Solving Challenging Flashing Applications
How to apply flashing on challenging metal roof
The majority of roof leaks
occur at flashing points. To prevent this water penetration,
flashing forms the intersections and terminations of roofing
systems and surfaces. Most professional roofers can easily install
a flashing membrane on the flat, open space of a metal roof and
then secure it. But roof lines and profiles are getting more
creative and complex. Installers are finding more challenging
flashing applications that require more technique and detailed
procedures to complete successfully.
In a perfect world, a standing seam metal roof would have zero
penetrations from the roof eve to the roof ridge. But these long
metal field runs used in lateral series are rarely unbroken. On
metal roofs, square ducts, skylights, large round pipes, expansion
joints, parapet walls, flashing dissimilar roofing materials and
profiles all pose challenging flashing applications, says Jim
Sharp, president of Deks
North America Inc., Chicago.
Jeff Haddock, technical representative at AEP Span, Fontana, Calif., believes dead
valleys, dormer valleys, slope transitions, field-fabricated roof
curbs, odd-shaped or oversized roof penetrations, large sheet metal
crickets, and ridge/ hip intersections are among the most difficult
conditions to properly flash for weathertightness. "Preventing
these conditions is extremely difficult as most buildings and
building conditions are designed with aesthetic features which must
utilize these conditions," he adds. "Designers do not typically
take application or installation of difficult conditions into
consideration when designing roof layouts." What follows is
information on challenging flashing applications.
All pipe and vent flashings that pass through metal roofs are
challenging. "The 'common art' employed by metal roofing companies,
installers, manufacturers and roofing specifiers for sealing pipe,
vent and penetrations that exit through the surface of metal roofs
are not even remotely acceptable to the National Roofing
Contractors Association (NRCA) or any other legitimate roof
specifying organization for other types of low-slope or steep-slope
applications," says Lawrence P. Evensen, All Style
Industries, Henderson, Nev. "Surface-mounted, face-fastened and
caulk-sealed flashings with up-slope edges are by definition, on
any roofing installation, extremely poor trade craft." Evensen
recommends cutting the pan at the penetration location, installing
a flashing and then continuing with the pan to the ridge using an
overlapping shingle fashion installation for pipe flashing
Challenging Thermal Movement
One of the most challenging flashing applications on metal roofs
involves thermal expansion and contraction. This is especially so
when tying a valley detail into the ridge detail as movement at
these junctures can be severe.
John L. Pierson, PE,
engineering services manager at The Garland Co. Inc., Cleveland, cites the
below real-world example of how he solved an expansion problem for
a roof owner who had been plagued with leaks for decades. It
involved replacing a standing seam metal roof system with a large
rooftop unit sitting across an expansion joint. The roof was
flashed successfully using Pierson's following advice:
1. Determine how the expansion joint detail can move at this
location. This is important for any metal flashing detail since
metal panels and the flashing materials will expand and contract
with temperature change.
2. Determine how water will drain across the flashing. Change of
slope and change of plane in a flashing detail can force water,
snow or ice in different directions.
3. Eliminate through fasteners.
4. Cover sealants, expandable flashing materials and
underlayment membranes with the metal flashing material. When these
components are exposed to the sun's ultraviolet rays, they will
fail before the metal roof.
5. Figure out the detail before the job is bid. Provide a
project-specific detail to all bidders to avoid costly change
orders. The detail should be provided by the manufacturer to ensure
compliance with warranty terms.
6. Provide inspection of the detail during installation to
ensure compliance and assist the installer as needed.
When Todd Miller, president of Isaiah
Industries Inc., Piqua, Ohio, replaced an asphalt shingle roof
with a metal shingle system on a very large building with a unique
construction, he faced a similar situation. "They had included
expansion joints in the roof which, with the asphalt shingles, were
being handled with a thick rubber material," Miller says. "When
changing to a metal system, we did not want to trust the rubber
material to last as long as the metal shingles. So, we created
accordion-style expansion pleats out of 0.032-inchthick aluminum
and then flashed the metal shingles against those similar to an
open style of valley. We had the opportunity to re-visit this
project a few years later when the building was added on to, and
the expansion joint was performing very well."
Dormer flashings on the high side of the dormer, into the panel
that transitions, require valley detailing at the correct width,
flat panning at the high side of the dormer and tying the
transition back into the downward slope. "Often diverters are
installed or hand fabricated with literally no attention or the
proper rise back up-slope on the dormer," says Mike Gwizdala,
director of product management at Duro-Last Inc., Saginaw, Mich. "The cricket is
not welded, which can allow moisture migration, dirt or debris
build up. Contractors will seek the lowest cost curb, fabricate a
diversion, all of which is improperly flashed and fabricated to
meet known Sheet Metal & Air Conditioning Contractors' National
Association (SMACNA) standards for water diversion or snow build
Flashing curbs is requisite for most roof assemblies, but can be
problematic. According to Gwizdala, the challenge occurs when
trying to provide the proper clearance on either side of the curb,
creating a cricket to divert snow and water (and still high enough
on the up-slope side of the curb), followed by the correct spacing
on the low side of the slope along with the setting of the curb,
into the seam of the newly installed standing seam roof. "Curb
flashings and size of the curb are very critical, and often
overlooked due to cost controls that fail over and over again," he
says. Nathan Plaxco, sales and marketing manager at Design
Components Inc., Fayetteville, Ga., cautions that, "Many times,
installers are not installing fasteners on the actual high rib [of
the curb] and they are relying on the gasket and caulk to keep
Rounded surfaces on metal roofs offer challenges when flashing
around them. And, "Obtaining arched or tapered standing seam panels
is not real easy in some cases, and we often find that metal
shingles offer greater flexibility for round or arched surfaces
than do vertical seam panels," Miller says. "However, in all of
these cases, it is critical to ensure that the minimum roof pitch
required by the roof system being installed is never violated. In
cases like that, going to a soldered copper system for the
low-pitch area may be wise."
