Guidelines for Achieving Long-performing Seams for Granule-surfaced Roofing Materials

Guidelines for Achieving
Long-performing Seams for
Granule-surfaced Roofing Materials

By Karl A. Schaack, P.E., RRC
A common protective surfacing or finishing ply
that is utilized throughout the industry in the
application of bituminous roof systems is a gran¬
ulated cap sheet. These sheets, whether in sin¬
gle- or multi-ply assemblies, include modified bitumen
products (either SBS or APP polymer) and asphalt-coat¬
ed fiberglass reinforced roll roofing (cap sheets).
According to the NRCA, modified bitumens now
account for approximately 20 to 25% of the commercial
roofing market. Not all modified bitumen systems incor¬
porate a granulated cap sheet. However, “hybrid” builtup
roof systems, which incorporate a cap sheet and mul¬
tiple layers of built-up roofing plies, are grouped in with
the built-up category. Therefore, roof systems utilizing a
granulated cap sheet more than likely comprise one¬
fourth of the commercial roofs installed today.
A critical step in the application of granule-surfaced
sheet materials is the proper adhesion of lap seams that
occur at both the sides and ends of sheets. According to
NRCA’s “Project Pinpoint Analysis, 1983-1992,” lap
seam deficiencies were considered to be the number
one problem associated with modified bitumen mem¬
branes, accounting for approximately 43% of the prob¬
lems associated with them. Modified bitumen roof sys¬
tems made up approximately 14% of the problem roofs
recorded in the Project Pinpoint survey.
There are several basic techniques and general appli¬
cation methods typically used to achieve long-term per¬
formance of lap construction on modified bitumen sheet
products. The type of product or process used to adhere
the cap sheet determines the technique/procedures.
When using traditional mopping grade asphalt as the
“adhesive,” the highest optimum temperature should be
maintained for the asphalt at the point of application
flash point, etc. This technique will allow the hot
asphalt to “melt” the coating on the backside of the
sheet at the time of the application. Both ASTM D 312,
Type III and Type IV asphalt are recommended by the
membrane manufacturers for the application of cap
sheets. A minimum temperature of 400 degrees
Fahrenheit is recommended by the membrane manufac¬
turers to be maintained for the asphalt at the point of
application of cap sheets. By using Type IV grade of
asphalt for the application of granulated sheet materials,
the highest optimum temperature can be obtained at
the point of application. The EVT (Equiviscous Temp¬
erature) of Type IV asphalt varies depending on the
manufacturer, but is approximately 450 degrees
Fahrenheit, versus 430 degrees for Type III. Some
manufacturers recommend that only Type IV can be
used to install the cap sheet. This is a two-fold reason¬
ing with the first being the higher EVT and the second
being the higher softening point. These two characteris¬
tics aid in achieving optimum adhesion and avoiding
slippage.
There are several practices that can be followed to
assist in maintaining optimum temperatures during the
application of roofing materials. These include: 1) set¬
ting the kettle/tanker in close proximity to the point of
application, 2) using insulated piping and luggers, 3)
minimizing holding time of asphalt in luggers/mop carts
and 4) keeping the roll in close proximity to the applied
asphalt.
When using cold process materials as the adhesive,
the material is dispersed onto the substrate and the
sheet is applied onto the adhesive. The cold process
adhesive can be dispersed onto the substrate by pouring
the material from pails and distributing across the sub-
Installer heating granule surfacing at end lap of bottom sheet
prior to applying overlying sheet.
24 • Interface April 1997
Fusing the side laps of a torch-applied modified bitumen cap
sheet utilizing hand-held torching equipment, rolling the heated
seam and embedding granules into the bleed-out.
ject area utilizing either a squeegee, roller or notched
trowel. The material can also be dispersed utilizing
spray-type equipment. After the adhesive is appropriate¬
ly applied, the sheet is laid onto the adhesive and then
rolled. A large weighted roller is typically used in the
field of the sheet and a hand-held roller is used at the
lap seams. A roller, hand-held or with handle, can be
used during the application of cap sheets to assist in the
bonding of side laps. Care should be taken when using a
roller during the construction of seams. The roller can
become “contaminated” with bitumen and granules,
creating unsightly markings or causing the roller to
“jump up” as the accumulations are rolled over. In addi¬
tion, if the sheet or roller becomes too hot, displacement
or damage to the protective granule surfacing can occur.
