A strip of material that overlaps flashing in roofing or masonry to create a waterproof seal by directing water away from joints.
From counter- + flashing (from flash, possibly related to Middle English flaschen, meaning to splash). This roofing technique became standardized in the early 20th century.
Roofers discovered that one layer of flashing wasn't enough—by overlapping flashing strips so water flows downward like roof shingles, they could finally stop those annoying leaks at chimneys and walls.
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