on the overflow outlet so it cannot back-flow Amazon
- COREA drainage path to a swale, garden or stormwater line, not to a building's footings Amazon
- A visible level gauge on the tank so overflow can be anticipated, not discovered Amazon
Pump, power and distribution
Getting the water out of the tank is another small project.
- COREPump choice between a pressure pump, a garden pump or gravity, based on distance and head Amazon
- Power to the pump: mains, solar, battery, or generator, decided in the plan not after Amazon
- A non-return valve on the pump suction so a prime is not lost every shut-off Amazon
- COREA distribution line planned to garden taps and house cold-water if used indoors Amazon
The sizing calculation is the first decision
A tank is sized by the roof it drains and the rain it catches. Not by the number in the catalogue that looks right next to the house. Get it wrong and either the tank overflows every storm or runs dry by February.
The calculation is simple. Roof area in square metres multiplied by annual rainfall in millimetres multiplied by a runoff efficiency factor of 0.8 gives the theoretical annual harvest in litres. That is the ceiling.
Demand is the floor. A household that gardens through summer, flushes toilets and washes cars can use 150 to 300 litres per day in a hot month. Multiply by the dry season's length for a real consumption figure.
The right tank sits between the two. A tank bigger than the harvest wastes money. A tank smaller than the demand is a barrel that is empty for half the year. The calculator at our rainwater sizing page does the arithmetic honestly.
The dry-season test: A tank that fills every month in the wet but is empty for two months in a dry summer is a garden tank, not a household tank. Plan the volume against the longest dry spell.
Harvest is the ceiling, demand is the floor. The right tank sits between them, and the calculation falls out of roof area and rainfall, not the supplier's shelf.
Site selection has four masters
The best site is the one where four competing priorities agree. The downpipe decides gravity flow and pipe cost. The grade decides whether you need a pump. The house wall decides setbacks and fire-rating. The maintenance access decides whether you will actually clean the tank in year five.
Downpipe distance is often the deciding factor. Every extra metre of guttering is a cost, a leak risk, and a sediment trap. A tank close to the main downpipe wins on installation cost and on water quality.
Grade is second. A tank placed higher than the house taps delivers water by gravity for decades without a pump. A tank in a dip needs a pump from day one and forever.
Setbacks and access are third. Most jurisdictions require a distance from a boundary or a specific foundation for tanks above a volume. A tank placed too tight to a fence becomes unmaintainable within two metres. Check the local rules before the delivery is booked, not the day before.
The 600-millimetre rule: Six hundred millimetres of clearance on every side of the tank is the minimum to open a manhole, inspect a wall and replace a gasket. Anything less, and the tank becomes hard to maintain.
Downpipe, grade, setbacks and maintenance access. The best site is where all four agree. Compromise any one of them deliberately, not by accident.
The foundation is the whole installation
A 5,000-litre tank full of water weighs five tonnes. A 10,000-litre tank is ten. That weight sitting on a compacted gravel pad is fine if the pad was properly prepared. It is not fine if the pad is a slight slope or if the soil underneath has a soft spot.
Concrete slabs are for larger tanks or sites with a weak soil profile. Compacted hardcore is enough for most household tanks up to 5,000 litres if the site is genuinely level. Follow the manufacturer's specification; it is a liability document and a good guide.
Level is the word. A few millimetres of slope across the footprint creates an uneven stress distribution, and a full tank will pick up the weak corner. A spirit level on a straight edge, checked at four corners and the middle, is the acceptance test.
Anchor straps or ring locks are worth fitting in any region with cyclonic or very high wind. An unanchored tank in a storm becomes a five-tonne projectile. The anchors cost less than the insurance conversation.
Wait seven days for the concrete: A fresh concrete slab needs a full cure before the tank goes on it, even if it looks hard. A premature load cracks the slab and voids the warranty on both parts.
A full tank weighs thousands of kilograms. Level, compacted and load-rated. Follow the manufacturer's spec. Anchor it in high-wind zones.
Delivery day is a small construction operation
A full-size tank arrives on a truck with a crane, or a hiab arm, or in a sectioned flat-pack that gets assembled on site. The method is decided by the site access and the tank size, and it has to be booked in advance based on real measurements, not an estimate.
The path needs no low wires, no soft ground, and a turning circle the truck can manage. A truck that cannot reach the site backs out and the tank has to be walked or rolled the last fifty metres, which is a different operation and a different cost.
The working space around the pad should be roughly three times the tank footprint so the crane or the rollers have swing room. Marking it out with site paint on the day before delivery makes the operation obvious to the driver.
