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Flashing refers to thin continuous pieces of sheet metal or other impervious material installed to prevent the passage of water into a structure from an angle or joint. Flashing generally operates on the principle that, for water to penetrate a joint, it must work itself upward against the force of gravity or in the case of wind-driven rain, it would have to follow a tortuous path during which the driving force will be dissipated. Exterior building materials can be configured with a non-continuous profile to defeat water surface tension.

Flashing may be exposed or concealed. Exposed flashing is usually of a sheet metal, such as aluminium, copper, painted galvanized steel, stainless steel, zinc alloy, terne metal, lead or lead-coated copper. Metal flashing should be provided with expansion joints on long runs to prevent deformation of the metal sheets. The selected metal should not stain or be stained by adjacent materials or react chemically with them.

Flashing concealed within a construction assembly may be of sheet metal or a water proofing membrane such as bituminous fabric or plastic sheet material, depending on the climate and structural requirements. Aluminium and lead react chemically with cement mortar. Some flashing materials can deteriorate with exposure to sunlight.

Flashing can assume a number of forms:

  • Roof flashing is placed around discontinuities or objects which protrude from the roof of a building (such as pipes and chimneys, or the edges of other roofs) to deflect water away from seams or joints.
  • Wall flashing may be embedded in a wall to direct water that has penetrated the wall back outside, or it may be applied in a manner intended to prevent the entry of water into the wall. Wall flashing is typically found at interruptions in the wall, such as windows and points of structural support.
  • Sill flashing is a concealed flashing that is typically placed under windowsills or door thresholds to prevent water from entering a wall at those points.
  • Base flashing is found at the base of a wall, and usually incorporates through-wall flashing with weep holes to permit the escape of water. Base flashings may be placed at the building grade or at a point where a roof intersects a wall.

Its name may derive from the fact that metal flashing (typically copper or aluminum) reflects flashes of sunlight. Modern flashing may also be rubber or other waterproof synthetic materials, but metal, such as stainless steel, is still frequently used.