Flashing

Plain-English definition

The waterproofing detail work where a roof meets anything that is not roof — walls, curbs, pipes, drains, edges. The open field of a membrane is a simple factory-made sheet; flashings are where that sheet has to be cut, shaped, and sealed by hand around every interruption.

Why it matters to you

Verified: Manufacturers describe these transitions — where the flat roof ties into walls, parapets, and penetrations — as vulnerable areas for water intrusion whose detailing is critical to system performance (GAF — roof-to-wall flashing). It is a long-standing rule of thumb among roofing professionals that most leaks originate at flashings and penetrations rather than in the membrane field; we have not found an independent published tally that quantifies this, so treat it as trade experience, not a statistic. The practical takeaway is the same either way: when you walk a roof or read an inspection report, the flashings — not the wide-open membrane — are where problems concentrate, and flashing workmanship is one of the clearest separators between a careful installation and a cheap one.

Technical detail

Flashing is not one product but a family of details: membrane base flashings turned up at walls and curbs, metal counterflashings and copings that shed water over them, pre-molded boots at pipe penetrations, and edge metal at the roof perimeter. Details should shed water over — never behind — the layer below, and roof-to-wall details need to accommodate differential movement between the roof and the wall structure. Each membrane manufacturer publishes its own required flashing details, and warranty coverage generally depends on following them.