When the sheet is adhered by either torching or
“heat-welding” methods, two basic techniques are uti¬
lized. One method involves heating the entire width of
the sheet with either a hand-held torch or wagon¬
mounted type equipment. Upon heating/melting the
coating, the sheet is adhered to the substrate. The sec¬
ond method involves heating the width of the sheet
except for the last three to four inches or width of the
selvage edge. The remaining edge can then be
addressed utilizing a smaller or “detail” torch. Torchapplied
sheet goods typically are supplied with a poly¬
ethylene film. During application, proper torching tech-
Installer applying downward pressure on cap sheet roll while
maintaining roll in close proximity to asphalt mopping source.
niques will result in melting or burn-off of the polyeth¬
ylene film. If the film is not burned off, it will act as a
bond breaker and prevent proper adhesion of the sheet.
Other techniques involve actual methods that can be
implemented during the application of the sheet goods.
One such method involves the “mop and flop” tech¬
nique. This technique involves applying the hot asphalt
directly to both the backside of cut sections of the sheet
that are positioned granule-side down prior to applica¬
tion, and adjacent to the subject area to receive the
product. After the asphalt is applied to the sheet it is
then “flopped” into place. The basic principle of this
technique is that the hot asphalt is applied directly to
the bituminous coating on the sheet, providing more
opportunity for the coating to be “melted.” The practice
of maintaining the highest optimum temperature for the
asphalt should also be used in conjunction with the
“mop and flop” technique.
Another common recommended application method
is to maintain a constant downward pressure on the roll
while the roll is being laid into the asphalt mopping,
adhesive and/or molten coating. This can be accom¬
plished with a properly-positioned foot or hand on top
of the roll. During the initial installation, the actual
weight of the roll provides relatively good downward
pressure. However, as the roll is unrolled and the weight
diminishes, manually applied downward pressure
becomes more critical.
In addition, the roll should be maintained in a rela¬
tively tight configuration during the application to aid in
achieving the proper downward pressure and subse¬
quent adhesion. “Kicking out” a roll, such as that com¬
monly used with felt products, should be avoided during
the application of cap sheet products. A cap sheet mate¬
rial will in most instances sit on top of the mopping if
the roll is not wound relatively tight and downward
pressure is not exerted on the roll. This will result in
inadequate adhesion (voids). Cap sheet products are
often unrolled, sometimes cut to specific lengths, and
laid out to “relax.” This practice is used to assist in
eliminating “tension” that may have been introduced
Granules embedded in asphalt
bleed-out and rounded corner
of end lap.
into the sheet as it was
wound and bound at the
manufacturing facility.
Laying the cap sheet gran¬
ule-side down on the sub¬
strate promotes a quicker
or shorter relax time. The
black underside of the
sheet more readily absorbs
heat than the reflective
top side, consequently
allowing the sheet to
relax. Prior to installation,
the sheet should be re¬
rolled using the cardboard
insert that was originally
April 1997 Interface • 25
with the roll. Using the insert aids in maintaining a
more round profile and somewhat “tight” configuration
throughout the roll, primarily towards the end of the
roll.
Gap sheets, like most roofing products, are installed
in a shingle fashion with the sheets extending over the
preceding adjacent sheet. This overlap (side lap) is typi¬
cally a minimum of 3 or 4 inches or the width of the
selvege edge (non-granulated edge). At the end of a roll,
the cap sheet installation is continued by overlapping an
end of a new roll on top of the end of the previously
installed sheet. This overlap (end lap) is typically a min¬
imum of 6 inches. Since end laps are constructed by
overlying the cap sheet onto the granule surfacing of the
underlying sheets, attention to detail and a “light”
application of primer on the subject area can aid in posi¬
tive adhesion of end laps. With torch applied products,
the subject area of the underlying sheet can be heated
and the granules “pressed down” into the sheet with a
trowel. This technique creates a bituminous surface on
which to apply the overlapping sheet.
Care and
attention should
be directed at
end lap construc¬
tion of modified
bitumen sheets
that utilize poly¬
ester reinforce¬
ment. These
types of sheets—
predominantly
those adhered in
hot asphalt—
have exhibited
some instability
after installation.
