Flexible rainwater tank: choosing the capacity, installing and maintaining it

An underground concrete tank means an excavator, a two-meter-deep hole, and a construction site that turns the garden upside down. Many homeowners have neither the space nor the desire for that, yet they watch every shower run into the sewer. A flexible tank is precisely the answer for this situation: a large watertight bladder laid flat in a crawl space, garage, beneath a terrace, or at the back of the garden, filling as it rains and emptying as watering requires. Well chosen and properly installed, a flexible tank stores several cubic meters of rainwater without excavation, at a much lower cost and with a much smaller footprint than an underground tank. The volume still needs to be sized correctly, the base prepared, the inlet and overflow carefully installed, and a suitable pump provided.

Flexible water tank: what exactly are we talking about?

The principle is simple. A highly resistant, high-frequency-welded fabric forms a large enclosed “mattress” fitted with several connections: an inlet at the top, an outlet at the bottom, an overflow, and sometimes a vent. When empty, the bladder is flat and foldable. When full, it takes the shape of a cushion whose height depends on the volume and the dimensions of the ground surface. Manufacturers offer models ranging from a few hundred liters to several dozen cubic meters; for domestic use, the most common range is between 2 and 20 m³.

The usual material is PVC-coated polyester fabric, treated with UV protection for outdoor models. Some bladders intended for drinking water or agricultural uses use other coatings, but for garden rainwater, polyester-PVC remains the standard: it is flexible, weldable and repairable. Allow for greater fabric thickness and weight for larger capacities, as the pressure exerted on the seams increases with volume.

A flexible water tank is sometimes confused with two other solutions. An above-ground rigid tank (made of polyethylene, often vertical) takes up less floor space but is much taller, and its capacity rarely exceeds a few cubic metres. An underground tank, meanwhile, provides cooler water protected from light, at the cost of major construction work. The flexible bladder sits between the two: large capacity, low height, reversible installation.

How many cubic metres do you really need?

Sizing is based on a very simple physical rule: 1 millimetre of rain falling on 1 m² of roof produces 1 litre of water. The rest is a matter of common sense.

In central Belgium, annual rainfall is around 800 to 900 mm, with significant variations from one year and region to another. Not everything that falls on the roof reaches the tank: some evaporates, some overflows from the gutters during storms, and some is removed by the filter. A collection coefficient of around 0.8 to 0.9 is therefore applied to a tiled or slate roof, and considerably less to a green roof, which retains a lot of water.

These figures do not mean that you need to store all the year's water: the tank fills and empties continuously. The right volume is more closely related to what you use during a dry period of three to four weeks. A family-sized vegetable garden and a few flower beds may require around a few hundred litres per week in midsummer; a regularly watered lawn requires much more. If you also connect the toilets, allow for a few dozen litres per person per day.

The classic mistake is to oversize the bladder compared with the roof area: a 20 m³ tank supplied by a garden shed will almost never be full, and the water will stagnate there for a long time. It is better to have a correctly filled and regularly renewed bladder than a large volume that is half empty.

Where should you install a flexible water tank?

The bladder needs floor space, not height. This is its main advantage in Belgian homes, where ventilated crawl spaces 60 to 80 cm high beneath floors are common. A 5 m³ tank may, depending on the model, occupy only about sixty centimetres in height when full, but it then requires around ten square metres of floor space. Before buying, measure three things: the usable length and width, the clear height beneath the beams or ducts, and the width of the access through which the folded bladder will be brought in.

The most common locations:

• Crawl space: protected from frost and light, cool water, no visible footprint. Access for maintenance is sometimes acrobatic.
• Garage or cellar: easy installation and inspection, but the area is occupied.
• Under a wooden deck: a good solution if the underside is accessible and the ground is stabilised.
• Outdoors, at the bottom of the garden: possible with a UV-treated liner, ideally in a shaded enclosure or under a cover to limit heating and algae growth.

One point is often overlooked: the load. One cubic metre of water weighs one tonne. In a crawl space with an earth or concrete floor, this is not a problem. On an upper floor or a wooden floor, it is out of the question without an engineer's advice.

Prepare the base: the step that determines its service life

A tank liner withstands evenly distributed water pressure very well, but is much less resistant to localised punctures. A sharp stone, a forgotten screw or a root beneath the bladder, multiplied by several tonnes, will eventually puncture it. Preparation therefore always follows the same order:

1. Clean the surface of all debris, sharp gravel, nails and shards.
2. Level the ground: a sloping surface makes the water migrate to one side, deforms the bladder and puts the load on a single weld. A layer of stabilised fine sand or a lightweight screed will do the job.
3. Protect it with thick geotextile or a ground mat supplied by the manufacturer, extending a few dozen centimetres all around.
4. Clear the walls and protruding corners so that the inflated bladder does not rub against anything.

Outdoors, add protection against rodents and garden tools, and avoid direct proximity to trees whose roots lift the ground over the years.

Connect the inlet: filter before storing

The quality of the stored water is determined before it reaches the tank. The roof collects leaves, moss, dust, bird droppings and pollen. If all this enters the tank, the organic matter decomposes at the bottom, consumes oxygen and eventually produces the notorious rotten-egg smells. A filter collector or basket filter is therefore installed on the downpipe, sending debris to the sewer and allowing only water through.

A few connection rules:

• The inlet is positioned at the top of the tank, with a flexible hose to absorb movement of the fabric during filling.
• The overflow is mandatory. A full flexible tank continues to receive water during every shower; without an outlet, the pressure rises and the seams suffer. The overflow must be connected to the sewer, a soakaway or an infiltration area, depending on what your local authority permits.
• A vent allows air to escape during filling and enter during emptying.
• All connections must be made using the flanges and rings provided by the manufacturer, never with a makeshift glue fix.

