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Repair of Above-Roofline Parapet Walls on Vintage Structures

May 15, 2012

Our nation’s inventory of vintage
structures has evolved
to include a broad range of
signature features. Many
vintage structures have
crenellated above-roofline
parapet walls with heights ranging from 12
in. up to 10 ft. above the roofline, with the
latter commonly referred to as battlements.
Historically, the regular intervals of projected
high (merlons) and low (crenels) points
provided cover and openings from which
combatants could exchange fire with the
enemy. Architects of the 19th and 20th centuries
included these features, focusing on
the form as an architectural embellishment
rather than the original function, which
went the way of moats, drawbridges, and
dungeons. This type of construction features
a geometric, symmetrical-toothed
appearance at the top of the parapet wall,
commonly as in-fill between projected buttresses
and neighboring outcroppings of
even higher towers and turrets.
It is not unusual for a design professional
to perform a due-diligence evaluation
on a vintage structure for an owner considering
roof replacement. Often, the mind’s
eye turns from the involved roof area, only
to find that the adjacent near- and aboveroofline
parapet walls are exhibiting varied
levels of distress. This place marker
in a building’s history brings renewed
meaning to the “out of sight, out of mind”
adage often thought reserved for the
roofing side of the building envelope industry.
In some in stances, the conditions (outof-
plane, deteriorated brick masonry backup,
evidence of movement) have evolved to a
point resulting in challenges for the project
participants. The seemingly routine task of
roof design expands to include the potential
for masonry restoration, minimally to an
extent that will offer the new roof cover the
promise of a service life consistent with the
effort. At the extremes of the conditional
variables are the low-profile installations,
perhaps 12-18 in. above a partially hidden
integral gutter, to parapet wall installations
that reach heights of 6-10 ft. above the
adjacent roofline. Lower parapets are commonplace
at the gutter line on steep-sloped
roofs. The higher parapet walls offer continuity
to the gothic theme while offering a
means to conceal the presence of a lowsloped
interior roof area.
Vintage structures—seemingly simple
compared to current construction practices
(e.g., air and vapor barriers, rain screens,
cavity walls, through-wall flashings)—rely
on the condition and performance of varied
interfaced components working together as
an assembly for optimum performance. In
the absence of current-era sophisticated
building envelope features, this type of construction
is inherently more susceptible to
the elements than any other building envelope
component, being subjected to weather
6 • IN T E R FA C E MA R C H 2012
Photos 1A and 1B – Examples of a
“crenellated” parapet wall or
battlement as identified by the
alternating high (merlon) and low
(crenel) points of the feature.
extremes on both faces.
Depending on the height
and overall condition of
the feature, the key parameters
of performance
move from a water management
problem centered
on roofing and accessory
flashings to a structural
issue. Com po nents of
these stone-clad outcropping
walls usually consist
of a full-depth capstone
dressing the upper limits
and providing transition
from the exteriorcut/
carved stone wall to
the brick masonry. The
underlying multiple-wythe
brick backup wall on the
interior and the cut-stone
ashlars on the exterior
support the capstone
assembly. Ac knowledg ing
the skyward-facing joinery
of the stone copings,
craftsmen of the era occasionally set sheet
copper flashing be tween the stone copings
and underlying coursing of stone and brick.
If present, this flashing may represent the
only effort made by the original construction
team to manage water.
Upon discovery, the design professional
would reasonably be expected to provide
the owner with some form of supplemental
report of findings. Generally speaking, it is
prudent to—at the very least—have a
licensed structural engineer perform a
review of the conditions where any ornate
above-roofline stone/masonry work is considered.
The structural engineer has the
responsibility to ensure that the components
of the building envelope are safe and
conform to the applicable building codes.
This can be a challenging task and requires
particular attention.
Typically, when a deficient masonry
component is identified, its repair is somewhat
intuitive. Degraded mortar in masonry
MA R C H 2012 I N T E R FA C E • 7
Photo 2 – Cut/carved stones labeled
for correct spotting during
reconstruction.
Photo 3 – During demolition,
the transition from singlewythe
backup (A) to multiplewythe
(B) is observed.
A
B
joints requires routing and tuck-pointing. Cracked
bricks or limestone ashlars end up being replaced or
repaired. However, parapet walls in need of repair cannot
always be merely replaced in kind. Parapet walls,
like all other exterior cladding, must be capable of resisting
stresses induced by horizontal wind and seismic
loads. Since parapets are laterally unsupported at the
top, they behave as cantilevers. Some current building
codes require that parapet walls be built to withstand
wind loads of a larger magnitude because they are subject
to simultaneous windward and leeward wind forces,
both acting in the same direction. For example, the current
Chicago Building Code stipulates
that a wind load of 40 psf
should be applied to parapet walls.
This is more stringent than the typical
wind load applied to building
structures.
Vintage structures were built in
an era prior to the sophisticated
masonry codes that we have now.
Up until the middle of the 20th century,
masonry structures were
designed empirically. Architects
used rules-of-thumb based on
height-to-thickness ratios of the
wall in order to design the masonry.
