Overflow (Secondary) Roof Drainage

Overflow drainage, also called secondary or emergency drainage, provides a second path for water to leave a roof when the primary drains are blocked or overwhelmed. Without it, water on a parapeted roof can rise until it overloads the structure or overtops flashings.

Forms

• Overflow scuppers through the parapet, with their inlet set above the roof surface.
• Overflow drains beside the primary drains, with a raised dam or standpipe so they only take water once it reaches a set depth, piped separately from the primary system.
• Lowered parapet sections or edge openings on some roofs.

Code basis

In North America, the building and plumbing codes require secondary drainage on roofs where parapets or other construction would otherwise trap water. Structural design for rain loads assumes the primary drains are blocked and accounts for the depth of water needed to reach and flow through the secondary system; for roofs with slopes below 1/4 in per 12 in, the IBC requires a ponding-instability analysis using that blocked-drain assumption.

In the UK, BS 6229:2025 requires emergency overflows on roofs with only one outlet and on all blue roofs, sized to match the full capacity of a blocked outlet, and recommends additional overflow routes for internally drained roofs where possible.

Design points

• overflow inlets must sit below the top of flashings and upstands
• discharge should be visible, so that flow from an overflow alerts the owner to a blocked primary drain
• pipes from overflow drains should discharge separately from primary pipes
• overflows must be kept clear in maintenance

Relationship to blue roofs

Blue roofs deliberately hold water using flow-restricting outlets, so overflows are a key safety feature in their design.