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The Art of Detailing and Specifying

May 15, 2005

Several resources are available
for providing general guidance
specific to the preparation of
details and specifications for
roof flashing systems. Typical
resources include manufacturer
and industry entities. Manufacturers
usually provide generic guideline details
and specifications for installation of the
materials they produce. Common related
items, such as sheet metal, sealants, fasteners,
etc., are often not depicted or are
simply pictorially represented by these
details, since they are usually not considered
the responsibility of the manufacturer.
The National Roofing Contractors Association
(NRCA) publishes standard construction
details in its Roofing and
Waterproofing Manual1 for conditions that
would typically occur on a roof. The Sheet
Metal and Air Conditioning Contractors
National Association, Inc. (SMACNA) provides
recommended details in its Architectural
Sheet Metal Manual2 for a wide variety
of conditions with regard to sheet metal
components. Since sheet metal is the purpose
and responsibility of this entity, the
roofing membrane and flashing material related
issues are not presented by SMACNA
As valuable as these resources are, critical
aspects such as terminations or transitions
of details are typically not represented
in standard details. The information available
from these sources typically consists of
either cross-sectional or isometric views of
standard conditions. However, quite often,
the atypical condition (i.e., termination of
base flashing or end of parapet wall) can
create obstacles and difficulties for the
installer, resulting in a substandard detail
and possible water infiltration. In addition,
the flashing conditions presented in a published
detail may be minimum standards
that are suitable for the materials promoted
but may neither be suitable for the existing
field conditions nor meet the requirements
of the building owner or the intended use or
serviceability of the roof.
Therefore, it is the responsibility of the
designer to detail these conditions in order
to achieve the desired finished product. If
these specific elements are omitted, the
constructability is left to interpretation by
the field mechanic, which could jeopardize
the integrity of the installation through a
lack of proper attention at a particular location.
The intent of this article is to provide
general insight into the technical content
that can be contained in a standard detail,
the potential oversight of common conditions,
and helpful hints to enhance the specific
The subject detail is a typical flashing
condition at a load-bearing wall. The wall
construction could be either a parapet wall
or a building rise wall that is constructed of
either concrete or masonry and extends vertically
a minimum of four feet above the
deck. The roof system selected is a multiply,
built-up roof with a granule-surfaced
modified bitumen cap sheet and similar
base flashing. The type of deck and presence
of insulation are not relevant to this
discussion. A two-ply modified bitumen system
has similar characteristics to the builtup
roof discussed herein.
Masonry walls can be a challenging substrate
on which to install flashing materials.
Masonry located on the inboard faces of
parapet walls is often not constructed as
true and clean as exposed face brick on the
exterior of the wall. Irregular surfaces, fractured
partial bricks, bug holes, and
untooled mortar joints are common conditions
on the inside of masonry parapet
walls. Plywood or gypsum sheathing can be
anchored to the wall to provide a smooth,
sound, and monolithic substrate.
If plywood is installed, it is recommended
that the top edge be chamfered, sloped
upwardly and back to the wall on a 30- to
A P R I L 2005 I N T E R FA C E • 5
45-degree angle. This honed edge will provide a smooth transition
to extend flashing material or three coursing, applied
along the leading edge of the base flashing up the interior of
the masonry wall. A mechanically-attached base sheet is typically
anchored to the plywood prior to application of other bituminous
With the improvement of gypsum sheathing, this type of
product provides a sound, suitable substrate to receive the
various types of flashing materials now available. Direct application
by the various installation methods (i.e., torch, adhesives,
etc.) is possible while providing a substrate that is inorganic
and not susceptible to the effects of moisture, fire, and
insects. A uniform coating of plastic roof cement can be
applied to the masonry wall or sheathing prior to installation
against the wall to serve as an additional layer of waterproofing.
The initial step of constructing the flashing system involves
installation of the cant strip at the deck and wall juncture.
