“Low-Slope Roofs are Rotting: Case Study Resolution” By Dwight Benoy, Pamela Jergenson, and Gary C. Patrick

“Low-Slope Roofs Are Rotting”
was an article published
in July 2016 in Interface
journal about three buildings
in a northern United
States climate experiencing
premature deterioration of their roofs
due to moisture condensation within the
roof assembly. The full paper can be found
on IIBEC’s website here: http://rci-online.
org/wp-content/uploads/2016-07-benoy-
jergenson.pdf. The following article is
a description of the challenges faced to
design and construct the repair at one of the
buildings that was the basis for the original
article.
The buildings that were repaired were
actually two four-story, wood-framed buildings
called Coborn Plaza Apartments in St.
Cloud, MN. Each building has retail space
on the first floor with student housing for
nearby Minnesota State University on the
upper three floors. J.A. Wedum Foundation
is the owner of the complex. Granite City
Roofing, Inc. of St. Cloud was the reroofing
contractor. Inspec was the architectural/
engineering firm. This reroofing project
proved to be one of the most challenging in
Inspec’s 45-year history from a design and
constructability standpoint.
BACKGROUND
The three buildings from the original
article had similar nonventilated roof
assemblies comprised—from interior to
exterior—of gypsum board ceiling, vapor/air
barrier (polyethylene sheeting), wood structural
trusses, blown-in fiberglass insulation
to fill the truss cavity, oriented strand board
(OSB) structural roof decks, rigid board
insulation, and a roof membrane. Coborn
Plaza differed from the other two buildings
in that it had a tapered polystyrene board
insulation system over the structural deck
and a plate-bonded thermoplastic olefin
(TPO) single-ply roof membrane. All three
buildings are multistory wood-framed structures
housing retail on the first floor with
apartments on the upper floors.
The essence of the problem in all three
buildings was that moisture-laden air
migrated into the truss space and condensed
in the upper reaches of the roof
assembly. This resulted in excessive moisture
buildup, mold, and rot of the OSB
structural roof deck and structural trusses
in a substantial portion of the roof
area. Discontinuities in the vapor/air barrier
(polyethylene sheeting), which allowed
moisture to migrate into the roof assembly,
occurred at interior partition and demising
walls and at penetrations through the ceiling,
such as sprinkler heads and electrical
boxes for light fixtures.
The problem was discovered approximately
five years after the buildings were
constructed when tenants of the top-floor
apartments on the Coborn Plaza Building
observed mold on the gypsum ceiling and
complained of musty odors. A mold remediation
project was undertaken that included
removing the gypsum ceiling, vapor/air
barrier, and blown-in insulation. It was discovered
that the exhaust ducts for the bathroom
and dryer vents were poorly installed
in some of the units. These ducts ran
through the structural trusses and exited
the exterior walls through the rim area. This
duct layout also bypassed the ceiling vapor/
air barrier, contributing excessive moisture
to the truss space.
The remediation work included cleaning
and sealing these ducts, which were
thought at the time to be the only cause of
the problem. The moldy framing and structural
roof deck were cleaned and painted
with an antimicrobial paint. Some of the
rotted deck was reinforced from below with
additional OSB sheathing and framing.
After the remediation project, inspection
openings from the interior were made to verify
whether the remediation was effective. It
was discovered that excessive moisture was
present, having redeveloped in a matter of
months following the remediation. Another
source for the moisture was investigated.
Hygrothermal modeling was conducted as
part of the investigation to provide information
to confirm or deny the theory that the
vapor/air barrier was inadequate. Results
indicated a propensity for moisture to accumulate.
DESIGNING THE REPAIRS
Due to the damages already experienced
and the potential for more to develop, it
was determined that Coborn Plaza needed
to have a complete roof replacement. The
J a n u a r y 2 0 2 0 I I B E C I n t e r f a ce • 2 5
primary challenge was to develop a complete vapor/air barrier below the dew point temperature that also tied into the wall vapor/air barrier to envelop the building.
Repair options were developed, with hygrothermal modeling conducted for each. The owner required all work to be conducted from above the ceiling to minimize disruption to the tenants. The options included:
Option 1
This option (Figure 1) was intended to create a complete vapor/air barrier by installing spray foam over the existing polyethylene sheeting and bottom chord of the truss. This required the removal of the existing roof system down to the structural roof deck and removal of a significant portion of the roof deck to facilitate vacuuming the existing blown-in insulation out of the truss space, and installation of the spray foam insulation and new blown-in insulation. New tapered insulation and roof membrane above the structural roof deck were part of this solution.
