What is stormwater retention?
Stormwater retention is holding rainwater back in an area, rather than draining it away immediately. The aim is to limit flooding during heavy rain and keep more water available in dry periods. It works at landscape scale, with retention basins and flood storage areas, and at the scale of a single house and garden.
Retention, attenuation and infiltration
In UK practice the terms are used a little differently. Attenuation, or detention, means storing water temporarily and releasing it at a controlled rate; a retention pond usually holds a permanent body of water. Infiltration means letting water soak into the ground. Sustainable drainage systems (SuDS) combine these.
What the rules say
In England, Approved Document H sets an order of preference for rainwater: first an adequate soakaway or other infiltration system; where that is not reasonably practicable, a watercourse; and only then a sewer. Local authorities and water companies may add their own requirements for new development, so check before you design.
Application around the house
- Water butt or rainwater harvesting tank for garden use.
- Green roof: the substrate holds water; how much depends on substrate type and depth, the drainage layer and planting.
- Blue roof or polder roof: a roof with extra water storage and a controlled outlet.
- Rain garden or swale: a planted hollow that fills during a storm and lets water soak away slowly.
- Soakaway crates or a gravel-filled soakaway below ground.
- Less hard paving, or permeable paving.
Rule of thumb: work out the volume
Run-off from a roof is roof area × rainfall. A 50 m² roof in 20 mm of rain produces 50 × 0.020 = 1 m³, or 1,000 litres. A 200-litre water butt is full after just 4 mm. Use this to check whether your storage matches what actually comes off the roof.
Common mistakes
- An overflow that discharges towards foundations, a cellar or the neighbours.
- Infiltrating without checking soil and groundwater; clay soils and high water tables drain poorly, so a percolation test is worth doing first.
- Adding a blue or polder roof without a structural engineer checking the roof can carry the extra load.
Related terms
- Infiltration
- Green roof
- Polder roof
- Cistern
- Climate-adaptive building
- Rainwater drainage
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.