Penetrations through the built-up roof membrane are usually flashed in one of two ways. Individual pipes and small vents usually use flat, metal flange flashings that are placed directly on the last ply of roofing material and are stripped in with felts and mastic or felts and bitumen.
Larger penetrations and groups of smaller penetrations usually use curbs constructed of wood, metal or concrete, flashed with bituminous base flashing and metal counter flashings.
Common penetration flashing problems are:
- The failure to properly design the flashing for the penetration. - Open or broken seams in metal curbs caused by expansion and contraction. - Standing water behind penetration curbs caused by the omission of crickets. - Sagging or separating base flashings caused by omission of top wood nailers. - Missing or deteriorated counter flashing. - Splitting or separation of the felt stripping over the edge of metal flanges. - Improper priming and stripping of metal surfaces. - Fastener backout and separation of the metal flashing flange from the roof around penetration flashings. - Movement between stack vents or pipes and the flashing.