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…".
Flashing is a material used whenever two different materials need to be joined together, applied in strips, in such areas as joining a brick/masonry chimney with roofing shingles; or where siding ties in with a window frame. 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. The introduction of manufactured flashing decreased water penetration at obstacles such as chimneys, vent pipes, walls which abut roofs, window and door openings therefore making buildings more durable and reducing indoor mold problems.
Traditionally flashing materials are metal, plastic, rubber or impregnated paper. Metal flashings include materials such as: aluminum, copper, stainless steel zinc alloy, and other aerchtectural metals with a coating such as galvanized steel or lead coated copper, etc. As with all materials, these metal flashings have certain ‘restrictions’ to consider in their use such as ‘expansion and contraction’ needs for long uns of metal sheets, as well as, staining or premature corrosion when certain metals are used adjacent to each other. Aluminum will be damaged with wet mortar and wet plaster. Salt spray in coastal areas may accelerate corrosion to certain metals and aluminum and zinc flashing used in contact with pressure treated wood will promote rapid corrosion.
Flexible flashing products include rubberized asphalt, butyl rubber and acrylic. The different types have different applications, somewhat like metals, as to their tolerable temperature ranges, material adhesion compatibility, chemical compatibility, resistance to ultraviolet light exposure, etc. No flexible flashing material is designed to remain exposed like metal flashing materials are, and, while adhesive backed materials are useful during installation the purchaser/owner needs to be aware that the adhesive is not guaranteed for long-term water-resistance.
As for the metal application, 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. This enables a roof to be built without weak points. Since flashing is expensive to replace if it fails therefore copper’s long life provides a cost advantage. Cold rolled (to 1/8-hard temper) copper can be recommended for most flashing applications. This material provides greater resistance than soft copper to the stresses of expansion and contraction while soft copper can be specified where extreme forming is required, such as with complicated roof shapes.
Another flashing alternative gaining popularity in some applications is soft zinc. Soft zinc is exceptionally malleable, making it extremely useful for complex roofing connections. It provides for normal soft soldering capabilities and delivers easy folding. Soft zinc is an environmentally friendly solution for replacing lead flashing as it is completely recyclable and provides 100% clean runoff
Flashing is a crucial component of all exterior work and is essential to keep a structure watertight since most leakage will occur where the differing types of materials meet. Which type of product is best in your building or remodeling situation can best be answered by your contractor based on your specific application needs and your geographic local.