GREEN & BLUE INFRASTRUCTURE
A YEAR OF THE 5MM REQUIREMENT:
WHAT WORKS FOR RAINWATER INTERCEPTION?
Just over a year has passed since England’ s revised National Standards for Sustainable Drainage Systems put greater emphasis on everyday rainfall. At their heart is a simple requirement: for the majority of rainfall events, the first 5mm should not leave a development.
The question now is less about what the rule means and more about how we meet it effectively.
In practice, the simplest solutions are often those that intercept water closest to where it falls. Permeable paving, rain gardens, SuDS planters, tree pits and areas of soft landscape can all capture rainfall before it enters the conventional drainage network. Local authority guidance is increasingly reflecting this approach, identifying features such as rain gardens, attenuating tree pits and permeable paving as straightforward ways of meeting interception requirements.
At building level, green roofs provide another effective first line of defence, retaining rainfall and enabling evapotranspiration before runoff reaches ground level. Rainwater harvesting can go further by capturing roof water for reuse, particularly where there is a reliable demand for irrigation, toilet flushing or other non-potable uses.
The important lesson from the past year is that there is rarely one‘ best’ technology. The most successful schemes combine several simple interventions into a management train, distributing interception across roofs, streets and landscape rather than relying on a single large storage feature. This reflects the National Standards themselves, which favour drainage features operating as close to the source of runoff as practicable.
Space remains the challenge. On dense developments, every square metre is contested. That makes multifunctional systems particularly valuable: a rain garden can provide planting and biodiversity; a tree pit can manage runoff while supporting canopy cover; harvested rainwater can maintain landscapes through dry periods.
A year on, the 5mm requirement is beginning to encourage a useful change in mindset. Rainfall interception is no longer simply an engineering calculation at the end of a project. Done well, it becomes part of the architecture and landscape from the outset.
The simplest route to compliance may therefore be the most integrated one: capture rain where it falls, use it where possible, and make every component of the landscape work harder.
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