External drainage outlet and DPC clearance
150 mm
External rainwater downpipes must discharge at least 150mm below the damp proof course (DPC) level, typically into a gulley or soakaway, to prevent moisture bridging across the DPC into the substructure
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Read the full answerExternal rainwater downpipes must discharge at least 150mm below the damp proof course (DPC) level, typically into a gulley or soakaway, to prevent moisture bridging across the DPC into the substructure. Subfloor ventilators should not be located directly beneath door openings where they could become obstructed or allow water ingress.
Basis in regulation
How to check it on site
Verify that all external drainage outlets are at least 150mm below the DPC and that no subfloor ventilators are positioned under doorways.
Common failure
Drainage discharges too close to or above DPC, causing damp, or ventilators are placed in vulnerable positions under door thresholds.
Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.