Rainwater Storage Tank Installation: The Pad, the Overflow, and the 12,400-Pound Problem

A rainwater storage tank installation looks like plumbing and is really carpentry and dirt work. The pipes are the easy part. The hard part is that the thing you are about to fill weighs more than a pickup truck, and it is going to sit in one spot on your property through every freeze, thaw, and gully washer for the next 20 years.

I have seen more rainwater systems ruined by a settling pad than by any pump or filter. So let's start at the bottom and work up.

How much does a full rainwater tank actually weigh?

Water weighs about 8.34 pounds per gallon. The Texas Water Development Board's Rainwater Harvesting Manual puts it as "just over 8 pounds per gallon," and works the example: a 1,500 gallon tank holds about 12,400 pounds of water.

Run that for whatever size you are considering:

• 55 gallon barrel: about 460 pounds.

• 275 gallon tote: about 2,300 pounds.

• 500 gallon tank: about 4,200 pounds.

• 1,500 gallon tank: about 12,400 pounds.

• 2,500 gallon tank: about 20,900 pounds.

Two things follow from those numbers. First, a tank that is level when empty is not necessarily level when full, and a tank that leans gets worse, not better. Second, you are never moving it once it is full. Pick the spot carefully, because the decision is effectively permanent.

What should a rainwater tank sit on?

A level, stable base sized for the full load. For small tanks on firm ground, compacted sand or pea gravel can be enough. For anything substantial, pour a pad.

The TWDB manual is specific about the trade-off. Over a stable substrate such as caliche, "a load of sand or pea gravel covering the bed may be sufficient." Otherwise, "a concrete pad should be constructed." And when the soil conditions are unknown, the manual recommends bringing in a structural engineer rather than guessing.

Practical points the manual raises that are easy to skip:

1. Check the pad after intense rainfall. The storm that fills your tank is the same storm that can wash out from under it. The first few big rains are your inspection schedule.

2. Keep runoff from undermining the base. That includes your own overflow, which is the single most common way people erode their own foundation.

3. Think about truck access. The manual notes placement should account for hauled water delivery. If a dry spell means a water truck, make sure one can reach the fill.

One more I will add from plain experience: give yourself room to walk all the way around the tank. You will need to get at fittings, screens, and the drain valve, and a tank crammed against a wall becomes a tank you never maintain. That access matters most the day you have to clean the sludge out of the tank, which is a job best done entirely from outside it.

How big should the tank be?

Start from your roof, not from a catalog. The yield figure is about 0.62 gallons per square foot of collection surface per inch of rain.

The Texas A&M AgriLife Extension rainwater harvesting manual states it plainly: "In theory, approximately 0.62 gallons per square foot of collection surface per inch of rainfall can be collected." Theory is the key word. The same manual says "Most installers assume an efficiency of 75 percent to 90 percent," because you lose water to splash-out, evaporation, wetting the roof, and the first flush you deliberately throw away.

So the honest math for a 1,200 square foot collection area in a 1 inch rain looks like this:

• Theoretical: 1,200 x 0.62 = 744 gallons.

• At 85 percent efficiency: about 632 gallons.

Do that against your own local rainfall month by month, not the annual average. The annual number hides the dry stretch, and the dry stretch is the whole reason you are building this. For working the other direction, from household demand back to a tank size, see our guide to sizing a water storage tank for emergencies.

What goes between the roof and the tank?

Screens first, then a first flush diverter, then the tank inlet. Each one takes out a different problem.

The AgriLife manual describes leaf guards as typically quarter-inch mesh screens in wire frames, with other options including funnel-type downspout filters using stainless steel or brass screens, strainer baskets, and nylon mesh filter socks at the tank inlet. A roof washer may include a 30 micron filter for finer material.

Then the first flush. The rule of thumb in both manuals is to divert at least 10 gallons for every 1,000 square feet of collection surface, with an alternate framing of 1 to 2 gallons per 100 square feet. The TWDB manual also cites a preliminary Australian vendor study with a range of 13 to 49 gallons per 1,000 square feet, which tells you the 10 gallon rule is a floor and not a ceiling.

If you are building a standpipe diverter, the TWDB gives you the sizing directly: with 3 inch PVC or similar pipe, "allow 33 inches of pipe per gallon." So a 10 gallon first flush is roughly 27 feet of 3 inch pipe, which in practice means several parallel standpipes rather than one absurd tower.

Finally, mosquitoes. Both manuals say the same thing: tanks must be covered and vents screened to discourage mosquito breeding, and a screen trap at the water entry point helps. An opaque tank also inhibits algae. This is not optional maintenance. An unscreened tank turns into a mosquito nursery in one warm month, and we walk through the fixes for that and the other usual complaints in common rainwater collection problems.

Where should the overflow go?

Away from the pad, away from the house, and nowhere near a septic drainfield.

This is the part people treat as an afterthought and it is the part that does structural damage. The TWDB manual says overflow and drainage should be routed so they do not undermine the tank foundation or other structures, and states directly that "Water runoff should not enter septic system drainfields."

