Leakproof Flashing for Metal Buildings
Senior Editor, Mark Robins,
Posted
01/10/2012
Water is a threat to buildings, and building components are
designed to direct water away from or keep water out of buildings.
A well-constructed roof with a successful drainage system is the
first defense against water penetration. But, "joints in metal
panels and trim create the opportunity for water intrusion," says
John Pierson, PE, engineering services manager, The Garland Co. Inc.,
Cleveland. "Since they are exposed on two sides, the corners of the
roof also see the most abuse from wind, rain and snow."
As a result, a watertight building needs flashing. Flashing
keeps water from getting into places it doesn't belong. Flashing is
installed at or around valleys, corners, hips, eaves, vent stacks,
mechanical equipment, solar systems and other locations. When
installed correctly, flashing makes joints at these junctions
watertight, while allowing the natural expansion and contraction of
the building and its devices. Flashing also seals against
penetrating unwanted air from the outside or heat loss from within
a building. The Uniform Building Code in Section 1707 (b) stresses
the importance of flashing for exterior openings.
Flashing comes in many different shapes and sizes and in a
number of different materials, such as aluminum, tin-coated and
galvanized steel, copper, plastic and rubberized asphalt. There are
different types of flashing depending on its required function.
"Standing seam, flat seam and corrugated metal panels do require
different flashing," Pierson says. "The differences are in the
flashing height and locations of the sealants." Flashing materials
must have the same or greater life span as the building material
itself. They must be shaped easily and have the capability to be
easily soldered, brazed or welded.
Valley flashing is used at sections where a part of the roof
meets another section at an ascending angle. The flashing is
installed to prevent water flow from collecting in the "valley"
between the two slopes. Step flashing is installed around chimney
bases. Vent pipe flashing is installed over pipes or flutes which
crawl along the roof. In almost all cases, the top of the flashing
attaches to the building under the underlayment, and the bottom
overlaps the roofing material, so water is directed away from the
underlying structure of the building. Intermediate pieces of
flashing overlap in the same manner.
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Two-part flashing systems consist of a base or step flashing
that is usually in contact or laced into the primary roofing
material, and these are then covered by a counter (or cap)
flashing. The counter flashing is a separate piece of metal or
building material that laps over the base or step flashing. It
should not be attached (mechanically or soldered) to the base or
step flashing. The two materials must be able to move independently
of one another so they can expand and contract independently under
changing weather conditions. The counter flashing can be made of a
wide variety of materials. It can be wood siding, stucco, vinyl or
aluminum siding, or other material that forms the outside surface
of a wall. In chimneys and along brick walls, the counter flashing
is almost always the same metal that is used for the base or step
flashing.
Successful flashing installation
Successful flashing installation is a very complex and technical
construction aspect that will determine whether or not your
building will leak. This type of work is best left to experienced
professionals, not DIYers. The best way to learn correct flashing
techniques is to spend time with an experienced craftsman as he or
she installs a complete roof. Inexperienced craftsmen too often
place flashing against walls or bricks, hammer it into position and
caulk to seal. This gets the job done quickly, but creates a
relatively short-term leakproof sealant. The reason for this is
economics. With countless roofing companies competing and aiming
for the lowest quote, quality details are often overlooked. The
faster flashing is installed, the cheaper it costs; but often, the
less successful the result.
The art of installing flashings is so technical it cannot even
be fully described in this article. Many books do not fully cover
the subject and it would take several books to get complete
coverage of all flashing applications. However, there is a standard
installation practice for installing roof corner edges.
Don't cut corners
For corner edges, the flashing must be bent and overlapped
securely to ensure a watertight corner. Measure from the last piece
of flashing installed to the corner of the roof. Add two inches to
this measurement for the lap. Mark the new piece of metal flashing
with a black marker for the cut line. Make two full cuts in the
metal flashing with a pair of metal snips. Start at the mark on the
top of the flashing. Cut down at a 30-degree angle from this mark.
Begin at the same mark and cut down at a 30-degree angle on the
opposite side to form a triangle. From there, cut a 1/8-inch slit
at the bottom of the metal flashing face directly below the
original mark at the peak of the triangle. Bend the metal flashing
around the corner using the slit as the bending point at the roof
corner. Hammer roofing nails every 6 inches along the top and
bottom of the metal flashing until you reach the end. Lap the next
piece of flashing over the corner piece 2 inches and continue.
Simple basic hand tools, such as hand tongs, pliers, needle-nose
pliers, metal tongs and snips, along with common sense, are
required for corners, says Frank Albert, owner and operator of Albert's Specialty
Roofing, Richmond, Va. "The roof material is turned up against
penetrations to form a seamless water proof system, and the
counterflashing is attached to the penetrations to keep water from
entering behind the roof panels. They are 'apart' in that they do
not get caulked together. The roof, the roof deck and walls all
move independently of each other."
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While flashing installation is very technical and its methods
are very diverse, there are some common simple-to-follow standards.
Look for loose nails and any damage to the seals at the edges of
the flashing. Roofing cement can dry out and crumble away, exposing
joints to water. Fasten loose nails and cover exposed nail heads
with roofing cement. Renew flashing seals by chipping out old
caulking and mortar along the edges of the flashing. Recaulk the
joints between the roof and the flashing. It's much easier to
reseal the flashing than it is to patch a water-stained wall or
ceiling. Flashings rarely need to be replaced unless they've been
damaged, such as by a falling tree limb or other impact. Typically,
they need only to be refastened and resealed. Fasten flashings
using wide-headed roofing nails, or roofing screws with a washer
underneath. Don't drive existing fasteners back into existing
holes, as they'll just come loose again.
For a simple quality inspection, safely follow a drop of rain
water as it moves down the side of the roof. It should be carried
from surface to surface all the way down, never encountering an
open seam or an upturned lip that blocks its progress. The success
of a good metal corner is usually dependent upon good details.
"This means collaboration between the designer, manufacturer and
installer," Pierson says. "Collaboration is the best way to ensure
a successful project."
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Applying sealant
A continuous bead of sealant is often needed with flashing to
prevent water intrusion. Sealant fills imperfections present with
metal-to-metal closures. "The adhesive materials, better described
as sealants, are the true leakproofing of the corner flashing,"
says John Pierson, PE, engineering services manager, The Garland
Co. Inc., Cleveland. "They need to be placed so that they are not
disturbed by the thermal movement of the roof, and they cannot be
exposed to sunlight. UV exposure will cause sealants to fail
prematurely."
Flashing sealants are available in caulking tubes, and come in
silicone, butyl rubber, thiokol and styrene butadiene. Indoor
sealants must never be used for outdoor applications. To apply,
insert the sealant into a standard caulking gun applicator. Cut the
end of the sealant at a 45-degree angle, using a utility knife.
Squeeze the gun trigger until it is tight against the tube of
flashing seal. Apply the flashing sealant to the flashing area.
Apply a bead of flashing seal, similar to applying silicone to a
tub. Let dry for several hours without getting the flashing wet.
Don't confuse flashing sealant with roofing compound, which should
not be used to stop leaks. It's not a long-term repair and can
cause more damage to the core building material. Roofing compound
becomes brittle and inflexible within one year exposure to the
elements.