The cant strip could be a fibrous type, treated wood, or
a quick-set, cementitious material. Fibrous cants are the most
economical material. However, depending on the substrate
(e.g., concrete deck), a cementitious cant material can be
installed and remain in place for future roof replacement
Whether the substrate is rigid insulation board or base
sheet, a fibrous cant should be set in a uniform troweling of
plastic roof cement, a cold-applied adhesive, or mopping of hot
asphalt. If a wood nailer or wooden substrate is present, a
wood cant may be nailed to the substrate. Cants should be
installed in a manner similar to requirements for rigid board
insulation. Adjacent pieces of fibrous or wood cants should be
installed with joints butted snugly to provide a continuous
smooth substrate (Photo 1). Cants installed at inside and outside
corners of walls should be constructed with mitered corners
at the ends of the mating cant pieces. Forming smooth
mitered corners is difficult to achieve with wood due to the
rough, sharp edges formed by saw-cutting and the rigidity and
trueness of both the wood and the underlying substrate (Photo
2). Fibrous cants can be “shaved” and cementitious cants can
be manually shaped to form smooth transitions (Photo 3).
After installation of the cant, felt plies or the smoothsurfaced
modified bitumen base ply sheet should be installed
continuously from the field of the roof to the top edge of the
cant and terminated. In BUR application, if the wall is parallel
to the long dimension of the field plies, starter plies should
be installed along the cant in order to achieve proper ply count
over the cant. If the masonry wall in the area to receive roofing
is deemed to be a suitable substrate, primer should be
applied and allowed to dry prior to application of the flashing
Common details published by material manufacturers
depict the ply sheets extending up the wall a minimum of two
inches above the top of the cant. With fiberglass felt plies, particularly
Type VI, the vertical extension is difficult to properly
install. Due to the relative rigidity and “memory characteris-
Photograph 3: Cementitious cant at circular penetration. tics” of glass felts, conforming to the two transitions (horizon-
Photograph 1: Misaligned cut sections of fibrous cant along curved
Photograph 2: Wood cant at corner miter.
6 • IN T E R FA C E A P R I L 2005
tal-to-cant; cant-to-vertical) is difficult to
achieve in the relatively short dimension.
Bridging of felts or unadhered felts at one or
both of these transitions will most likely
occur. The author has observed roofing contractors,
during the installation of built-up
roofing felts, terminating the field plies
above the cant and leaving the felts in that
condition to serve as a “night seal.” This
condition rarely performs satisfactorily,
even if coated with plastic roof cement,
because the felts often become disbonded
from the vertical substrate, creating an
opening for water entry (Photo 4). Therefore,
felt plies of the field membrane should be
terminated at the top of the cant and properly
pressed into place to conform to the
Some modified bitumen manufacturers
recommend terminating the base ply of the
field membrane at the top of the cant. After
installation of the field membrane plies, the
author recommends installing two plies of
felts (Type IV preferred as they tend to be
more flexible) or backer sheets. The base
flashing felts should be cut into 6-foot
lengths and 12 to 18 inches width or the
dimension required for proper flashing
height and site conditions. One ply of
smooth-surfaced modified bitumen could
Photograph 4:
Disbonded felt
Photograph 5: Degranulating cap sheet to receive base flashing.
A P R I L 2005 I N T E R FA C E • 7
also be utilized in lieu of the felt plies. The
flashing plies should be installed over the
substrate, extending horizontally four inches
beyond the toe of the cant and vertically
four inches (minimum) above the top of the
cant. If the application
of the cap sheet
and the modified
base flashings are
delayed (which is
more common than
not), the backer plies
can be temporarily
sealed and made
watertight with a
troweling of plastic
roof cement applied
over the top edge.
Upon completion of
the backer plies, the
cap sheet is then
installed up to the
top edge of the cant
and terminated.
The granulesurfaced
base flashing
is then extended
horizontally four inches
beyond the toe
of the cant and vertically
above the top of
the cant a minimum
of four inches. Granule surfacing of the cap
sheet to receive the base flashing should
either be primed or degranulated to provide
a bituminous substrate (asphalt-to-asphalt
bond) to adhere the material (Photo 5).
Modified bitumen finish plies should be cut
into lengths of six feet (maximum). Flashing
sheets in lengths greater than six feet are
difficult for mechanics to handle and properly
install, particularly using hot asphalt
or torching techniques.
