Why are holding water back and using it two different jobs?
Most conversations about rainwater open with „how big a tank?“. The first question is a different one: what the water is meant to do. Holding it back and using it are two different jobs, and a tank that does one usually cannot do the other.
Retention means holding storm runoff on the plot so the sewer does not receive the whole peak at once, and increasingly that is what the municipality asks of you. Harvesting is the opposite job: water is collected in order to be used, and that is the part that saves you money. The difference shows in the state the tank should be in just before the rain: a retention tank empty, a storage tank full. A full harvesting tank has no retention capacity left at the moment of a cloudburst.
One cistern can do both jobs only if it is sized and controlled for both. Either the volumes are stacked: the lower part is the usable store, above it sits the retention volume with controlled discharge. Or the tank is partly emptied under active control before expected rainfall. Anyone who orders „a rainwater tank“ without saying which job it is for gets a device that performs one and pretends at the other.
Why does the municipality care, and where does the requirement come from?
Spatial planning is governed by Act No. 200/2022 Coll. on spatial planning, and the permitting of construction by Act No. 25/2025 Coll. on construction, in force since 1 April 2025. The condition applying to your parcel, though, is not in the statute. It is in the municipality's documentation and in the opinions of utility operators.
Besides land-use designation, the municipal land-use plan carries general regulations for the whole territory, and stormwater handling now belongs among them in a growing number of municipalities. The second lever is the green space coefficient: the more of the plot you must leave planted, the less you may build over and pave, and the less water runs off. The third point is the permitting itself, where the building authority and the sewer operator both ask where the water from the roof and the hardstanding goes. The answer „into the storm sewer“ often no longer passes, or passes with a restricted discharge attached.
What volume you will be required to provide and what discharge you will be allowed differ between municipalities and cannot be guessed in advance. Get it in writing before the siting of the house is drawn. How to reach the binding text is described in How to Read a Municipal Land-Use Plan.
How is the retention decision actually made?
The survey first, the catalogue second
The order never changes. A percolation test and a hydrogeological assessment first establish the permeability of the soil and the groundwater level. Cohesive clay subsoil and a high water table are the two commonest reasons infiltration does not work on Slovak plots, and that has to be found out before the design, not after the infiltration boxes have been bought.
Infiltration, or retention with controlled discharge?
If the ground accepts water, infiltration is the first choice: a trench, a modular soakaway or a rain garden, complemented by permeable paving on the driveway and the terrace. The water stays where it fell, which is the principle behind a sustainable drainage system. The infiltration structure must sit at the distance from the foundations the assessment specifies, otherwise you are routing water towards your own substructure.
If the ground will not take water, what remains is a retention tank with controlled discharge: the volume absorbs the peak and releases only a small flow, in the order of a few l/s, set by the operator of the network or the receiving watercourse. A green roof lowers the peak too, retaining part of the rainfall and delaying the rest.
A safe overflow route must always exist
Every system eventually receives rainfall beyond its design. It therefore needs an emergency overflow and a surface route falling away from the house, along which the water leaves under control and never onto the neighbouring plot. The overflow is the one part of the design that must not be dropped.
| Property | Retention (holding back) | Harvesting (using) |
|---|---|---|
| Purpose | Reduce the runoff peak leaving the plot | Replace drinking water where it is not needed |
| Who asks for it | Municipality, building authority, sewer operator | The owner |
| Ideal state before rain | Empty tank | Full tank |
| What sets the volume | Drained area and the authority's condition | Irrigation and in-house demand, length of dry spells |
| Benefit | The permit, and a less waterlogged plot | A lower water bill |
| Risk if designed badly | The tank is full, the peak runs to the sewer | The tank runs dry in July, when irrigation matters most |
What can harvested water be used for, and what not?
Rainwater is soft and free of lime scale, and that is its practical advantage. It is not drinking water, however, and must never be presented as such.
| Use | Suitability | What it requires |
|---|---|---|
| Garden and lawn irrigation | The best use of all | Pump, hose or irrigation circuit |
| Toilet flushing | Suitable, year-round steady demand | Separate circuit, mains top-up |
| Laundry | Conditionally, soft water saves detergent | Good filtration, an informed owner |
| Washing the terrace, cars, tools | Suitable | A clearly marked outlet tap |
| Filling a swimming pool | Not advisable without treatment | Filtration and disinfection, assessed separately |
| Drinking, cooking, dishwashing, hygiene | Not permissible | Do not do it. The circuit must stay separate |
Reusing water from basins and showers is a separate system: greywater recycling has different technology, hygiene requirements and maintenance, and it is not mixed with rainwater in one tank.
