Plastic, concrete, self-supporting: which rainwater tank to choose

In online shops, choosing a tank looks like picking a volume and colour. In reality, the decision is between three designs that can differ in installation cost by as much as 25,000 Kč, and which one is feasible often depends not on you, but on what lies beneath the garden.

Plast versus beton

Plastic tanks (polypropylene, polyethylene) weigh tens of kilos, are installed by two people with an excavator and are cheaper. They do not corrode, do not burden the structure and are manufactured in volumes from 1 to 15 m³. Disadvantage: an empty plastic tank is essentially a large air bubble, in waterlogged ground buoyancy can push it upwards.

Concrete tanks They weigh tons, so buoyancy poses no risk and they can withstand overhead loads (vehicles can drive over them). However, you will need heavier machinery for installation and crane access. Additionally, concrete slightly alkalises water, which is not an issue for irrigation or flushing. It would only be a problem for drinking water, which rainwater is not used for anyway.

Self-supporting or requiring concrete encasement

With plastic tanks, this is the decision that most affects the budget.

Self-supporting tank It features a reinforced structure (ribs, thicker walls) and after settling is simply backfilled with soil or sand-gravel mix. It has a higher purchase price, but saves you the entire concrete pouring item, in practice 15 to 25 thousand Kč for concrete, formwork and labour.

Tank for concreting It is cheaper, but without a concrete casing it will deform in the ground under soil pressure or be lifted by groundwater. The concreting is done in layers while simultaneously filling the tank with water to equalise the pressure, this causes a one-day delay and cannot be skipped.

Groundwater matters, not the catalogue

During the survey, our main concern is the groundwater level. When it is high, a situation arises that laypeople do not expect: An empty tank behaves like a ship.Water pushes up around it, and if it is not held down by sufficient weight: a concrete casing, a foundation slab with anchoring, or the mass of the concrete tank itself. It will lift, tear off the inlet pipe and deform the piping.

That is why you cannot "save on self-supporting" when the water table is high. It must be either a concrete tank or a plastic one on an anchored slab and encased in concrete. Anyone offering a cheap solution in such terrain has either not measured the water level or is counting on being untraceable by the time the problem manifests within a year.

Questions to ask the supplier

• Have you measured the groundwater level, or are you estimating it based on the surroundings?
• Is the offered tank self-supporting? If not, is concrete encasement included in the price?
• What is the load above the tank: will it be drivable or only walkable?
• Is the foundation slab included in the price, or is only a gravel-sand bed provided?
• Who will provide the documentation of implementation required for the grant?

The final question is often the one where e-shop offers end. They deliver the tank, not the installation, and no one can issue you with a certificate of completion.