Poradna
Rainwater infiltration: when it is sufficient and when it is not
The tank fills up within a few hours during heavy rain. The overflow must have somewhere to drain; otherwise, the water will find its way to the house foundations.
The tank fills up within a few hours during heavy rain. The question of "where to put the surplus" is therefore not a detail at the end of the project. It is a matter that determines whether the system works or whether water will undermine your foundations.
Three ways a takeover can lead
Soakaway on your own plot of land
The most common and usually the cheapest solution. Water from the overflow goes into infiltration pits, tunnels or blocks, from where it percolates into the subsoil. Condition: the subsoil must be permeable and the groundwater level must be sufficiently deep: a clearance of at least one metre is required between the bottom of the infiltration object and the water table.
Connection to the rainwater drainage system
Where the municipality has a stormwater sewer and grants connection permission. This is usually more expensive (the connection is calculated per metre) and in some places there is a charge for rainwater drainage. However, it works even where the plot does not allow infiltration.
Controlled discharge into a watercourse
This is an option for plots of land near a stream or ditch. It requires the consent of the watercourse manager and usually a water law permit: the most administratively demanding variant.
How to determine whether a plot of land is permeable
There is a standard percolation test, but for a detached house a simple test often suffices: dig a pit about half a metre deep, fill it with water and measure how long it takes for the water level to drop.
| Subfloor | Vsakuje | What this means |
|---|---|---|
| Písek, gravel | excellent | a small soakaway pit is sufficient |
| Sandy loam | good | standard tunnels or blocks |
| Clay, loess | incorrectly | large volume, soil replacement, or another method |
| Rock beneath the surface | nevsakuje | sewer or watercourse |
Carry out the test outside the dry season. In August, after three weeks without rain, even clay absorbs water reasonably well, but by November you will find that the soakaway pit is full.
How large a soakaway do you need
The volume is calculated based on the drained area, the intensity of design rainfall and the infiltration capacity of the subsoil. As a rough guide: for a house with a roof area of around 120 m² in well-infiltrating ground, the infiltration object comes to around 2-3 m³; in clayey terrain it can easily be three times that. Therefore, on poorly infiltrating plots it often pays to install a larger storage tank: the more water you use, the less needs to infiltrate.
What causes the suction
- Soakaway pit with gravel - cheapest, but takes up a lot of space and gradually clogs with fine particles. Requires geotextile around the entire volume.
- Infiltration tunnels - plastic semi-circular components that assemble into a row. Better volume-to-space ratio, easy installation, expandable.
- Infiltration blocks - high-porosity blocks, maximum volume in the smallest footprint. The most expensive option, but the best choice where the garden is small or vehicles will drive over the unit.
What to watch out for
Distance from the building. The infiltration object must be located further from the foundations: usually at least five metres away and never above the level of the foundation joint. Infiltration right next to the wall is the surest way to get moisture into the cellar.
Distance from the property boundary. Water that seeps in near the fence may end up at the neighbour's. Besides being a technical issue, it is also a legal one.
Inspection shaft. Without it, you won't know the property is clogged until water starts pooling in your garden. A shaft on the inlet costs a few thousand Kč and saves future digging.
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.