The walls were generally robust and
did not contain any reinforcing
steel. However, the empirical design
did not account for special conditions
or unique calculable loads
8 • IN T E R FA C E MA R C H 2012
Photo 4 – Rebar set for forming
and pouring of a new concrete
beam above existing roofline.
Photo 6 – Stone requiring field cut to
accommodate new tolerances
established by the concrete beam.
Photo 5 – Forms stripped, exposing
interior face of new concrete beam
fitted with embedded weldments.
established by current provisions of code.
Current masonry codes require parapet
walls to have vertical reinforcement unless
the stresses are small enough to satisfy
unreinforced masonry criteria. Hence, there
is a need for a structural analysis of parapet
walls requiring reconstruction. The engineer
or architect proficient in structural
evaluation needs to determine if the existing
above-roofline feature has sufficient capacity
to safely resist the applicable lateral wind
and seismic loads imparted onto it. Shorter
parapet walls with a height of only a couple
of feet generally have adequate strength.
However, taller parapet heights cannot be
merely replaced in kind.
Most building codes require reconstruction
of components to satisfy the current
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Photo 7 – Reinstallation of cut/carved limestone cladding with stainless steel anchors.
Photo 8 – Fully integrated zinc- and tin-coated copper receiver flashings below stone copings.
MA R C H 2012 I N T E R FA C E • 9
requirements. That means that
parapet walls deficient by current
standards must be rebuilt to meet
the more stringent criteria. This
generally means that tall parapet
walls requiring reconstruction need
reinforcing with vertical rebar dowelled
into the wall below the roofline
in order to attain sufficient strength.
Therefore, the parapet wall must be
engineered such that vertical rebar
of adequate size and spacing can be
installed so that the stresses in the
wall are within the allowable limits.
Whether the newly rebuilt aboveroofline
feature requires vertical
reinforcing or not, it must be properly
fastened to the roof framing
such that the lateral loads can be
transferred into the roof deck
(diaphragm). In extreme cases, the
capacity of the existing roof
diaphragm is insufficient to transfer
the code-stipulated lateral loads and
requires reinforcement or possible
reconstruction.
Depending on the height of the
10 • I N T E R FA C E MA R C H 2012
Photo 9 – The introduction of new, galvanized structural framing elements above the original
roofline.
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parapet wall, the additional cost to rebuild
the wall to meet current codes can be substantial.
This need for a structural review
along with the understanding that mere inkind
replacement of deteriorated masonry
parapet walls is not always acceptable
should be taken into account during the
building’s condition assessment.
Based on the order of magnitude for the
work under consideration, it is also reasonable
to require that any scaffold/work platforms
be designed by a licensed engineer to
ensure that they have sufficient capacity to
support workers, equipment, and materials.
Most qualified scaffold services are familiar
with this and have a means to provide
drawings for the project record.
The anatomy of the walls in sections
below the capstone usually is composed of
random ashlars (referring to the coursing
and cut/carved stone cladding) in the field
of the wall, ranging in thickness from 4-8
in. nominally, of varying lengths, widths,
and, in some instances, ornamentation.
The 4-in. stones are typically mechanically
anchored to the brick backup materials
MA R C H 2012 I N T E R FA C E • 1 1
Photo 10 – Fully completed parapet and
new roof assembly.
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with mild steel straps resembling heavy staples.
Their engagement with the brick
masonry backup materials (as many as one
or more brick courses toward the interior of
the multiple wythe wall) and adjacent stone
is a clear indicator that the walls (backup
and stone cladding) were built concurrently.
The 8-in.-deep stones are set into pockets
of the brick masonry backup as a means
to further tie the interior and exterior
wythes together.
On projects in which the scope includes
the complete reconstruction of the brick
masonry backup wall—utilizing a steelreinforced,
fully grouted CMU wall section—
the 8-in. bond stones may need to be field
cut to 4-in. thickness so as not to interfere
with the continuity of a bond beam and/or
reinforcing steel set in the fully grouted
cells of the new backup. Any new steel (generally
limited to anchors for resetting of the
stone, in the form of pins and clips) should
be of stainless steel.
In summary, reroofing, partnered with
extensive exterior wall rehabilitations—
more notably on vintage structures—
requires building envelope consultants who
can convey to their clients the need for a
significant amount of forward planning and
to work with them to understand the handin-
glove relationship of seemingly disparate
elements of our built environment.
12 • I N T E R FA C E MA R C H 2012
Don Kilpatrick has been with Inspec, Inc. for 25 years, fulfilling
varied roles ranging from laboratory supervisor to project
manager. For the past eight years, he has been involved in
master planning of multiple projects at the University of
Chicago. Don is an active member of RCI, serving on the Peer
Review Editorial Board for Interface (to which he is a regular
contributor) and is a past recipient of the Horowitz Award.
Don Kilpatrick
Michael Wiscons, SE, PE, is the supervisor of the exterior wall
department in the Chicago office of Inspec, Inc. He has bachelor’s
and master’s degrees in civil engineering from the
University of Illinois at Urbana-Champaign. Mike is a licensed
structural engineer and professional engineer and has been
practicing since 1983. His work includes design and field
investigation of structures and building façades for commercial,
institutional, and residential buildings.
Michael Wiscons, SE, PE
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