Flashing refers to thin pieces of impervious material installed to prevent the passage of water into a structure from a joint.
Definition and etymology
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.
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 make buildings more durable and reduce 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:
- 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 and in valleys where the runoff is concentrated.
- 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 placed under windows or door thresholds to prevent water from entering a wall at those points.
- Roof Penetration Flashings are used to waterproof pipes, supports, cables, and all roof protrusions. Stainless steel penetration flashings have proven to be the longest lasting and most reliable roof flashing type.
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.
- Video on how to weld sheet lead
- Video series of still images showing some complex lead flashing work
- Oxford English Dictionary Second Edition on CD-ROM (v. 4.0) © Oxford University Press 2009. Flash, n.4, flash, v.1, flashing,
- http://dictionary.reference.com/browse/flashing referenced 04/12/2013
- http://encyclopedia2.thefreedictionary.com/counterflashing accessed 04/12/2013
- Loudon, J. C.. An encyclopædia of cottage, farm, and villa architecture and furniture; containing numerous designs for dwelling ... each design accompanied by analytical and critical remarks .... London: Longman, Rees, Orme, Brown, Green, & Longman, 1833. 126. Print.
- Old-time New England Journal of the Society for the Preservation of New England Antiquities, volumes 52-55 1961.19.
- Smith, Percy Guillemard Llewellyn. Notes on building construction. Part 1. London, Oxford and Cambridge: Rivingtons, 1875. Chapter 10, Plumbers work. Print.
- Multiple authors. Roofing, flashing & waterproofing. Newtown, CT: Taunton Press, 2005. Print.
- The glory of copper; Metal Roofing Magazine, December 2002/January 2003
- Flashings and copings, Copper in Architecture Design Handbook, Copper Development Association Inc., http://www.copper.org/applications/architecture/arch_dhb/flashings_copings/intro.html
- Sternthal, Daniel (2000). Copper flashings in contemporary construction, The Construction Specifier, Magazine of the Construction Specifications Institute, October 2000
- Ching, Francis D. K.; Cassandra Adams (2001). Building Construction Illustrated (3rd edition). John Wiley @ Sons Inc. ISBN 0-471-35898-3.
- Beall, Christine (1987). Masonry Design and Detailing (2nd Edition). McGraw Hill Book Company. ISBN 0-07-004223-3.
- Ramsey, Charles; Hoke, John Ray, Jr. (ed) (2000). Architectural Graphics Standards (10th Edition). John Wiley & Sons, Inc. ISBN 0-471-34816-3.