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
  Roof
  A roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather. Structures that require roofs range from a letter box to a cathedral or stadium, dwellings being the most numerous....
  
   of a building
  Building
  In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...
  
   (such as pipe
  Pipe (material)
  A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases , slurries, powders, masses of small solids...
  
  s and chimney
  Chimney
  A chimney is a structure for venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as near as possible to vertical, to ensure that the gases flow smoothly, drawing air into the combustion in what is known as the...
  
  s, or the edges of other roofs) to deflect water
  Water
  Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
  
   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
  Weep
  A weep or a weep-brick is a small opening that allows water to drain from within an assembly. Weeps are located at the bottom of the object to allow for drainage; the weep hole must be sized adequately to overcome surface tension....
  
   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

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 flashing (typically copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 or aluminum) reflect

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

s flashes of sunlight

Sunlight

Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

. Modern flashing may also be rubber

Rubber

Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...

 or other waterproof synthetic

Chemical synthesis

In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...

 materials, but metal, such as stainless steel

Stainless steel

In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

, or galvanized steel is still frequently used.

A structure incorporating flashing has to be carefully engineered and constructed so that water is actually directed away from the structure and not inside. Sometimes flashing is improperly installed by inexperienced laborers who do not understand its purpose, which can result in severe water intrusion and mold.

Environmental Impact

In the US and UK, at least, lead flashing and fittings are still readily available, despite the environmental concerns associated with bulk use of this heavy metal. The Lead Sheet Association touts its recyclability and extreme durability, 500 years, compared to modern materials that can fail within 20 years.http://www.lead4life.co.uk/green.htm

External links

• Roof Flashing Details Illustrated summary of various types of flashing
• Repairing and Replacing Flashing