Size the overflow at least as large as the inlet, because in a hard rain the overflow has to pass everything the roof is delivering. An undersized overflow backs water up into the tank lid, the screens, and eventually the downspout, which is how you find out about it.

Where does the water go instead? A swale, a rain garden, a gravel infiltration bed, or simply a run of solid pipe that carries it well downslope of everything you care about. Just make the destination deliberate.

Which tank materials are safe for drinking water?

Only materials with a food-grade or potable-approved interior, and only tanks that have never held anything toxic.

The AgriLife manual goes material by material. Fiberglass tanks for potable use "should have a USDA-approved food-grade resin lining" and must be opaque. Wood tanks need a food-grade liner. Galvanized metal usually has a food-grade liner or epoxy coating, and that coating "must be FDA- and NSF-approved for potability." Concrete tanks should have the interior "plastered with a high-quality material approved for potable use." Off-the-shelf plastic tanks must be opaque, and buried polypropylene tanks need specially reinforced walls.

Two hard rules from the same manual: verify that any repurposed steel drum has never held toxics, and use only new trash cans if you go that route. The underlying principle is simple and worth stating plainly. A tank that once held a chemical is never a drinking water tank again, no matter how well you rinse it.

If the system is for irrigation only, your material options widen considerably. Decide that question before you buy, not after.

How do you keep a rainwater tank from freezing?

In freezing country, empty and disconnect before the first hard freeze and leave it that way until spring.

The Texas manuals do not cover this, because Texas mostly does not need it. Penn State Extension does, and the guidance is specific: empty and detach the barrel before the normal first hard freeze date for your area, and wait until the median last hard freeze date in spring before hooking it up again. Penn State treats a hard freeze as temperatures falling to 28 degrees F or lower for a sustained period.

Why it matters: water expands about 9 percent when it freezes, which Penn State says can crack open a wooden or plastic barrel, especially a full one. Even when nothing cracks, ice can sit in the tank all winter, cutting capacity and making the thing hard to drain.

The winter checklist Penn State lays out is short:

• Empty it and disconnect it from the downspout.

• Clean out sediment, leaves, and sticks while it is empty.

• Replace leaky drain valves, torn screens, and worn diverter hoses now, not in April.

• Store it indoors, or leave it outside upside down so rain cannot collect in it.

• Strap or weight an empty plastic tank so wind does not walk it across the yard.

• Put a downspout extension on in the meantime so roof water does not dump against your wall or freeze on a walkway.

For a large fixed cistern that cannot be moved, the same logic applies in a different form. Drain the lines, valves, and pump (those are what split first), keep the tank partially full so it is not a sail, and insulate or bury what you can.

The numbers, in one place

Frequently asked questions

What does a rainwater storage tank need to sit on?

A level, stable base sized for the full weight. The Texas Water Development Board's Rainwater Harvesting Manual says a load of sand or pea gravel over a stable substrate may be sufficient for a small tank, and that otherwise a concrete pad should be constructed. If you do not know your soil conditions, the manual recommends consulting a structural engineer, and checking the pad after intense rainfall.

How much does a full rainwater tank weigh?

Water weighs just over 8 pounds per gallon, so a 1,500 gallon tank holds roughly 12,400 pounds of water according to the Texas Water Development Board. A 500 gallon tank is still over 4,000 pounds. That load is why the pad, not the tank, is the part that usually fails.

How much water can you collect from a roof?

About 0.62 gallons per square foot of collection surface per inch of rainfall in theory, per the Texas A&M AgriLife Extension rainwater harvesting manual. Real systems lose some to splash, evaporation, and first flush, so installers typically assume 75 to 90 percent efficiency.

How big should a first flush diverter be?

The common rule of thumb is to divert at least 10 gallons for every 1,000 square feet of collection surface, or 1 to 2 gallons per 100 square feet. For a standpipe diverter built from 3 inch PVC, the Texas Water Development Board says to allow 33 inches of pipe per gallon of diversion.

How do you winterize a rainwater tank or barrel?

Empty and disconnect it before your area's normal first hard freeze date, and do not reconnect until the median last hard freeze has passed. Penn State Extension notes that water expands about 9 percent when it freezes, which can crack a plastic or wooden barrel, and that ice left in a tank can persist all winter and cut its capacity.

Can you drink water from any rainwater tank?

No. For potable use the tank interior has to be food grade or potable approved, per the Texas A&M AgriLife Extension manual, and it must never have stored toxic materials. Collection is only half the job either way. Rainwater still needs appropriate filtration and disinfection before anyone drinks it.

Get the pad right and the rest of a rainwater system is just fittings and a weekend. Get the pad wrong and you will spend years chasing a lean you cannot fix without draining 12,400 pounds of water. Level it, drain around it, screen it, route the overflow somewhere you chose on purpose, and put it to bed before the first hard freeze.

You never know, but you can always be ready.

Sources: Texas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd edition · Texas A&M AgriLife Extension, Rainwater Harvesting Manual · Penn State Extension, Rain Barrel Winter Maintenance