Option 2
Option 2 (Figure 2) required removal of the existing roof system down to the structural roof deck, and replacement of any wet, rotted, and/or moldy deck and blown-in insulation. A roof vapor/air barrier would be applied on the structural roof deck.
Spray foam insulation of a minimum 3-in. thickness applied to the rim area wasdetermined to be the most effective way insitu to transition the vapor/air barrier (polyethylene sheeting) from the exterior walls tothe roof vapor/air barrier. The rim area isat the top of the exterior walls at the level ofthe 16-in.-deep roof trusses.
Sufficient insulation needed to be added above the structural roof deck to get the dew point temperature above the roof vapor/air barrier. This insulation also needed to be tapered to provide roof slope to the existing interior primary and secondary (overflow) roof drains. The hygrothermal anal-
ysis showed a minimum of 4 in. of isocyanurate insulation was required to keep the
dew point temperature above the roof vapor/
air barrier. This meant all roof drains would need to be raised to accommodate the increased insulation thickness.
Option 3
This option required removal of all the existing blown-in insulation in the truss space and installation of a sprinkler system to satisfy the fire code. A new roof assembly above the structural roof deck included a roof vapor/air barrier, tapered rigid board insulation, and membrane. This achieved the need to have the dew point temperature occur above the roof vapor/air barrier and minimize the amount of insulation required. This option also required the spray foam rim area described in Option 2.
This option was quickly eliminated from further consideration because the owner decided not to install a fire sprinkler system above the top floor ceiling due to the considerable disruption to the occupants and the cost. Therefore, the blown-in insulation in the truss space needed to be maintained by selecting Option 1 or 2.
26 • IIBEC InterfaceCEJanuary 2020
Figure 1
Figure 2
Figure 3 – Detail from winning Option 2.
THE SOLUTION
Option 2 is the solution that was ultimately selected and developed into construction documents for bidding and construction (Figure 3). This was the best solution to achieve the goal of a complete vapor/air barrier. It also exposed all the existing roof assembly to allow for the removal and remediation of wet, deteriorated, and moldy roof components. This option also maximized the reuse of the structural roof deck and blown-in insulation that was still in acceptable condition.
Vapor/Air Barrier Continuity
Vapor/air barrier continuity from the wall to the roof was the key consideration and the toughest challenge for the repair design. Installing the roof vapor/air barrier on top of the structural roof deck required transitioning the vapor/air barrier through the structural roof deck to the rim area to complete the envelope. This was solved by designing a U-shaped sheet metal to wrap around the structural roof deck edge, which provides a surface on the bottom to receive the spray foam insulation applied to the rim area, and a surface on top to which the self-adhering membrane roof vapor/air barrier could be bonded.
Other Considerations
In addition to selecting Option 2, other considerations included:
•The rim area had to be accessed fromabove, which required the removal ofsome of the structural roof deck andblown-in insulation along the roofedge parapet.
•The parapet varies in height, withsome of the low parapet design beingchallenged by the additional insulation thickness.
•The trusses run parallel and perpendicular to the parapets, which causesvariations in the rim area conditions.
•The structural roof deck removalalong the parapets compromised thestructural integrity of the roof perimeter at some conditions, so an engineered solution was required thatincluded continuous steel anglesand plywood sheathing being addedto reinforce the structure (Figure 3).
•An allowance was included in thebase bid for deck and blown-in insulation replacement. The allowanceamount was an educated estimateof how much replacement wouldbe required based on the previous investigation work. Unit priceswere requested to be used to chargeagainst this allowance.
•During the design process, inputwas provided by Horizon RoofingCompany. Collaboration amongstHorizon, the owner, and the A/Eworked to develop a construct28
• IIBEC InterfaceCEJanuary 2020
Figure 4 – Typical invasive inspection opening above trusses.
Figure 5 – Typical invasive inspection opening below roof deck sheathing.
ible design that achieved the
goals and minimized costs and
delays.
CONSTRUCTION CHALLENGES
Preconstruction
Three contractors were invited to
bid the project, and they provided
input during the bidding process. One
key, high-risk factor in constructing
Option 2 was that doing all the work
from the top side left the roof open
and vulnerable to the weather for a
substantial portion of time each day.
Some days had greater exposure than
others, depending upon how much
deck and blown-in insulation needed
to be replaced.