Two people at the receiving end guide the tank onto the anchor bolts and square it with the downpipe entry. One person with a phone and a shout is a very common source of a tank landed in the wrong orientation with an inlet on the wrong side.
Orientation is not free: A tank landed 90 degrees out of position has its inlet on the wrong side and its level gauge behind a shrub. Walk through the orientation with the driver before the hook is released.
Delivery day needs a clear path, three-times-footprint working space, and two pairs of hands. Book the crane method after a real site walk, not after an estimate.
First-flush decides water quality forever
The first millimetre of rain off any roof carries most of the dust, pollen, bird droppings and leaf matter that had accumulated on the surface. Sending that into the tank is the reason so many tanks smell within a year of installation.
A first-flush diverter holds that initial volume in a small pipe and drains it away before the cleaner water continues to the tank. Sizing is one litre per square metre of roof area, so a 100 square metre catchment needs a 100-litre diverter.
A leaf screen at the downpipe inlet catches the larger debris before the water reaches the diverter. It is not a substitute; both layers work together. A mesh at the tank inlet itself prevents mosquitoes from entering and breeding.
A calmed inlet inside the tank stops the inflow from disturbing the sediment that will accumulate on the floor over years. A swirl keeps particulates suspended where they enter the pump instead of settling where they can be cleaned out.
The diverter drain check: A first-flush diverter drains to the ground. Once a season, confirm the drain is not blocked and the diverted volume actually leaves. A plugged diverter sends all the dirt into the tank.
The first millimetre carries the dirt. A first-flush diverter, a leaf screen and a calmed inlet together decide whether the tank stores water or soup.
Overflow is a design feature, not a leak
Every full tank has to release the next storm somewhere. A tank that cannot overflow fast enough will lift a roof panel or push water into the house foundation. Overflow capacity is a real number, sized for peak storm flow off the roof, not a normal rain.
The overflow pipe goes to a swale, a garden, or a stormwater line. Never to a building's footings, a septic field or a neighbour's fence line without agreement. A full tank dumping 2,000 litres in an hour next to a house wall is a structural problem.
A mosquito screen fitted on the overflow outlet prevents the tank from filling someone else's pool of stagnant water in reverse. The same mesh at the inlet keeps the tank closed to insects.
A visible level gauge on the tank shows how close to overflow you are. A sight glass or a float gauge on the wall makes the whole water position readable from the house. Without it, overflow is discovered when it starts.
The pre-storm walk: Before a big rain, check the tank's level. A full tank meeting a storm dumps onto whatever is near it. A partly full tank absorbs the whole front and keeps the garden path dry.
Sized for peak storm flow, routed to a real drainage path, screened against insects, and monitored by a visible level gauge. Overflow is not an accident.
Pump, power and distribution: the last mile
The tank is only useful once the water gets to the taps, the garden line or the house plumbing. Pump choice is a distance-and-head calculation, not a shelf decision.
A gravity-fed system needs no pump at all if the tank sits higher than the outlets. A garden tap 30 metres away at the same level needs a small transfer pump. A house-fed system with pressure demands a pressure pump sized to the household's peak simultaneous draw.
Power is the next decision. A mains pump needs a socket, a weather-proof fitting and a run of cable. A solar pump shifts that dependency onto the sun and usually onto a tank that fills the header during the day. On a genuinely remote site, a solar pump into an elevated header is the simplest reliable answer.
A non-return valve on the suction line keeps the pump primed between starts. A lost prime is the most common pump complaint on a rainwater system, and it usually means a valve that was never fitted or has failed. A filter at the tank outlet protects the pump from the sediment the diverter missed.
Prime it once, think about it forever: Fit the non-return valve and the suction filter during installation, not after the first failed start. Ten dollars of extra parts saves a Sunday afternoon on the tank lid with a bucket.
Match pump type to head and distance. Decide power at design time. Non-return valve, suction filter and an elevated header solve most distribution problems.
Tank size versus roof catchment
| Roof area (m2) | Rainfall (mm/year) | Annual harvest (L) | Suggested tank |
|---|---|---|---|
| 50 | 600 | 24,000 | 3,000 L garden tank |
| 100 | 800 | 64,000 | 7,500 L household |
| 150 | 700 | 84,000 | 10,000 L household |
| 200 | 500 | 80,000 | 10,000 L + rain boost |
| 300 | 900 | 216,000 | Multiple tanks in series |
Illustrative sizing examples at a runoff efficiency of 0.8. Actual tank choice depends on household demand, dry-season length, and local rainfall patterns.
