Poradna
What retention tank capacity do you need?
An oversized tank is an expensive hole in the garden; an undersized one overflows in May and is empty by August. Here is the calculation that gives you the figure before you call anyone.
Tank capacity is the first figure people ask about, and also the only one that can be reasonably estimated from the desk. You need two things for this: how much water actually runs off your roof and how much you consume. The rest is calculation.
How much water runs off your roof
The basic formula is simple: roof plan area × annual rainfall total × runoff coefficient. The footprint area refers to the area occupied by the building on the plot, not the area of sloped surfaces: the slope does not affect the amount of water collected.
The annual total rainfall in the Czech Republic ranges from approximately 450 mm in the driest parts of Central Bohemia and South Moravia to 700 mm or more in the foothills. The runoff coefficient indicates how much water is not lost along the way: for fired clay tiles, concrete roofing, or sheet metal, use 0.8; for rougher surfaces, less.
Example: a house with a 120 m² footprint in an area with 550 mm of rainfall and a metal roof. 120 × 0.55 m × 0.8 = 52.8 m³ of water per year. That is over fifty thousand litres that are flowing down the drain today.
How much electricity does it consume
This is where estimates most often diverge from reality. Indicative figures we work with:
| Usage | Consumption |
|---|---|
| Lawn irrigation system | 3–5 l/m² and grouting |
| Edging flower beds and shrubs | 2–3 l/m² and grouting |
| WC flushing | 25-30 l/osobu a den |
| Washing | 10-15 l/osobu a den |
A household of four that uses water for flushing consumes around 4 m³ per month just on the toilet. In addition, a thousand square metres of lawn in a dry week can easily swallow up 4 m³ with a single watering.
How to determine the tank capacity
The tank cannot hold annual consumption. It is designed to bridge periods of shortage. dry spells between rains. Under Czech conditions, it is calculated to last approximately three weeks without rainfall. Practical rule: take the consumption for three weeks and verify that the volume fills from one heavy rain.
| Roof | Zahrada | Objem | Who is this for? |
|---|---|---|---|
| do 80 m² | do 200 m² | 3 m³ | side yard, smaller plot |
| 80-120 m² | 200-500 m² | 5 m³ | filling and flushing |
| 120-200 m² | 500-1 000 m² | 8 m³ | prolonged dry period without refilling |
| over 200 m² | over 1 000 m² | 10 m³ and more | the entire season, or a swimming pool |
Three most common mistakes
Tank sized according to the roof, not according to consumption
A large roof alone is not a reason for a large tank. If you use the water only to irrigate two flower beds, a 15 m³ tank will be full for nine months of the year and overflow with every rain. You pay for volume you never use, and the difference between 5 and 15 m³ in implementation is roughly twenty thousand Kč just for the tank, plus more for excavation.
Forgotten Winter
From November to March you do not need water in the garden, but it continues to rain. If you plan to use only outdoor systems, expect the tank to remain full for half a year and remember that the distribution pipes must be shut off and drained for winter.
Volume regardless of inflow
The tank is filled from a single roof, not from the sky in general. If you divert only half of the downpipes to it, it fills at half speed, and an eight-cubic-metre tank may not fill up within a month after a dry period. Therefore, during design we always determine how many downpipes can realistically be connected to the tank.
What to do next
The calculated volume is an input for the discussion, not the final figure. The final design also depends on the groundwater level (which determines the tank type and whether concrete lining is required), site accessibility for equipment, and the overflow discharge point. These three factors together determine the price more than the volume alone.
Are you dealing with a tank on a specific plot of land?
Tell us the size of your roof and garden and how you intend to use the water. We will get back to you with the volume, price and whether a subsidy is available in your case.