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.
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. Usage may come from Middle
English between 1350–1400 "...flasshen to sprinkle, splash [compare with] flask...".
Dictionary quotes refer to lead as the material used as a flash. Counter-flashing is when
there are two parallel pieces of flashing are used together 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, 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 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.
Birch bark was occasionally used as a flashing material. 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. Flashing may be exposed or concealed. Exposed
flashing is usually of a sheet metal such as lead, aluminium, or copper, galvanized
steel, stainless steel, zinc alloy, terne, lead-coated copper, or other architectural
metals. 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.
Flexible, adhesive backed, flashing materials may be used around wall penetrations such
as window and door openings. Types of flexible flashing products are rubberized asphalt,
butyl rubber and acrylic. 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.
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. 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 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 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. Flashing is used in different areas:
Roof flashing is placed around discontinuities or objects which protrude from the roof of
a building to deflect water away from seams 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 is a concealed flashing placed under windows or door thresholds to prevent
water from entering a wall at those points. Roof penetration flashings are 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.
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.[1]
See also Housewrap
Copper flashing Rainscreen
References
Ching, Francis D. K.; Cassandra Adams. Building Construction Illustrated. John Wiley @ Sons
Inc. ISBN 0-471-35898-3. Beall, Christine. Masonry Design and Detailing.
McGraw Hill Book Company. ISBN 0-07-004223-3. Ramsey, Charles; Hoke, John Ray, Jr.. Architectural
Graphics Standards. John Wiley & Sons, Inc. ISBN 0-471-34816-3.
External links Roof Flashing Details Illustrated summary
of various types of flashing Repairing and Replacing Flashing
Roof Penetration Flashing Types Photos and solutions for all roof protrusions
Video on how to weld sheet lead Video series of still images showing some
complex lead flashing work