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Flashing refers to thin pieces of impervious material installed to prevent the passage of water into a structure from a joint.
The origin of flash and flashing are uncertain but these words are related to a pool of water and the word splash. Usage may come from Middle English between 1350–1400 "...flasshen to sprinkle, splash [compare with] flask...". Dictionary quotes refer to lead as the material used as a flash. Counter-flashing (cover flashing, cap flashing) is when there are two parallel pieces of flashing used together such as on a chimney where counter-flashing is built into a chimney and overlaps a replaceable piece of base flashing. Strips of lead used for flashing an edge were sometimes called an apron, the term still used for the piece of flashing below a chimney. Flashing is often let into a groove in a wall or chimney called a reglet.
Before the availability of sheet products for flashing carpenters used creative methods to minimize water penetration such as angling roof shingles away from the joint, placing chimneys at the ridge, and building steps into the sides of chimneys to throw off water. Birch bark was occasionally used as a flashing material. The introduction of manufactured flashing decreased water penetration at obstacles such as chimneys, vent pipes, walls which abut roofs, window and door openings, etc. thus making buildings more durable and reducing indoor mold problems. In builders books, by 1832 Loudons An Encyclopædia of Cottage, Farm, and Villa Architecture and Furniture... gives instruction on installing lead flashing and 1875 Notes on Building Construction gives detailed instruction and is well illustrated with methods still used today.
Flashing may be exposed or concealed. Exposed flashing is usually of a sheet metal such as lead, aluminium, or copper, Galvanized steel, stainless steel, zinc alloy, terne or lead-coated copper and other architectural metals are also used. Metal flashing should be provided with expansion joints on long runs to prevent deformation of the metal sheets, and should not stain or be stained by adjacent materials or react chemically with them. Flexible, adhesive backed, rubberized, asphalt compounds are used around wall penetrations such as window and door openings.
Copper is an excellent material for flashing because of its malleability, strength, solderability, workability, high resistance to the caustic effects of mortars and hostile environments, and long service life (see: copper flashing). This enables a roof to be built without weak points. Since flashing is expensive to replace if it fails, copper’s long life is a major cost advantage. Cold rolled 1/8”-hard temper copper is recommended for most flashing applications. This material offers more resistance than soft copper to the stresses of expansion and contraction. Soft copper can be specified where extreme forming is required, such as in complicated roof shapes. Thermal movement in flashings is prevented or is permitted only at predetermined locations.
Soft zinc is another flashing alternative gaining popularity. Soft zinc is an exceptionally malleable material, making it extremely useful for complex roofing connections. This material provides normal soft soldering capabilities and delivers easy folding. Soft zinc is a sustainable solution for replacing lead flashing. this environmentally friendly material is completely recyclable and provides 100% clean runoff. 
Flashing is used primarily in three areas:
A structure incorporating flashing has to be carefully engineered and constructed so that water is directed away from the structure and not inside. Flashing improperly installed can direct water into a building.
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.