A stormwater drain is the underground component of a stormwater system that serves as a junction point between the downspouts and the surface water sewer or infiltration solution. The drain collects rainwater from the roof and yard areas, separates solid matter using a sand trap (sediment sump) and directs the cleaned water onwards. In Finland, stormwater management is a statutory obligation. According to the Land Use and Building Act and municipal building ordinances, rainwater must be directed off the property in a controlled manner. In most cities and built-up areas, properties must be connected to the municipal surface water sewer, and the stormwater drain is a central part of this connection. In rural areas where there is no sewer, water is directed to an infiltration field or soakaway well. The design and installation of a stormwater drain must meet the requirements of the municipal water utility. The location of the connection point, pipe size and drain type are defined in the connection statement. An inadequate stormwater system can cause basement flooding, foundation moisture problems and even overloading of the municipal sewer network.
Stormwater drain structure and types
A stormwater drain typically consists of a concrete or plastic drain body, a sand trap (sediment sump), inlet pipes, an outlet pipe and a cover. The drain diameter is usually 315–560 mm for plastic drains or 800–1,000 mm for concrete drains. Plastic drains (PE or PP) are currently more popular in single-family house construction due to their lighter weight and easier installation.
The sand trap is the drain's most important functional component. It is a recess at the bottom of the drain where solid matter — sand, leaves and other debris — settles by gravity. The sand trap volume is typically 30–50 litres in a single-family house drain. Without a sand trap, solids would travel into the pipe system and block it over time.
Special types include inspection chambers (inspection access only, without a sediment sump), combination drains (stormwater and foundation drainage in the same drain) and soakaway wells (bottomless drains from which water infiltrates into the surrounding soil). Soakaway wells work well in permeable moraine and sandy soils, but do not function in clay soils.
Installation and connections
Stormwater drain installation requires excavation work and precise height control. The drain is installed below the frost depth (in Finland typically 1.0–1.8 m depth depending on the region) or alternatively at a shallower depth with frost protection. A level base of gravel or sand is prepared at the bottom of the excavation.
The downspout connection to the drain is made with an underground pipe installed at a minimum 1% slope towards the drain. The pipe material is PVC or PP plastic pipe, 110 mm diameter. The sealing of the connection point is critical — a leaking connection washes fine soil material away from beside the foundation and can cause settlement.
Connection to the municipal surface water sewer requires a permit from the water utility. The connection height and location are defined in the connection statement. A typical connection fee is 2,000–5,000 € depending on the municipality. Installation of the property drain line from the downspout to the sewer costs 3,000–8,000 € including materials and excavation. The price varies significantly depending on distance and soil conditions.
Maintenance and sand trap emptying
The most important maintenance task for a stormwater drain is emptying the sand trap (sediment sump). The sand trap should be emptied at least once a year, preferably in spring after the meltwater period. If the surroundings have many trees or sand, emptying may be needed more frequently. A full sand trap no longer separates solids, and they travel into the pipe system, blocking it.
Emptying is done either by hand with a bucket or with a suction device. Professional emptying with a suction device costs approximately 80–200 € per drain. At the same time, the drain condition, tightness of connections and cover integrity are checked. A broken cover is a safety risk and must be replaced immediately.
The most common problems are blockage (debris, roots), frost damage (drain or pipe heaves) and odour nuisances (decomposing organic matter in the sediment sump). Root intrusion into pipes is a common problem, especially near willows and birches — root guard installation or a mechanical root barrier may be necessary. A camera inspection can check the pipe system condition without excavation.
Stormwater management in Finnish municipalities
Finnish municipalities have tightened stormwater regulations in recent years. With climate change, the intensity of heavy rainfall is increasing, adding to the load on surface water sewers. Many municipalities now require stormwater retention on the property before discharge to the sewer in new construction — retention wells or depressions are used for this purpose.
A retention well is larger than a normal stormwater drain and temporarily stores heavy rainfall, releasing it slowly to the sewer. A typical retention volume is 1–3 m³ per single-family plot. The requirement is defined in the municipality's stormwater strategy and varies significantly between municipalities.
Stormwater drain installation is licensed work. A building permit or action permit is required when installing a new stormwater system or connecting to the municipal sewer. The work may only be performed by a qualified contractor, and installations are inspected as part of the building's commissioning inspection. The municipal stormwater fee is typically 50–150 €/year per property.
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Updated: April 2026
