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 chimneys, vent pipes, walls, windows and door openings to make buildings more durable and to reduce indoor mold problems. Metal flashing materials include lead, aluminium, copper, stainless steel, zinc alloy, and other materials.
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 flaunching. 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. 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 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.
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, aluminium, copper, stainless steel, zinc 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. 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 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 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 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.
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 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.