S2: Management of Everyday Rainfall (Interception)

S2

What S2 requires

Source: Defra Non-Statutory Technical Standards for Sustainable Drainage Systems (2015, refreshed 19 June 2025), Standard S2. Reproduced in CIRIA C753 §3 (Designing for water quantity: interception criterion) and the Sewerage Sector Guidance (SSG) for adoption.

S2 addresses the most frequent rainfall events, the everyday showers and moderate rain that fall regularly throughout the year. The standard requires that the first 5 mm of rainfall must not result in runoff from the site to surface waters or piped drainage. This "lost rain" is captured, infiltrated, or evaporated on-site through a range of features.

Compliance Thresholds:

• Summer (May–October): At least 80% of rainfall events must be intercepted (not contributing runoff). Higher summer interception benefits from evapo-transpiration from vegetation.
• Winter (November–April): At least 50% of rainfall events must be intercepted. Lower winter threshold accounts for reduced vegetation activity and higher soil moisture.

Deemed Compliance Routes: The standard lists features that, if properly designed and maintained, automatically meet S2 compliance. These include:

• Green roofs (typically 50–80 mm depth, retaining 5 mm rainfall)
• Rainwater harvesting systems (sized to capture and store 5 mm across the roof area)
• Permeable surfaces (with 100+ mm permeable base and subbase)
• Soakaways and infiltration trenches (sized for 5 mm infiltration from contributing area)
• Swales with adequate depth and permeability
• Bioretention features with engineered growing media
• Detention basins (dry-weather mode)

Critical Note: Water butts alone do not comply with S2. A water butt typically captures only a few millimetres from a roof; once full, it provides no further retention. A compliant rainwater harvesting system must be sized to retain 5 mm from the full roof area and must have provision for overflow management.

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