Since modified bitumen products that
are to be applied with asphalt require that
the asphalt be of sufficient temperature in
order to melt the coating on the back of the
sheet, use of shorter lengths will help to
achieve this. Although more vertical seams
are created when using shorter lengths of
flashing sheets, the author feels that a better
installation will result. The flashing
sheet can be created by cutting sections
from the full-width material roll, resulting
in sheet lengths of three feet with a selvage
edge. If a selvage edge is not present on the
flashing sheet, granules should be primed
or degranulated in the overlapped (vertical
sidelap) portion.
Care should be taken to install the modified
bitumen flashing sheet so that it conforms
to the contour of the roof-to-cant-towall
profile. With low profile flashing
heights (i.e., less than 24 inches), the flashing
material should be installed on the roof
surface initially and proceed upward. For
taller flashing heights, the flashing sheet
(commonly torch or cold-adhesive application)
is typically installed from the top edge
and proceeds downward to the roof surface.
The modified bitumen sheet should be
installed without any pulling or stretching
(tension). The sheet should be allowed to fall
into place and then be hand pressed into
After adjoining sheets are installed, the
author recommends that the vertical seams
of the newly installed base flashing be treated
with three coursing (plastic cement, reinforcing
fabric, and plastic cement – Photo 6).
A technique that can be used to provide a
neat appearance of three coursing is
installing strips of an adhesive tape (i.e.,
masking or duct tape) parallel and on each
side of the lap seam
(approximately 6 inches
apart). After the
application of the
three coursing, the
tape is removed to
provide straight vertical
lines for the edges
of the treatment. After
this treatment, the
three coursing can be
finished by embedding
granules in the
wet cement or by
applying aluminized
coating after curing of
the plastic cement
(Photo 7). The three
coursing is not intended to substitute for or
conceal a poorly constructed lap, but rather
to provide an additional layer of protection
to the lap (exposed edge of the modified
bitumen sheet). Several manufacturers do
not specifically suggest the application of
plastic cement over modified bitumen
sheets — particularly APP modified products.
However, due to the advent of lower
solvent-bearing products and because the
plastic cement is topically applied, this concern
is not as critical.
After the flashing has been adhered to
the substrate and the lap seams treated,
8 • IN T E R FA C E A P R I L 2005
Photograph 7: Application of aluminized coating to three-coursing.
Photograph 6:
Application of
three-coursing to
vertical lap seams
of base flashing.
the top edge of the base flashing should
then be mechanically fastened. Individually
placed nails (spaced six to eight inches on
center) are often depicted in published
details (Photo 8). Although this approach
serves well to secure the material to the
substrate, the long-term seal of the material
located between the attachment points
can be jeopardized if it becomes disbonded.
Therefore, a continuous metal termination
or compression bar is recommended for
securing the flashing (Photo 9). The bar
should be a minimum 1 inch wide and 1/4
inch thick, with a flat profile and should
have slotted, pre-punched holes at the
desired spacing (the author recommends
six inches). The slotted or oval holes provide
room for differential movement of the bar
during changes in temperature. Termination
bars can “bow” outward between the
points of attachment when secured at
greater than six inches on center. The fasteners
used to secure the termination bar
into a masonry or concrete substrate
should be suitable for the substrate. The
author recommends a type of fastener that
can be removed via screwdrill for ease in
future renovation activities. Although drive
pins function well, removal of these fasteners
requires shearing of the head and either
drilling out the fastener or auguring another
hole. When installing the termination bar
over a masonry substrate, the author recommends
positioning the anchors into a
mortar joint as individual bricks may fracture
during the drilling or anchoring
After installation of the termination bar,
a treatment of three coursing is recommended
over the termination bar and top
leading edge of the base flashing (Photo 10).
Many in the industry accept base flashings
that are fully adhered to a wall and periodically
mechanically attached (even with a
termination bar). Over time, the base flashing
material can become disbonded from
the substrate between the fasteners, providing
avenues for possible moisture infiltration.
Continuous securement and three
coursing will prevent this from occurring
and embellish the long-term integrity of the
counterflashing element. The design of this
base flashing creates a watertight detail,
even if the counterflashing were to be purposely
or accidentally removed, dislodged,
or was missing. The NRCA detail depicts
three coursing as an optional item, and
material manufacturers vary in the requirement
for three coursing treatment of the
base flashing leading edge.
