6 Mistakes to Avoid When Installing a Flexible Tank

The 6 Mistakes to Avoid When Installing a Flexible Tank

How to avoid common mistakes before installing a flexible tank on your property to collect rainwater.

Installing a flexible tank: 6 mistakes to be absolutely avoided :

Repeated droughts, watering restrictions, rising water bills, collection and storage ofrainwater with a flexible tank is one of the most effective ways to achieve long-termwater savings.

The installation is simple, provided you avoid a few pitfalls. Here are six key mistakes to steer clear of when choosing and properly setting up your tank.

1— Undersizing (or oversizing) the tank capacity

This is the most strategic point. A tank that’s too small will fill up after the first rainfall and won’t meet your needs during dry spells.

On the other hand , a tank that’s too large costs more, takes up space, and will rarely (if ever) be fully filled.

Two factors should be the base of your choice :

Your rainwater storage needs : Estimate as accurately as possible, considering all intended uses, such as:

• Watering plants and flowers
• Irrigating the garden and vegetable patch
• Washing (cars, floors, etc.)
• Household use (washing machine, toilets, etc.)
• Filling ponds or basins

Your rainwater harvesting potential You would need three indicators :

• Catchment area (S): the roof surface (or surfaces if collecting from multiple buildings). This is usually equal to the building’s ground footprint
• Rainfall (P): average annual precipitation in liters per m².
• Runoff coefficient (D): measures collection efficiency. The steeper the roof, the higher the recovery rate. For example, a flat roof has a coefficient of 0.6, while a pitched tiled roof reaches Formula: S × P × D.

At SERENA, standard tank sizes range from few hundred liters to dozens cubic meters. Choose according to your actual needs.

2 — Installing the tank on an unprepared surface

Unprepared ground, stones, roots, or debris can puncture the tarpaulin, create wearpoints, and compromise stability. Water may pool, fittings may strain, and leaks can
occur.

The solution is to prepare a flat, stable, and clean base (e.g., a smooth concrete slab or compacted sand bed). We recommend laying down geotextile (SERENA offers 650 g/m² PVC ground mats) to reduce friction and protect against sharp surfaces. Our installation manual provides full details.

3 — Neglecting rainwater filtration

From roof to tank, rainwater carries leaves, dust, and grit that accumulate in gutters and inside the tank. This leads to blockages, dirty water, pump damage, foul odors,
and contamination. Poorly managed overflow can even cause flooding, backflow to the roof, or tank damage.

It is essential to install simple, accessible pre-filtration upstream (such as gutter guards, downspout filters, or fine mesh baskets). At the tank inlet, use an additional
filter and a floating strainer at the suction point to capture the cleanest water between the surface and the bottom.

4 — Choosing the wrong pump and piping

An undersized pump delivers inadequate flow, making watering impractical. Anoversized pump consumes too much energy and may run dry, causing wear and
inefficiency. Similarly, pipes that are too narrow create pressure losses, strain the pump, and shorten its lifespan.

Selecting the right pump is straightforward: once you’ve defined your intended uses(drip irrigation, sprinklers, washing, etc.), calculate the total manometric head
(TMH = elevation + pressure losses + required pressure). This will identify the type of pump (submersible or surface).

For piping, use appropriately sized diameters (e.g., PE32/PE40 for long distances) andquality fittings (brass or stainless steel, new seals). If supplying multiple outlets,
install a pressure switch or booster pump.

5 — Ignoring rainwater collection regulations:

A flexible tank whose water is used only outdoors (watering, washing, filling ponds, etc.) does not require any formalities. However, if you use the collected water inside your home, certain restrictions apply: it may only be used for non-drinking purposes. In this case, a declaration to the local town hall is mandatory, under penalty of fines. Municipal authorities must be able to verify that the system complies with regulations.

In addition, for obvious health reasons, it is essential to maintain a physical separation between the rainwater network and the drinking water supply (using a backflow
preventer). Clearly mark all outlets with the label “Non-potable water.”

6 — Forgetting tank maintenance

Leaves and sediments inevitably build up. Without regular inspection, blockages form, flow decreases, and the pump deteriorates. Winter frost can also damage valves
and fittings if left unprotected.

SERENA tanks are low-maintenance, but vigilance is key. We recommend a visualinspection every 2–3 months (check connections, valves, filters, strainers). Drain the
tank if water remains unused for long periods.

SERENA flexible tanks withstand frost and summer heat (up to 70°C on the liner) without needing to be emptied. However, the end valve must be protected from
freezing. Insulate it with protective foam, and if temperatures fall below –10°C, drain the water circuit at the valve.

In summary
Installing a flexible tank is straightforward when the right factors are anticipated: capacity, base preparation, filtration, overflow, connections, and pump. By avoiding
these six common mistakes, you ensure a simple, durable, and efficient rainwater harvesting system, maximize water savings, and gain independence during droughts.

Need advice on tank sizing, roof water collection, or equipment (filters, pumps, accessories)? The SERENA team is here to guide you every step of the way, from
planning to installation.

Internal & external links

• See our flexible rainwater tanks
• World Water Report– ONU