This instability is
typically charac¬
terized by shrink¬
age. Shrinkage,
whether induced
during the manu¬
facturing or application process, currently occurs in the
longitudinal direction of the sheet and shows up at the
end lap with approximately 1/4 to 1-1/2 inch reduction
in the length of the installed sheet. Since the ends of
the sheets are overlapped approximately 6 inches, this
shrinkage is not a significant concern at this location.
However, the concern of shrinkage is at the adjacent
side lap interface with the end lap. This interface is
commonly referred to as “T-joint.” When shrinkage
occurs, a “channel” or void is created under the adjacent
sidelap seam, which could allow possible moisture
migration under the sheet. One step to maintain conti¬
nuity of these laps and sheet alignment during the
application is to lay out chalk lines on the substrate in
Polyester reinforced modified bitumen
cap sheet exhibiting shrinkage at end lap.
order to line up the side and end laps. Another tech¬
nique that can be utilized during the installation of
modified bitumen sheet is staggering of lap seams. With
two-ply modified bitumen roof systems (smooth sur¬
faced base ply and granule surfaced cap sheet), the side
laps of the cap sheet should be offset from side laps of
the base ply. Due to the relative thickness of modified
bitumen sheets, side lap alignment will result in a build¬
up and more difficult level of construction of the sidelap
in the cap sheet. The cap sheet will lay flatter when
side laps are offset.
During the installation of the cap sheet, the asphalt,
adhesive or molten coating is typically extended or
exposed beyond the edge of the sheet. This “exposed”
bituminous material is commonly referred to as “bleed
out.” The actual asphalt applicator or “mop man” will
apply the mopping of hot asphalt on the substrate
extending approximately 1/4 to 1/2 inch beyond the
chalk line and/or selvage edge of the previously laid
sheet. A technique to minimize the flow of asphalt
across the previous sheet is to position a sacrificial piece
(approximately 18 inches wide) with the granule side
down and located approximately 1/2 inch from the outer
edge of the subject selvage edge. If the asphalt mopping
is extended up to the sacrificial piece, a relatively
straight line or strip of bleed-out can be achieved. With
cold process adhesive applications, the adhesive is typi¬
cally extended past the edge of the sheet, commonly
approximately 1/2-inch. With torch-applied methods,
the molten back coating is typically “pressed” out
beyond the edge of the sheet during application. For
torch applications, a bleed-out of 1/4 – 1/2 inch in width
is an indication that the proper temperature was
obtained during the torching process.
If, during the initial application process, proper lap
seam adhesion is not achieved, there are several process¬
es that can be followed to rectify the condition. One
process involves heating a metal trowel with a torch and
applying the “heated” trowel under the disbonded or
unbonded area in order to melt the coating. After melt¬
ing the coating, the subject area can then be rolled or
pressed into place. Another method involves inserting a
cold process adhesive into the subject area and then
“stepping-in” or rolling the lap seam together.
Several techniques can be utilized during application
that can assist in attaining well performing end lap
seams. During the application process, as a sheet is
overlapped on to the end of the preceding sheet, the
outside corner of the upper sheet can be trimmed to cre¬
ate a rounded finished corner. A non-rounded corner can
act like a piece of “dog-eared” paper and the corner can
be pulled up or become readily disbonded. Another
method that can be utilized at end laps is to trim the
outside corner of the underlying sheet. Starting at the
end of the sheet and at 3 or 4 inches from the outside
corner, the corner should be trimmed at a 45 degree
angle. This particular technique helps in achieving a
26 • Interface April 1997
Alligatoring of exposed asphalt bleed-out along edge of cap
sheet.
well adhered and superior watertight “T-joint” at the
end lap.
A common practice that is often implemented during
the installation of cap sheets and the adhering of lap
seams is the broadcasting of loose granules into the
bleed out. This practice can lend itself to creating well
constructed and performing lap seams. Loose granules
are typically manufactured by either 3M or GAF and
supplied by the manufacturer of the cap sheet in 100
pound paper bags. The granules are small mineral fines
that have a colored ceramic coating that is “baked-on.”
There are two critical steps in this process. The first
step requires that the granules are broadcast into the
“bleed-out” while the bituminous material is sufficient¬
ly hot, or in the case of the cold process material, prior
to initial set or “skinning over.” The second important
step is ensuring that the granules not only get broadcast¬
ed, but are embedded into the bleed out. This can be
accomplished by the applicator “tamping” or “stepping”
the granules down into the bleedout. Since the granules
are relatively light in nature, simply broadcasting them
will not result in complete adhesion of the granules to
the bleedout. When broadcasted, the granules come to
rest on the top surface of the bleedout. If the granules
are not embedded into the bleedout, once the bleedout
cools or cures, wind or water will readily displace the
granules toward the low part of the roof (i.e. drains, gut¬
ters).