After the base flashing is completed, the
sheet metal counterflashing should be
installed. A two-piece counterflashing, consisting
of a receiver and counterflashing, is
recommended. This type of assembly allows
for removal of the counterflashing segment
without affecting the anchorage to the wall.
Removal of the counterflashing segment will
allow personnel to perform specific roof-related
work and re-use components in the
future. A receiver can be installed into a sawcut
reglet, secured in place, sealed, and then
the counterflashing is secured to the receiver.
A saw-cut reglet is preferred over a surface-
mounted type (often depicted in published
details) for several reasons. A surface-
mounted counterflashing has a caulk
trough on the top edge to receive sealant at
the wall interface. This detail relies heavily
on the performance of the sealant to maintain
the waterproof integrity of the counterflashing.
The trough of a surface-mounted
counterflashing is constantly exposed to
water running down the face of the wall.
Proper sealant application is extremely difficult
to achieve on a masonry substrate
due to the irregular surface (i.e., brick and
A P R I L 2005 I N T E R FA C E • 9
mortar joints). Proper bonding of the
sealant to the masonry and concrete substrate
is also difficult due to other soiled or
contaminated surfaces (i.e., bituminous
A caulk trough results in a fillet bead
profile, which is a less than optimum
sealant configuration. With a saw-cut
reglet, the sealant is installed in proper configuration
(between parallel planes), and the
majority of it is recessed behind the face of
the wall. The tooled face of the sealant is
exposed, providing little resistance to water
running down the face of the
wall. A saw-cut reglet also
requires proper techniques to
achieve the desired performance.
The reglet should be
of proper depth (minimum 1
inch) into the wall and proper
width (minimum 1/2-
inch). The noted depth allows
for adequate embedment of
the metal flange of the receiver,
and allows for the installation
of lead wedges, backer
rod, and sealant. The noted
width allows for the creation
of a proper sealant joint
The reglet should also be properly
cleaned to remove all debris from the sawcutting
process. The cleaning process
should use forced air, such as that created
by a commercially available “leaf blower.”
Compressed air is not recommended, as
moisture can be created by this process and
introduced into the joint. The flange of the
receiver should be fabricated to provide a
downward-sloping facing at the point of
protrusion from the wall. This profile provides
the least resistance for water cascading
down the wall.
The actual receiver should also be of
sufficient length (2 inches minimum) to
allow proper attachment of the counterflashing.
The counterflashing is then inserted
and attached to the receiver. This can be
accomplished with either pop rivets or selftapping
screws. Self-tapping screws are
preferred, as the screw can be removed and
reinstalled to disengage the counterflashing
in order to perform maintenance and
repairs to the base flashing or roof in the
Adjacent sections of counterflashing
should be lapped a minimum of four inches
with a bead of sealant sandwiched between
the mating surfaces. The hemmed portion
(backside) of the drip edge on the underlying
section of the counterflashing is typically
notched or removed on the lapped area so
that the two adjoining pieces can nest. The
lapped section can be secured, preferably
with a grommeted fastener installed
through an enlarged or oval hole to allow for
If a surface-mounted counterflashing is
the preferred or the only viable option, then
the following detailing is recommended:
• Apply a pre-formed sealant tape
between the backside of the vertical
flange of the counterflashing and
concrete wall;
• Utilize a continuous, heavy-gauge
bar secured six inches on-center, to
provide uniform attachment;
• Utilize appropriate fasteners with
steel washers backed with rubber
If the counterflashing is designed, fabricated,
and installed appropriately, the
metal should hug the base flashing and
eliminate the need for wind clips. The
receiver should have a length of two inches
and extend out of the reglet and be bent
downward on a 45-degree angle with the
vertical wall. The return should be a minimum
of one inch in length to engage the
counterflashing. The counterflashing
should have a receiver leg
approximately two inches in
length (one inch engaged in
receiver; one inch extending
out of receiver). The angle
between the receiver leg and
fascia should be approximately
90 degrees. The
length of fascia of the counterflashing
should be approximately
three inches. At these
dimensions, the counterflashing
should provide
approximately 4-1/2 inches
of coverage over the top edge
of the base flashing, and the
hemmed drip edge should
almost be touching the finished
surface of the base
flashing. The drip edge
should be a hemmed return,
approximately 1/4 inch in length, and bent
on a 45-degree angle.
