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 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 typic ...

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 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 seco ...

problems. Metal flashing materials include

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

copper Copper is a chemical element with the symbol Cu (from la, cuprum) 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 pinkis ...

stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...

zinc alloy Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc 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 ta ...

, 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 A reglet is found on the exterior of a building along a masonry wall, chimney or parapet that meets the roof. It is a groove cut within a mortar joint that receives counter-flashing meant to cover surface flashing used to deflect water infiltrat ...

''.

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 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 integrity of the roof prior to a

solar panel A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...

installation. 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 and 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 Birch bark or birchbark is the bark of several Eurasian and North American birch trees of the genus ''Betula''. The strong and water-resistant cardboard-like bark can be easily cut, bent, and sewn, which has made it a valuable building, crafti ...

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 with the symbol Zn and atomic number 30. Zinc 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 periodi ...

alloy, other

architectural metals Metals used for architectural purposes include lead, for water pipes, roofing, and windows; tin, formed into tinplate; zinc, copper and aluminium, in a range of applications including roofing and decoration; and iron, which has structural and o ...

or a metal with a coating such as galvanized steel, lead-coated copper, anodized aluminium,

terne Terne plate is a form of tinplate: a thin steel sheet coated with an alloy of lead and tin. The terne alloy was in the ratio of 10-20% tin and the remainder lead. The low tin content made it cheaper than other tinplates. Terne plate was used for ...

-coated copper, galvalume (aluminium-zinc alloy coated sheet steel), and metals similar to

stone-coated metal roofing A stone coated metal roof is a roof made from steel or some other metal; the metal is then coated with stone chips and attached to the steel with an acrylic film. The goal is a more durable roof that still retains the aesthetic advantages of a more ...

. 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, in the presence of an electrolyte. A sim ...

. 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 Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...

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 Rubberized asphalt concrete (RAC), also known as asphalt rubber or just rubberized asphalt, is noise reducing pavement material that consists of regular asphalt concrete mixed with crumb rubber made from recycled tires. Asphalt rubber is the la ...

butyl rubber Butyl rubber, sometimes just called "butyl", is a synthetic rubber, a copolymer of isobutylene with isoprene. The abbreviation IIR stands for isobutylene isoprene rubber. Polyisobutylene, also known as "PIB" or polyisobutene, (C4H8)n, is the ho ...

polyvinylidene fluoride Polyvinylidene fluoride or polyvinylidene difluoride (PVDF) is a highly non-reactive thermoplastic fluoropolymer produced by the polymerization of vinylidene difluoride. PVDF is a specialty plastic used in applications requiring the highest pur ...

(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 compounds, and resistance to ultraviolet light exposure. No flexible flashing materials are designed for long-term exposure as are metal flashing materials. Adhesive backed materials are convenient during installation, but such adhesives are not intended for long-term water-resistance.

Copper Copper is a chemical element with the symbol Cu (from la, cuprum) 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 pinkis ...

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

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 temper ...

of a

building A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and fun ...

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. ; 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 Environmental issues are effects of human activity on the biophysical environment, most often of which are harmful effects that cause environmental degradation. Environmental protection is the practice of protecting the natural environment on th ...

associated with bulk use of this heavy metal. The Lead Sheet Association touts its recyclability and extreme durability.

References

* * *

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