During the design phase, based
on investigation-generated test results and
observations, it was decided to make 60
invasive inspection openings prior to the
start of construction to provide an idea of
where the deck and insulation would need
to be replaced (Figures 4 and 5). This would
help the contractor better plan the construction
work. The contractor awarded the
reroofing project would make and repair the
inspection openings.
Because litigation had been initiated,
parties involved with the original construction
had an interest in observing the existing
construction. To minimize the disruption to
the contractor’s operations during the roof
replacement, all interested parties were
allowed to observe and conduct moisture
testing at each of the 60 invasive inspection
openings. The owner hired IEA, an environmental
consulting firm, to conduct moisture
tests and sampling for fungal analysis on its
behalf. This consultant provided a report,
including a roof plan showing the results of
their testing.
Moisture Content
Based on the 60 invasive inspection
openings, test results, and observations,
a roof plan was developed showing the
approximate areas where roof deck sheathing
and blown-in insulation would most
likely require replacement (Figure 6). The
final determination of what needed replacement
would be made by the contractor
when each area was opened daily. While
on site, performing their periodic observations,
the A/E assisted the contractor
to determine what needed to be replaced.
A hand-held moisture meter was utilized
daily, which worked well in determining the
moisture content of the OSB structural roof
deck. Industry convention indicates that a
16% moisture content would be the threshold
for requiring replacement.
The moisture meter did not provide
useful readings for determining the need to
replace the blown-in fiberglass insulation.
Samples of insulation were taken to determine
an oven-dried moisture content by
weight to develop a correlation with moisture
meter readings. A correlation could not
be determined, so the decision to replace
insulation was somewhat subjective. First,
wherever mold was detected on the OSB
deck, the underlying insulation was also
replaced, because mold spores can migrate
into the insulation. Second, the contractor
determined whether excess moisture was
present by sight and touch.
Construction
The contractor elected to do the perimeter
work prior to the replacement work in
the field of the roof (Figure 7). The perimeter
work proved to be time-consuming and
would have significantly reduced the size of
the area that could be reroofed on a daily
basis if it was done in conjunction with the
field of the roof. The contractor could also
schedule the perimeter work on days when
J a n u a r y 2 0 2 0 I I B E C I n t e r f a ce • 2 9
Figure 7 – Typical work at roof perimeter.
Figure 6 – Approximate areas of roof deck and insulation replacement.
3 0 • I I B E C I n t e r f a ce J a n u a r y 2 0 2 0
Figure 9 – Two-piece U-shaped
vapor/air barrier transition metal.
Figure 8 –
Z-shaped
vapor/air
barrier
transition
metal.
the weather forecast was a bit questionable, as the
perimeter could be enclosed rapidly should precipitation
be imminent.
The contractor fabricated a Z-shaped transition
metal instead of a U-shaped transition metal that
served the same purpose as a vapor/air barrier transition
material (Figure 8). However, there were areas
of the previous mold remediation where additional
framing done as part of that work interfered with the
installation of the Z-shaped metal. Therefore, a twopiece
U-shaped metal was installed with the connection
between the pieces accomplished with aluminum
tape (Figure 9).
After the Z-shaped transition flashing was
installed, a short width of vapor/air barrier was
installed (Figure 10), and then a parapet reinforcing
assembly of plywood and sheet metal angle was
installed (Figure 11), followed by the field of the roof
vapor/air barrier (Figure 12).
The contractor had on-call local insulation and
plumbing subcontractors under contract and available
to complete varying amounts of work, depending
on what was uncovered and anticipated each day.
J a n u a r y 2 0 2 0 I I B E C I n t e r f a ce • 3 1
Figure 10 – Roof
vapor/air barrier at
roof perimeter.
Figure 11 – Plywood
and sheet metal
angle parapet
reinforcement.
Figure 12 – Vapor/
air barrier applied
to field of roof.
Figure 13 – Blown-in insulation.
Figure 14 – Mold remediation paint.
32 • IIBEC InterfaceCEJanuary 2020
“The owner,
the contractor,
and the architect/engineer worked to develop a constructible design that achieved the goals and minimized costs and delays.”
Perimeter work required the insulation subcontractor
to be on site to vacuum insulation,
install the spray foam insulation
in the rim area, and install new blown-in
insulation on each day of perimeter work
(Figure 13).