Underground or above ground, and what does winter do to it?
An above-ground plastic butt at the downpipe is the cheapest way into the subject, but in the Slovak climate a seasonal one. It has to be drained for the winter, otherwise repeated freezing and thawing will damage it, so from November to March it does nothing.
An underground tank below the frost line works all year and keeps the water cold and dark, so algae develop less readily. The price is the excavation, protection against flotation where the water table is high, a lid rated for the loading above it, and poorer access for cleaning.
Slovak spring is exactly where retention and harvesting collide hardest. Snow melts onto frozen ground that will not accept it, so infiltration does not work and the runoff travels across the surface. If the tank has stood full since autumn, the peak arrives at a system with nowhere to give. That is why the retention volume is calculated separately and must never be confused with the storage volume.
| Comparison | Above-ground tank | Underground tank |
|---|---|---|
| Winter operation | Drained, out of service | Year-round, below the frost line |
| Fitness for retention | Low, the volume is small and seasonal | High, can be stacked with a storage volume |
| Water quality | Warmer, daylight encourages algae | Cold and dark, more stable |
| Installation cost | Minimal, no earthworks | Excavation, bedding, surround, lid |
| When to choose it | Irrigation only, small plot, low outlay | An authority condition, WC flushing, year-round use |
Why does a tank silt up, and what does filtration change?
It is not only water that comes off a roof, but dust, pollen, conifer needles, moss debris and bird droppings, and all of it settles in the tank as sludge. A silted tank loses usable volume, the water starts to smell, and the pump draws sludge.
The sequence of elements that prevents this is always the same: leaf catching in the gutter and the downpipe, a filter ahead of the tank inlet, diversion of the first flush (the most contaminated part of the rainfall after a long dry spell), and a calmed inlet that does not stir up settled sludge. On the outlet side belongs a floating suction filter, drawing water from near the surface rather than from the bottom.
Why must the non-potable circuit have no connection to drinking water?
This is the one part of the system where a mistake costs health rather than money. The rainwater circuit is separate, routed apart from the mains, the pipework and every outlet must be marked as non-potable, and there must be no point at which the two networks could be joined, not even temporarily by a hose.
If the system is topped up from the mains in a drought, it is topped up through a free discharge with an interruption, that is an air gap, never by a direct pipe connection. Protection of drinking water against backflow is covered by a separate European standard, and compliance with it is worth requesting from the installer in writing. The pump is either submersible in the tank or a suction unit with a pressure vessel, in both cases with dry-run protection. Pipework for WC flushing has to be prepared during the shell stage: retrofitting it into a finished house is the most expensive item of the solution.
How do the roof and the gutters affect water quality?
Water is best collected from smooth, chemically neutral coverings: clay and concrete tiles, or coated sheet metal. Copper and zinc coverings release metals into the water that do plants no good, and bitumen shingles or old sheets of unknown composition have no place in a system harvesting for use inside the house. A green roof retains and slows the water, but what leaves it is coloured and carries nutrients: acceptable for irrigation, not for laundry.
The gutter is the bigger culprit, though. A downpipe under conifers silts up all year, and without a catcher the problem passes into the tank. A downpipe from a terrace or a driveway does not belong in a harvesting system, although for retention it is a legitimate inflow.
What does the maintenance really involve?
- Gutters and leaf catchers: inspection and cleaning at least twice a year, more often under conifers.
- Filter ahead of the tank: flushed through during leaf fall, then as contamination dictates.
- Suction filter and pump: annual check, worn parts replaced.
- Sludge on the bottom: checked once a year, pumped out according to what you measure, not to the calendar.
- Above-ground tank: drained and disconnected from the downpipe before the first frosts.
- Infiltration and overflow: after every heavy downpour, check the water is leaving where it should.
When does this pay for itself, and when is it done because it must be?
Let me be direct. Harvesting on its own pays back slowly in a single house: water here is still cheaper than the technology replacing it, and one household's consumption is small. The return is best with a large garden under real irrigation and with year-round WC flushing. On a small plot with two vegetable beds it is an investment made out of conviction rather than calculation, which is legitimate as long as it is named honestly.
The economics change the moment the municipality requires retention. You are paying for the volume and the excavation anyway, and the difference between a retention tank and a combined one is only the extra technology, which makes it the best moment to add harvesting. The same holds where the house relies on its own well of limited yield, because there rainwater spares a resource money cannot top up.
In one sentence: retention is an obligation towards your surroundings, harvesting is an opportunity for you, and a single tank satisfies both only when somebody designed it from the start as two jobs.