Pumping water: which pump and why use a float

A flexible water tank is generally installed at the same level as the garden, or even lower. The water therefore will not flow out on its own, or only at a ridiculous flow rate. A pump is needed. Two types dominate the field.

The submersible rainwater pump is placed in the cistern or in a connected pumping chamber. It is quiet, simple and sufficient for supplying a garden hose or sprinkler. A model such as the Oase ProMax Rain 4000 is designed for this purpose. In a flexible tank, do not place the pump directly on the fabric: install it in a separate chamber or use a floating intake.

The automatic booster pump is installed outside the cistern and draws water through a hose. It maintains pressure in a network and starts automatically when a tap is opened: this is the solution when supplying several outlets or toilets.

In both cases, the floating intake makes all the difference. A float keeps the intake strainer a few centimetres below the surface: the pump draws the clearest water, away from the deposits that accumulate at the bottom and the particles floating on the surface. The pump experiences less wear, and sprinklers and drip irrigation systems clog much less often.

What can water from a flexible water tank be used for?

Rainwater is soft, practically free of limescale, and slightly acidic. Plants appreciate it, and it leaves few marks on windows or car bodies. Reasonable uses:

• watering the vegetable garden, flowerbeds, planters, and patio plants;
• topping up a garden pond, provided the water is clear and the KH is monitored, as rainwater provides almost no buffering capacity;
• cleaning patios, tools, and the car;
• Toilets and washing machines, only via a separate, clearly identified network, with no connection whatsoever to the drinking water distribution network.

This last point deserves emphasis. European regulations on water intended for human consumption and Belgian regional regulations require strict separation of the networks. A simple connection tap between the tank and mains water can cause non-potable water to flow back into the public network. Have the installation checked by a plumber if you supply water to the inside of the house.

Water from a flexible tank is not drinking water, neither for humans nor, without precautions, for pets: it may contain bacteria from droppings, metals washed off the gutters, and roofing residues.

Maintaining the flexible tank throughout the seasons

A well-protected flexible tank requires little maintenance, but not none.

In spring, clean the filter collector and gutters, check the condition of the overflow, and walk around the bladder to look for any signs of moisture on the ground, indicating a slow leak.

In summer, the tank empties quickly. Do not let the pump run dry: a float switch or low-water safety device protects it. If the bladder is outdoors, monitor the temperature: warm, sunlit water turns green and develops an odor more quickly.

In autumn, it is leaf season. The upstream filter becomes clogged within a few days beneath a large tree; clean it often, otherwise the water will overflow beside it.

In winter, a bladder installed in a crawl space or garage is generally not at much risk. Outdoors, water freezing at the surface generally does not damage a flexible bladder, which deforms, but outdoor fittings, taps, and pumps must be drained or protected.

Every few years, a complete draining removes the sediment accumulated at the bottom. This is an opportunity to inspect the liner, welds, and flanges. A small tear in a PVC model can generally be repaired with the kit supplied by the manufacturer, on a dry, clean liner.

Odours, cloudy water, fabric that “sweats”: common problems

The most common complaint concerns odour. It almost always comes from organic matter decomposing without oxygen at the bottom of the bag. The solution involves three steps: improve upstream filtration, remove the sludge when draining the tank, and do not leave the water stagnant for months without renewal. We explain the causes and solutions in detail in our article about rainwater odours and what they mean.

Cloudy water after a storm is normal: heavy rain washes the roof and stirs up sediment. It clears within a few days. With a floating intake, the pump continues drawing reasonably clean water during this time.

A bag that appears damp on the outside is not necessarily punctured: in a crawl space, condensation forms on the cold fabric when the air is warm and humid. To be sure, wipe it dry, place a dry sheet of paper under a fold, and check the next day.

Flexible tank, rigid tank, or underground tank: how to choose

In summary, the flexible tank is the best choice whenever a flat surface is available and you want to avoid construction work. The underground tank remains the benchmark solution for a new home with a large roof, especially if it supplies water indoors. The rigid tank is suitable for small volumes and tight spaces.

Also read

Does your rainwater tank smell bad? Bad smells in my rainwater tank: what should I do?

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Frequently asked questions

— Can a flexible tank remain outdoors all year?

Yes, if the fabric is UV-treated and intended for outdoor use. It will nevertheless last longer in the shade or under a cover, and the connections, pump, and outdoor taps must be protected from frost.

— Is a permit required to install a flexible tank?

A tank installed in a crawl space or garage generally requires no formalities. Connecting the overflow and supplying toilets may, however, be subject to municipal or regional regulations: check with your local authority.

— How long does a flexible tank last?

Several years to one or two decades, depending on the quality of the fabric, exposure to sunlight, and care taken with the support. Soil preparation and the presence of an overflow matter more than anything else.

— Can a fish pond be filled with water from a flexible tank?

Yes, as a supplement, if the water is clear and odorless. Rainwater is very soft: monitor the pond's KH and pH, especially after significant top-ups.

In summary

• The flexible tank stores rainwater flat, without excavation, in a crawl space, garage, or garden.
• Size it based on the roof area (1 mm of rain on 1 m² = 1 liter) and your needs during a dry period.
• A level, clean and protected surface, a connected overflow, and an upstream filter: these are the keys to longevity.
• A suitable pump and a floating intake provide cleaner water and protect the equipment.
• The water is never potable; for toilets, a compliant separate system is essential.

Last updated: October 3, 2026.