#40 – Roof Flashing: Lineal vs. Step Flashing Explained
I recently had an interesting conversation with a contractor who has installed traditional roofing for many years. He was familiar with a concept called step flashing that is used with many types of roofing. Step flashing may be used alongside a chimney or wall where the roof intersects it. Step flashing is a series of short 90-degree angle metal flashing pieces that go up the wall or chimney and extend out onto the roof, often being interlaced with each course of roofing.
Metal Roofs and Step Flashing
This is not the norm with metal roofing and this contractor was quite surprised when I told him that. Rather than use these series of small flashings, metal roofs rely upon lineal flashings, often formed in 10 or 12-foot lengths. A lineal flashing does not have all of the little joints where water, and especially water impeded from traveling down the roof by ice or snow, can work its way into the flashing system and enter the structure.
Additionally, because of the linear design of vertical seam metal roofs and the interlocking nature of most metal shingles, step flashing would trap the water on a metal roof rather than return it to the surface of the roof the way it does with other roofing materials.