Key terms
- Catchment area
- The effective roof surface that contributes rain to a tank, measured in square metres and split by downpipe where different roofs feed different tanks.
- First-flush diverter
- A section of pipework that holds the initial dirty volume of rain off the roof and drains it away before cleaner water reaches the tank.
- Calmed inlet
- The pipe entry inside the tank designed to release inflowing water gently, so that settled sediment on the tank floor is not stirred back up.
- Pressure pump
- A pump that maintains a set pressure in the distribution line so multiple taps can run at once, mimicking a mains supply.
- Non-return valve
- A one-way valve on a pump's suction line that keeps water in the pipe when the pump stops, so the next start does not require re-priming.
- Runoff efficiency
- The fraction of rain landing on a roof that actually reaches the tank after losses to evaporation, gutter hold-up and the first flush. Typically 0.8 for a clean pitched roof.
Tips and common mistakes
Field-tested tips
- Install the level gauge before the tank is filled for the first time. Once the tank is full and dark, a small fitting job becomes a puddle in the shed.
- Fit a second shut-off valve on the pump suction so the pump can be serviced without draining the tank. Ten dollars now, a day saved later.
- Label every pipe at the tank: inlet, overflow, first-flush, pump suction, drain. Future-you will not remember which is which when standing in the dark.
- Ask the installer for a maintenance access on both sides of the tank, not just the pump side. Cleaning the inlet screen is very different when you have to climb the tank to reach it.
Common mistakes
- Buying the biggest tank that fits the site instead of the right tank for the roof's harvest and the household's demand.
- Placing the tank on a slope and assuming a compacted pad will level it out under load.
- Skipping the first-flush diverter because it looks like plumbing clutter, then finding a green tank in the second year.
- Connecting the overflow to a soak-well right beside the house's footings and discovering the connection only when the wall cracks.
Frequently asked questions
What size rainwater tank do I actually need?
Calculate the annual harvest first: roof area times average rainfall times 0.8. Then measure the household or garden demand, in litres per week through the driest month. The tank needs to cover the dry-season gap, not the annual average. A typical household with a garden and a moderate climate lands between 5,000 and 15,000 litres.
Can I install a rainwater tank myself, or do I need a licensed plumber?
A small garden tank on a prepared pad with a simple inlet and overflow is often within a competent DIY scope. A tank connected to household plumbing, stormwater drainage, or any potable crossover almost always requires a licensed plumber and a permit. Check local rules before buying; the requirements vary by jurisdiction and are not universal.
Do I really need a first-flush diverter?
Yes, for any tank whose water you plan to use for anything beyond rough garden work. The first millimetre of rain carries most of the roof's accumulated dirt. Without a diverter, that dirt settles on the tank floor and eventually contaminates the water. The diverter is one of the most cost-effective components in the whole system.
How far from the house should a tank go?
Far enough for easy maintenance access on every side, close enough to the downpipe to avoid long gutter runs, and clear of any fire-rated wall or septic field. Most codes allow a tank near a boundary with specific foundation rules. Check local setbacks; they are real constraints and usually enforced at the permit stage.
Do rainwater tanks need to be anchored?
In any region with cyclonic or high winds, yes. A half-empty tank weighs much less than a full one and can lift off a pad in a strong wind. Anchor straps or ring locks are cheap compared to the damage an unanchored tank causes. Even inland areas with occasional storms justify the anchors.
How does a solar rainwater pump work, and is it better?
A solar pump runs directly from a panel during sun hours, filling an elevated header or the main tank. On a site without reliable mains power, a solar pump is often simpler than a fuel-driven generator. It has fewer failure points and no running costs. On a site with reliable mains, a conventional electric pressure pump is usually cheaper and delivers higher instantaneous flow.
About this checklist
- Prepared by
- Spare Theory editorial · research-based
- Last checked
- 2026-10-16
How we choose what earns a spotWe treat a rainwater tank as a fifteen-year construction project planned in a fixed order: sizing from roof and rainfall, site selection, foundation, delivery and placement, inlet and first-flush, overflow and drainage, and pump and distribution. Harvest arithmetic follows published rainwater-collection engineering practice. Foundation and anchoring guidance follows manufacturer specifications and standard site-preparation rules. We state no prices, no brand endorsements and defer permit requirements and potable-connection rules to the local authority and a licensed installer.
Reference basisUS Environmental Protection Agency rainwater harvesting system guidanceNational Weather Service and NOAA rainfall intensity references