Mold remediation was handled by the
contractor, alleviating the need for a specialty
contractor. This eliminated coordination
and delay issues. The contractor cleaned
any discolored areas that were within
moisture content limits, then painted
these areas with an antimicrobial
paint (Figure 14). Most of the parapet
that was left in place was remediated
when the perimeter work was constructed,
which proved to be the most
efficient.
The estimated amount of existing
roof deck sheeting removal, based on
the 60 invasive inspections openings,
was 8,000 sq. ft. The actual amount
of existing roof deck
sheeting removal was
6,000 sq. ft.
While conducting
the invasive inspection
openings, and
subsequently during
the reroofing work,
it was observed that
the plate-bonded
TPO roof membrane plates were severely
corroded in much of the roof area. This
significantly reduced the wind uplift resistance
of the roof membrane. The contractor
was conscious of the need to respond
quickly, should a high wind event occur.
Fortunately, the reroofing work was completed
without incident.
The contractor removed tear-off debris
from the site daily. The debris was lowered
by crane into dump trucks. New materials
were hoisted daily with only a one- to twoday
stockpile on the roof. The crane and
roofing materials were staged on the streets
running adjacent to the building, but only
at certain locations, which resulted in long
travel distances across the existing roof in
some areas. The city of St. Cloud allowed
Figure 16 – Completed roof.
J a n u a r y 2 0 2 0 I I B E C I n t e r f a ce • 3 3
Figure 15
– Perimeter
safety rails.
the streets to be temporarily closed. Access
to the retail establishments and egress from
the buildings was continuously maintained
but was an ongoing public safety challenge.
Perimeter safety was primarily accomplished
with rails attached to the parapet
(Figure 15). A safety monitor was also assigned
to work with the crew applying the low-rise
foam adhesive for the insulation attachment.
The fully adhered EPDM membrane
over the tapered insulation system provided
a fully draining roof with a finished
appearance (Figure 16). Even with all of the
construction challenges, the roof was completed
in a timely manner.
REMARKS
The owner, the contractor, and the
A/E worked together to achieve the goal of
taking a sick building and making it well.
All parties understood from the start that
shortcuts couldn’t be taken. As with most
projects, some surprises were encountered,
but these were quickly resolved with input
from all parties. Cost efficiencies were considered
and implemented only if they didn’t
compromise the design intent. The project
was completed with minimal disruption to
the operation of the building and its occupants.
3 4 • I I B E C I n t e r f a ce J a n u a r y 2 0 2 0
Pamela Jergenson,
CCS, CCCA,
BECxP, CxA+BE,
is a senior building
enclosure consultant
and expert
in exterior walls
with Inspec. She is
an expert in hygrothermal
analysis.
Jergenson
has served on the
Existing Masonry
Committee of the
Masonry Society.
Pamela Jergenson,
CCS, CCCA,
BECxP, CxA+BE
Gary C. Patrick,
RRC, AIA, is an
executive vice president
of Inspec,
with which he has
been since 1977.
He oversees the
roofing services
area of the company,
including evaluations,
design,
peer reviews, construction
observation
and testing,
and forensics. He is a registered architect
in five states, and he is a Registered Roof
Consultant (RRC) with IIBEC.
Gary C. Patrick,
RRC, AIA
Dwight D. Benoy,
PE, was employed
at Inspec, a building
enclosure consulting
engineering
and architectural
firm in
Minneapolis, for
over four decades.
He is a licensed
professional engineer
and has
focused his practice
in forensic engineering of the building
enclosure. He received a BS in civil engineering
from the University of Minnesota.
Dwight D. Benoy, PE
In 1988, Joe Hale agreed
to chair RCI’s (now IIBEC’s)
Asbestos-Containing Roofing
Material Committee, the first
new committee to have been
formed by the association since
its inception. Another new joint
committee was formed with the
National Roofing Contractors
Association (NRCA) to explore
mutually agreeable professional
relationships between roof
consultants and roofing contractors.
In November 1988, the
association hired its first fulltime
paid employee, Paula
Baker, as administrative assistant
to the executive director
(Bob Phillips) at the headquarters
office in Raleigh.
Blast From IIBEC’s Past: 1988
Attendees at the Roof Consultants Institute’s (now IIBEC’s) board meeting in Washington,
D.C., in August 1988. Seated, left to right: Second VP James E. Magowan, Treasurer Richard
Horowitz, and First VP George F. Kanz. Standing, same order: Executive Director and Immediate
Past President Bob Phillips, President D.B. Hales, and Secretary Donald E. Bush.