If wind clips are desired, the author recommends
utilizing 1-inch wide strips of the
same type of metal as the counterflashing.
It is recommended that the wind clip be
secured to the termination bar (after three
coursing is applied) or height that the top
edge of base flashing is secured. Typical
10 • I N T E R FA C E A P R I L 2005
Photograph 8:
Base flashing
nailed along
top edge.
Photograph 9:
Installation of
termination bar.
SMACNA details depict wind clips
that are installed into the base flashing
material just above the bottom of
the counterflashing (Figure 11).
However, the previous installation
scenario has exhibited similar capabilities
of restraining the counterflashing
while the point of securement
is located above the line of possible
windblown water and snow,
and it does not penetrate the base
flashing material.
When the wall terminates, particularly
at the edge of the roof, a
unique flashing detail occurs to
transition the wall to the metal edge.
Sealed terminations should be
installed on each flashing component
(i.e., counterflashing, base
flashing, metal edge). When the base
flashing is terminated at the edge of
the wall, a termination bar should
also be positioned vertically along
the edge of the material, extending
from the top of the cant to the horizontally
positioned bar across the
top edge of the base flashing. Three coursing
can then be applied over the corner,
extending three to four inches each side of
the corner for the full height of the flashing
material, overlapping the three coursing
applied at the top leading edge.
For the metal edge, a specialty piece
should be fabricated that extends up the
Photograph 10: Application of three-coursing over termination bar.
A P R I L 2005 I N T E R FA C E • 1 1
cant and over the flashing and provides an
end cap for the fascia. The flange of this fabrication
should be anchored and then
stripped into the flashing. (See Figure 12.)
An outwardly deformed, hemmed kick-out
should be formed on the outer edge of the
metal cap to provide a trough to receive
sealant. This fabrication can be made to extend
up and under the counterflashing, or a
secondary piece of metal flashing could be
installed on top of (in shingle fashion) the
top edge of this unit and extend under the
The counterflashing should be wrapped
around the end of the wall extending the
same distance as the base flashing and
metal edge. The counterflashing segment to
be installed on the receiver should extend
approximately 1/2-inch to 3/4-inch beyond
the end of the receiver. This extended portion
can then be cut along the break lines
and hand-tonged downward to form an end
closure for the counterflashing.
There are many different field conditions,
materials, design intents, installation
nuances, and specific experiences that
influence why details are constructed as
they are. The purpose of this text is to show
ideas and reasons for designing and constructing
long-term, serviceable details and
to present multiple areas than can be
affected if not appropriately identified. It is
the opinion of the author that the information
and resources available to the industry
are guidelines for minimum standards. It is
the duty of the specifying community to present
the proper information so that the
product can be properly installed. Proper
detailing will result in competitive and
defined bidding, minimize multiple submittal
processes, eliminate ambiguity between
project personnel, and provide the owner
with a roof system suitable for the intended
purpose of the design.
1 Roofing and Waterproofing Manual,
Fifth Edition, NRCA.
2 Architectural Sheet Metal Manual, Fifth
Edition, SMACNA.
3 Schaack, Karl, “The Use of Sealants in
Roofing,” Interface, May 2001.
12 • I N T E R FA C E A P R I L 2005
Karl A. Schaack, RRC, PE
Figure 11: Courterflashing; SMACNA Figure
Figure 12: Metal Edge/Risewall Termination
Karl A. Schaack is president of Price Consulting, Inc., a roofing
and waterproofing consulting firm located in Houston,
Texas. Mr. Schaack has a B.S. in Civil Engineering from
Clemson University. He is a registered professional engineer
in Texas, South Carolina, and North Carolina. Mr. Schaack is
an RCI Registered Roof Consultant. He has been employed in
the consulting industry since 1983 with emphasis on building
envelope technology by providing condition assessments,
remediation and new design, material testing, and construction
monitoring and administration. He is a member of RCI,
the Roofing Contractors Association of Texas, and the Gulf Coast Chapter of RCI.