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 as

chimney A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typical ...

s, vent pipes, walls, windows and door openings to make buildings more durable and to reduce indoor

mold A mold () or mould () is one of the structures that certain fungus, fungi can form. The dust-like, colored appearance of molds is due to the formation of Spore#Fungi, spores containing Secondary metabolite#Fungal secondary metabolites, fungal ...

problems. Metal flashing materials include

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

stainless steel Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...

, zinc alloy, and other materials.

Etymology and related terms

The origin of the term ''flash'' and ''flashing'' are uncertain, but may come from the Middle English verb ''flasshen'', 'to sprinkle, splash', related to ''flask''. ''Counter-flashing'' (or ''cover flashing'', ''cap flashing'') is a term used when there are two parallel pieces of flashing employed together such as on a chimney, where the counter-flashing is built into the chimney and overlaps a replaceable piece of ''base flashing''. Strips of lead used for flashing an edge were sometimes called an ''apron'', 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 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

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 mortar . 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

problems. It is also essential to prevent leaks around skylights or roof windows. Moreover, flashing is important to ensure the integrity of a roof prior to a

solar panel A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. These electrons flow through a circuit and produce direct ...

installation. Builders' books, such as Loudons ''An Encyclopædia of Cottage, Farm, and Villa Architecture and Furniture...'' gave instructions on installing lead flashing by 1832 and in 1875 ''Notes on Building Construction'' provided 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, while concealed flashing may be metal or a flexible, adhesive-backed material, particularly around wall penetrations such as window and door openings.

Materials

In earlier days, birch bark was occasionally used as a flashing material. Most flashing materials today are metal, plastic, rubber, or impregnated paper. Metal flashing materials include lead,

,Multiple authors. ''Roofing, flashing & waterproofing''. Newtown, CT: Taunton Press, 2005. Print.

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

alloy, other architectural 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 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 Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, different metal, when both in the prese ...

. 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 Sea spray consists of aerosol particles formed from the ocean, primarily by ejection into Earth's atmosphere through bursting bubbles at the air-sea interface Sea spray contains both organic matter and inorganic salts that form sea salt aerosol ...

in coastal areas may accelerate corrosion; so stainless steel, copper, or coated aluminium are recommended flashing materials near salt water. Types of ''flexible flashing'' products are rubberized asphalt, butyl rubber, polyvinylidene fluoride (sometimes known as kylar or hylar), and acrylic. The different types have different application temperature ranges, material adhesion compatibility, chemical compatibility, levels of

volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to Indoor mold, house mold, Upholstery, upholstered furnitur ...

s, and resistance to ultraviolet light exposure. Adhesive backed materials can aid installation, but such adhesives are not intended for long-term water-resistance.

Copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

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. 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. "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.

Types

Georgia-Pacific ForceField moisture barrier sheathing

Flashing types are named according to location or shape: ; Roof flashing: Placed around discontinuities or objects which protrude from the

roof A roof (: roofs or rooves) is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of tempera ...

of a

building A building or edifice is an enclosed Structure#Load-bearing, structure with a roof, walls and window, windows, usually standing permanently in one place, such as a house or factory. Buildings come in a variety of sizes, shapes, and functions, a ...

to deflect water away from

seams Seam may refer to: Science and technology * Seam (geology), a stratum of coal or mineral that is economically viable; a bed or a distinct layer of vein of rock in other layers of rock * Seam (metallurgy), a metalworking process the joins the ends ...

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 (or sill pan): A concealed flashing placed under windows or door thresholds to prevent water from entering a wall at those points. ; Roof penetration flashing: 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. ; Channel flashing: Shaped like a “U” or channel to catch water (e.g., where the edge of a tile roof meets a wall). ; Through wall flashing: Spans the thickness of the wall and directs water to weep holes. ; Cap flashing (drip cap): Often used above windows and doors. ; Drip edge: A metal used at the edges of a roof. ; Step flashing (soaker, base flashing): Pieces of flashing material which overlap each other in "steps". ; Counter flashing (cap flashing): Covers a base flashing. ; Pipe flashing (pipe boot, vent boot, pipe flange): A product used where pipes penetrate roofs. ; Chimney flashing: A general term for flashing a chimney to cover the intersections of the chimney and install a damp proof course (DPC) ; Kickout flashing: At the very bottom of a roof/wall intersection, the lowermost step flashing specially formed to deflect water away from the wall. ; Valley flashing: In the valley of two intersecting roof planes. 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.

References

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External links

Roof Flashing Details
Illustrated summary of various types of flashing
Video on how to weld sheet lead

Video series of still images showing some complex lead flashing work
{{Roofs Moisture protection Building engineering Building materials