When flashing around large round objects, Miller suggests making
sure the underlayment system is well sealed to the penetration.
"Then, the metal panels can be cut, flashed and sealed as best as
possible," he says. "If the watertightness of the system is
dependent upon sealant or a neoprene boot of some sort, covering
that sealant or boot with matching metal in order to protect it
from the sun's rays will extend its life."
Exact formation and exact positioning of the flashing in
challenging applications is critical. Creating a flashing with
improperly sized sections can lead to failure. "Ensure the roof
flashing has the proper pleat design at the base; this will ensure
the roof flashing provides a watertight seal when installed on
various roof pitches and accounts for any roof movement," Sharp
says. "A roof flashing with aluminum on the base edges will ensure
the product can be formed to the metal roof profile."
Additionally, "Many of these situations [improperly sized
sections] can result in flashings with negative pitch if they are
not properly dimensioned and positioned on the roof," Miller says.
"Therefore, good bending equipment is essential. If good portable
bending equipment is not available, then it is best to have the
flashings made in a sheet metal shop rather than try to make do in
the field." Haddock advises using small hand folders, material
benders, hemming tools and shears for successful field fabrication
of custom trim and challenging flashing work.
|For a 80,000 square foot office and manufacturing
facility'addition in Sharonville, Ohio. a custom flashing
detail was used where a taller new building abutted
the existing PEMB metal roof system. (Photo courtesy
of Cincinnati Commercial Contracting)
To alleviate challenging flashing applications, Haddock agrees
the layout of the panel system and roof geometry must be well
thought out prior to any application. "Panel coverage, rib
placement and the size of the condition should all be taken into
consideration when dealing with difficult flashing applications,"
he adds. "Balancing panel layout around penetrations and rooftop
equipment is essential to ensure the integrated flashing will tie
in properly. Field fabrication capability also has a huge benefit
for the installer as it allows the installer to fabricate flashing
on-site to meet as-built conditions rather than manufacturing
flashings off of the plans by a manufacturer or sheet metal
Installers should always work with a material supplier that has
an understanding of how to install challenging flashing. "Get the
roof system manufacturer involved whenever possible," Miller says.
"While manufacturers may not have the field experience that a
seasoned contractor has, they do know their products well and they
have seen a wide variety of installations in all situations.
Getting their input and approval in advance can be very helpful."
Gwizdala says manufacturers should provide engineered designs,
dimensions and approved details for their warranty to perform.
Also, most roof plans do not match as-built conditions or show
how to correctly install the conditions that have purposely been
designed into the project. "The manufacturer is relied on heavily
to provide supplemental details for handling tricky or difficult
flashing applications," Haddock says. "Ensuring that you are using
a qualified manufacturer with consistent details and a technical
staff that understands how to install difficult conditions will
greatly aid in the final installation." Plaxco agrees that
installation instructions are key to solving challenging flashing
applications, if installers will, "Take the time to read them," he
Sidebar: Chimney and Skylights
All Metal Roofing Specialists feels roofing penetrations such as
chimneys and skylights present some of the most challenging
flashings on a metal roof. A stone-and-mortar chimney has an uneven
surface, and it is difficult to cut into. You want the counter
flashing to look good, be watertight and not take away from the
appearance of the stone. We like to take a diamond cutting wheel
with guides, cut into the stone keeping the line as straight as
possible, fill our cut with a bead of butyl caulk and then take a
preformed flashing with a 1/2-inch bend and form the flashing into
the groove. We then install a mortar anchor into the joint of the
concrete holding the stone. This ensures that our step flashing
looks neat and clean.
Chimney placement also makes a big difference in how and where
we flash. If a chimney is below the ridge line there is water that
runs down the metal panels against the back side of the chimney.
Some guys like to build a cricket. We choose instead to counter
flash up the back of the chimney, then we will leave the panels
about 1 foot from the back of the chimney making sure that we lay
the panels out so that the ribs fall to the outside of the chimney.
Water and snow will naturally flow around the chimney.
Skylights also present some interesting installation issues.
First and foremost, we lay our roofing panels out so that they look
symmetrical and so that the panels fall the best way possible
behind the skylights. Our crews like to take the panels and bend
them up alongside the skylight. We then will counter flash under
the skylight cover and then over the panel. We do this on two sides
and the bottom. At the top of the skylight we leave the panels back
approximately 1 foot, taper the panels, and install a flashing up
under the skylight cover and then under the standing seam panels,
which are hooked to a metal z.
Neil Drebushenko, owner, All Metal
Roofing Specialists, Womelsdorf, Pa.