Flashing is 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 system. In modern buildings, flashing is intended to decrease water penetration at objects such aschimneys, vent pipes, walls, windows and door openings to make buildings more durable and to reduce indoormold problems. Metal flashing materials includelead,aluminium,copper,[1]stainless steel,zinc alloy, and other materials.
The origin of the termflash andflashing are uncertain,[2] but may come from the Middle English verbflasshen, 'to sprinkle, splash', related toflask.[3]
Counter-flashing (orcover flashing,cap flashing) is a term used when there are two parallel pieces of flashing employed together[4] such as on a chimney, where the counter-flashing is built into the chimney and overlaps a replaceable piece ofbase flashing. Strips of lead used for flashing an edge were sometimes called anapron,[5] and 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 withcricket flashing or on narrow chimneys with no cricket aback flashing orback pan flashing. Flashing may be let into a groove in a wall or chimney called areglet.
Before the availability of sheet products for flashing, builders used creative methods to minimize water penetration. These methods included angling roof shingles away from the joint, placing chimneys at the ridge, building steps into the sides of chimneys to throw off water, and covering the seams between roofing materials with mortarflaunching. 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 indoormold problems. It is also essential to prevent leaks around skylights or roof windows.[citation needed] Moreover, flashing is important to ensure the integrity of a roof prior to asolar panel installation.[citation needed]
Builders' books, such as LoudonsAn Encyclopædia of Cottage, Farm, and Villa Architecture and Furniture... gave instructions on installing lead flashing by 1832 and in 1875Notes on Building Construction provided detailed instruction and is well illustrated with methods still used today.[6]
Flashing may be exposed or concealed. Exposed flashing is usually of a sheet metal, while concealed flashing may be metal or a flexible, adhesive-backed material, particularly around wall penetrations such as window and door openings.
In earlier days,birch bark was occasionally used as a flashing material.[7] Most flashing materials today are metal, plastic, rubber, or impregnated paper.[8]
Metal flashing materials include lead,aluminium,copper,[1]stainless steel,zinc alloy, otherarchitectural metals or a metal with a coating such as galvanized steel, lead-coated copper, anodized aluminium,terne-coated copper,galvalume (aluminium-zinc alloy coated sheet steel), and metals similar tostone-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 isgalvanic corrosion. Copper and lead cannot be used in contact with or even above aluminium, zinc, or coated steel without an increased risk of premature corrosion. Also, aluminium and zinc flashing cannot be used in contact with pressure treated wood due to rapid corrosion. Aluminium is also damaged by wet mortar and wet plaster.Salt spray in coastal areas may accelerate corrosion; so stainless steel, copper, or coated aluminium are recommended flashing materials near salt water.
Types offlexible flashing products arerubberized asphalt,butyl rubber,polyvinylidene fluoride (sometimes known as kylar or hylar), and acrylic.[9] The different types have different application temperature ranges, material adhesion compatibility, chemical compatibility, levels ofvolatile organic compounds, and resistance to ultraviolet light exposure. Adhesive backed materials can aid installation, but such adhesives are not intended for long-term water-resistance.[citation needed]
Copper is an excellent material for flashing because of its malleability, strength, solder-ability, 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 a newer, proprietary flashing material. It is a relatively malleable material, making it useful for complex roofing connections. It provides normal soft soldering capabilities and delivers easy folding. Soft zinc is said to be an "environmentally friendly" replacement for lead flashing; like lead, it is recyclable, while avoiding lead-contaminated runoff.[13]
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Flashing types are named according to location or shape:
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 waterinto a building.
In the US and UK, at least, lead flashing and fittings are still readily available, despite theenvironmental concerns associated with bulk use of this heavy metal. The Lead Sheet Association touts its recyclability and extreme durability.[14]
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