The broadcasting technique typically involves a roof¬
ing laborer, with granules in a readily accessible contain¬
er, following in close proximity of the roofing technician(
s) rolling out the sheet goods. Once the sheet goods
are applied, the “granule person” broadcasts the gran¬
ules into the bleedout. An ample amount of granules
should be broadcasted into the bleedout and then be
stepped-in to ensure embedment of the granules. A lin¬
ear mound of granules piled on top of the bleedout
allows the applicator to “step-in” the granules without
getting hot asphalt and granules stuck to the bottom of
the applicator’s shoes. Contractors have utilized various
containers for carrying the granules. Common “nail
pouches,” metal/plastic buckets or pails and/or cus¬
tomized holders have been used. Once a respective area
is completed, the excess granules should be removed
from the roof surface. This can be accomplished with
“leaf’ blower type equipment or a soft bristled broom.
These procedures will remove a large percentage of the
excess granules. If the excess granules are not removed,
they will collect at low points or in drainage mediums,
possibly resulting in restricted flow of rooftop drainage.
Significant amounts could also be considered unsightly
if deposited into parking or walk areas located adjacent
to the building.
The broadcasted and embedded granules provide a
two-fold function, the first being a cosmetic or aesthetic
function. By embedding granules in the bleedout, a
more homogenous surface appearance of the roof sur¬
face is achieved. Together with the alignment of end
laps and rounded corners of end laps, embedded gran¬
ules help to result in a finished roof surface that by
appearance indicates an attention to detail. The finished
appearance is often important to the designer, the con¬
tractor and/or the owner, as the appearance of the com¬
pleted roof is often the final “judge” of the performance
of the contractor. In addition, these roof areas can often
be visible from higher adjacent viewing points, therefore
a pleasing appearance can be in the best interests of the
project participants.
The second function is more performance based. The
granules can provide both a stabilizing factor and protec¬
tive factor for the bituminous bleedout. This effect can
be compared to the similar effects gravel has when
embedded in a bituminous floodcoat. The granules can
be a stabilizing factor or act like a “reinforcement”
which can minimize movement due to shrinkage as the
bleedout material cures and weathers. In addition, the
granules can also reflect ultraviolet rays and promote
moisture evaporation from the roof surface. If the bleed¬
out remains exposed, it can become readily affected by
the weathering elements. Direct exposure to ultraviolet
and moisture will result in increased weathering, conse¬
quently resulting in oxidation, embrittlement, and
cracking. These cracks in the bleedout (which are com¬
monly referred to as alligatoring in asphaltic flood coats)
are like “fissures,” as they can extend into and/or under
the overlying sheet, consequently allowing moisture
migration beneath the lap seam.
If granules are not placed in the bleed-out in a timely
manner during the application, it is possible to adhere
them after completion of the installation of the cap
sheet. A hand-held detail torch can be utilized to “re¬
melt” the bleedout and then the granule application can
take place. However, this process is not advisable, as
damage to the surfacing sheet can occur if extreme care
is not taken. Topically applied bituminous or modified
bituminous cements/mastics with embedded granules
April 1997 Interface • 27
can be used to conceal bleed-out and other surface
irregularities (i.e. bitumen spillage, scars, etc.). The
membrane manufacturer should be consulted regarding
compatibility of these products with the respective cap
sheet.
In summary, simple techniques can be implemented
during the application process of cap sheet materials in
order to achieve the best possible lap seam construction
and minimize those problems that are typically associat¬
ed with these products.
References:
NRGA “Project Pinpoint Analysis: Ten-year
Performance Experience of Commercial Roofing 1983-
1992,” 1993.
“Roofing Material Specification and Details Manual”
1995 – BITEC, Inc.
“Technical Guide” 1996 – Siplast, Inc.
About The Author
Karl A. Schaack received a bach¬
elor of science degree in civil engi¬
neering from Clemson University and is a professional
engineer in the states of South Carolina, North Carolina,
and Texas. He is Vice President of Houston Operations
for Price Consulting, Inc., and is a Registered Roof
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28 • Interface April 1997