Flashing (weatherproofing)

Flashing refers to thin pieces of impervious material installed to prevent the passage of water into a structure from a joint or as part of a weather resistant barrier (WRB) system.

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.[1] Usage may come from Middle English between 1350–1400 "...flasshen to sprinkle, splash [compare with] flask...".[2] 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[3] 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,[4] the term is still used for the piece of flashing below a chimney. The up-hill side of a chimney may have a small gable-like assembly called a cricket with cricket flashing or on narrow chimneys with no cricket a back flashing or back pan flashing. Flashing may be let into a groove in a wall or chimney called a reglet.

Purpose

Chimney flashing on a tile roof on the Island of Jersey.
The flashing visible here is the apron below the dormer and the valley flashing in the open valley. The step flashing (soakers) are properly installed below the roof and wall shingles and is not visible. Hôtel Demoret Moulins, France

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, 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.[5]

Flashing may be exposed or concealed. Exposed flashing is usually of a sheet metal and concealed flashing may be metal or a flexible, adhesive backed, material particularly around wall penetrations such as window and door openings.

Materials

Birch bark was occasionally used as a flashing material.[6] Most flashing materials are metal, plastic, rubber, or impregnated paper.[7]

Metal flashing materials include lead, aluminium, copper,[8] stainless steel, zinc alloy, other architectural metals or a metal with a coating such as galvanized steel, lead-coated copper, anodized aluminum, terne-coated copper, galvalume (aluminum-zinc alloy coated sheet steel), polyvinylidene fluoride (sometimes know as kylar or hylar), and metals similar to stone-coated metal roofing. Metal flashing should be provided with expansion joints on long runs to prevent deformation of the metal sheets due to expansion and contraction, and should not stain or be stained by adjacent materials or react chemically with them. An important type of potential chemical reaction between metal flashing materials is galvanic corrosion. Copper and lead cannot be used in contact with or even above aluminum, zinc, or coated steel without an increased risk of premature corrosion. Also, Aluminum and zinc flashing cannot be used in contact with pressure treated wood due to rapid corrosion. Aluminum is also damaged by wet mortar and wet plaster. Salt spray in coastal areas may accelerate corrosion so stainless steel, copper, or coated aluminum are recommended materials.

Types of flexible flashing products are rubberized asphalt, butyl rubber and acrylic.[9] The different types have different application temperature ranges, material adhesion compatibility, chemical compatibility, levels of volatile organic compounds, resistance to ultraviolet light exposure. No flexible flashing material is designed to remain exposed like metal flashing materials. Adhesive backed materials are useful during installation but that adhesive is not guaranteed for long-term water-resistance.

Also, flashing cement (roof cement) is a paste applied as an adhesive between roofing materials or for temporary repair of damaged roofing materials.

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.[10][11] Cold rolled (to 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.[12]

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 an environmentally friendly solution for replacing lead flashing; it is completely recyclable and provides 100% clean runoff.[13]

Types

Flashing types are named by their location of uses or shapes:

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.

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.[14]

See also

References

  1. ↑ Oxford English Dictionary Second Edition on CD-ROM (v. 4.0) © Oxford University Press 2009. Flash, n.4, flash, v.1, flashing,
  2. ↑ http://dictionary.reference.com/browse/flashing referenced 04/12/2013
  3. ↑ http://encyclopedia2.thefreedictionary.com/counterflashing accessed 04/12/2013
  4. ↑ 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.
  5. ↑ Smith, Percy Guillemard Llewellyn. Notes on building construction. Part 1. London, Oxford and Cambridge: Rivingtons, 1875. Chapter 10, Plumbers work. Print.
  6. ↑ Old-time New England Journal of the Society for the Preservation of New England Antiquities, volumes 52-55 1961.19.
  7. ↑ Schmid, Karl F. "Flashing". Concise Encyclopedia of Construction Terms and Phrases. New York: Momentum, 2013. Print.
  8. ↑ Multiple authors. Roofing, flashing & waterproofing. Newtown, CT: Taunton Press, 2005. Print.
  9. ↑ Charles Wardell, "Flexible flashing:Tips For Getting the Most Out of These Useful Products". Journal of Light Construction. vol. 32. no. 7. April, 2014. 51-58. Print.
  10. ↑ The glory of copper; Metal Roofing Magazine, December 2002/January 2003
  11. ↑ Flashings and copings, Copper in Architecture Design Handbook, Copper Development Association Inc., http://www.copper.org/applications/architecture/arch_dhb/flashings_copings/intro.html
  12. ↑ Sternthal, Daniel (2000). Copper flashings in contemporary construction, The Construction Specifier, Magazine of the Construction Specifications Institute, October 2000
  13. ↑ http://www.metaltech-usa.com/soft-zinc.html
  14. ↑ http://leadsheet.co.uk/leads_green_credentials

External links

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