Stormwater management on properties

The property owner or holder is responsible for the management of stormwater generated on the property. The property must be connected to the HSY’s stormwater sewer network if it is located in the stormwater sewerage area. Otherwise stormwater can be treated on the property for example through infiltration. Stormwater can also be conveyed away from the property to the city's stormwater system (e.g. an open trench).

It is forbidden to convey stormwater to someone else’s property or to the street. Stormwater must not cause harm to neighbours. 


Picture: Where is stormwater conveyed to on your property?

  1. Stormwater is collected from the roof into the stormwater sewer
  2. Underdrain water is conveyed via an inspection well to the basic water chamber.
  3. All stormwater generated on the property and foundations’ drainage water is collected into the basic water chamber.
  4. Stormwater and foundations’ drainage water are conveyed to the connection chamber of the stormwater sewer. From there, water is led to the water body.
  5. Wastewater is conveyed in its separate system via the inspection well to the wastewater sewer and wastewater treatment centre.
  6. It is recommended to choose permeable materials for yard structures instead of asphalt so that as much stormwater can infiltrate as possible.

Because stormwater ends up in nature as such, it is important that toxic or harmful substances are not released into the stormwater sewer. For example, the washing water used for washing cars and its treatment is regulated by the cities’ own environmental protection provisions. The environmental protection regulations may determine that it is forbidden to convey wastewater, generated on the property through car wash, untreated to the stormwater sewer. If the washing water does not include any detergents, it can be infiltrated into the soil on the property.

Familiarise yourself with the cities’ environmental protection regulations: 

Read more about the cities’ instructions for stormwater management:


Infiltration and detention of stormwater


The drainage of a new building should be planned in the context of building planning. Water management planning requires an expert. Cities’ building control authorities provide instructions and information. For example, the Building Control Service of the City of Helsinki has prepared instructions for stormwater management.

When it concerns an old property and its renovation, it is worth checking how the property's sewerage has been implemented in the first place and if necessary change it so that stormwater will be conveyed to a stormwater sewer or into a trench near the house. 

Infiltration and detention of stormwater on the property are both good proven methods for managing stormwater, provided they do not cause a flood risk or other harm. There are various methods for infiltration and detention. Usually, subterranean systems are applied. Stormwater can also be managed through small actions: for instance, rainwater and melt water running down from the roof can be collected into containers and use for watering the garden.


Yard and garden materials and vegetation


Choose vegetation and permeable materials for your yard and garden (e.g. turf, gravel or paving slabs with drainage holes) instead of impermeable materials such as asphalt or pavement with no holes. 

It is also recommended to include bioswales, i.e. indentations covered by vegetation, or small basins into which water can be conveyed in the yard planning. Vegetation binds nutrients from different water types and depending on soil properties, water in the bioswales and basins can also be infiltrated into the soil. Bioswales serve to slow down stormwater flows.

Green roofs detain rainwater falling on the roof and melt water. A green roof infiltrates, filters, evaporates and treats water with the help of plants.

There are several different natural, ecological and technical solutions for stormwater management available. A lot of business companies offer various kinds of services and solutions for different situations and needs.