Historic Roof Decks: Roof Design Issues and Considerations

ABSTRACT
On many older buildings, frequent
water leakage into the roof assembly over
the years results in deterioration of the roofing
system, structural deck, and exterior
walls. Evaluation of the condition of the
building components that interrelate with
the membrane, especially flashings, is
required for the successful installation of a
new system. Simply replacing the membrane
and disregarding direct or indirect
issues such as deteriorated parapets, structural
deck deficiencies, excessive deflection,
drainage line corrosion, drainage system
capacity, and conformance with current
code requirements eventually results in
poor stewardship of the assets that a historic
property affords. It also supports
short-term thinking that ultimately results
in future performance problems and
advanced and accelerated decay.
This paper focuses on problems and
issues associated with substrate conditions
that are hidden by materials and flashings
and some of the pitfalls associated with
them. Guidelines for evaluating the condition
of masonry walls, lightweight concrete,
and structural clay tile roof decks and their
impact on performance are provided.
Selection of the proper materials, given the
condition of many older underlying substrates,
is also discussed. In addition, the
authors (based on their experiences) present
suggested practices for obtaining a
successful installation of a new roof system
on an older, historic building.
WHY DO A THOROUGH INVESTIGATION?
Reroofing an historic structure should
begin with a carefully thought-out plan so
that a successful outcome will result for the
building owner, designer, contractor, and
general public. A thorough investigation
should be the first step in developing a
replacement program. The health, safety,
and welfare of the building users are maintained
if detailed and reliable information
about as-built conditions is obtained, particularly
for the roof deck. Reroofing specifications
that are prepared based upon holistic
interpretation and detailed knowledge of
as-built conditions indicate to all concerned
that the plan of action and approach to a
project are professional, knowledgeable,
and responsible. A prudent doctor would
not proceed with surgery on a patient without
running the necessary tests first to
diagnose the problem. In fact, it may be
considered professionally irresponsible for
designers not to perform a thorough investigation
of the interrelated roof system components
to formulate a proper plan of
action. The time invested in a detailed
investigation is almost always recovered in
more complete and accurate bids from contractors.
Through experience, the authors have
found structural decks for historic structures
to be relatively unique. When substrate
conditions are properly assessed, a
relatively small amount of unforeseen conditions
will result in a construction project
completed with relatively few change orders.
Unpleasant surprises and cost extras are
likely to occur if investigative activities are
deferred to the construction phase.
Frequently, older structures have a history
of prior leaks. The combination of deteriorated
conditions at parapet walls and
decks necessitates repair of these elements
in conjunction with the roofing system
replacement to achieve a successful project
outcome. An inspection of interior spaces
and attics will help to reveal areas of leakage.
Water stains on wall and ceiling surfaces
can be carefully recorded and superimposed
with the relative position of specific
areas of the roof. Focusing investigative
attention on the areas of apparent deterioration
increases the likelihood that worstcase
scenarios will be revealed.
Inspection openings in the roofing system
are necessary to identify the type and
number of membrane layers, how the system
is attached to the deck, whether insulation
is present and what type was used,
and the condition of the top surface of the
structural deck. The openings should be
relatively large so that a reasonable examination
of the top surface of the deck can be
made. Selecting an opening adjacent to a
parapet wall has the benefit of revealing the
condition of the wall below the flashing as
well as the roof deck-to-wall interface.
Conditions uncovered may substantially
influence the new flashing design.
In many older buildings, the original
architects designed the roof structure with
liberal slope for drainage. Typically, lowslope
decks employing masonry or cementitious
materials were protected by built-up
4 • IN T E R FA C E A P R I L 2009
coverings or sheet metal. A common practice
was to utilize an organic felt bituminous-
membrane system adhered directly to
the deck, without rigid insulation. In our
experience – and depending on the age of
the structure – it is not unusual to discover
several membrane layers applied one over
the other, sometimes with rigid insulation
installed between some of the layers, as
well. Also, older building designs often
include attic spaces. Attic spaces have the
benefit of accommodating access to
mechanical, plumbing, and electrical services.
In our experience, roof drainage was
given careful attention and consideration in
many older buildings. The manner in which
slope was achieved was either through sloping
the structural system or by adding
slope. Frequently, loose cinders were used
as sloped fill over the structural deck, similar
to today’s factory-tapered insulation.
Saddles and crickets were often made from
wood and/or cementitious fills.
TYPICAL SUBSTRATES CIRCA 1900
Common materials used for roof substrates
in older historic buildings included
structural clay tile, lightweight concrete
decks, precast gypsum planks, and masonry
parapets. This paper focuses on structural
clay tile and lightweight concrete
decks and the interaction of these decks
with masonry walls.
Structural clay tile is characterized by
machine-made hollow units with parallel
spaces. These units were available in a variety
of shapes and sizes. Tiles were first
manufactured around 1875. Several floor
and roof designs were patented during this
time. In 1903, the National Fire proofing
Cor poration of
P i t t s b u r g h
published a
handbook and
catalogue illustrating
products
and presenting
data
for use in the
design of segmental
and
flat-arch floors.
The dead weight of structural clay tile systems
often ranged from 35 to 45 lbs/sq ft.
The main advantages provided by these
floor and roof systems were ease and speed
of erection (independent of temperature limitations),
and fireproofing for structural
steel framing. Structural clay tile may be
classified into four groups:
• Flat arch
• Segmental arch
• Combination tile and concrete
• Book tile
Flat-arch and segmental-arch systems
rely upon arch action for strength and rigidity.
For these arches, tiles are placed
between steel beams, forming a flat arch.
Figure 1 illustrates both of these.
Combination systems rely upon the
composite interaction of clay tile units, concrete,
and steel reinforcing bars to carry
tensile and bending stresses, as shown in
Figure 2. These decks often utilized a 2-inthick
plain or cinder concrete topping over
the clay tile as a composite component of
the roof-deck system. The combination system
is analogous to a modern-day concrete
pan joist or waffle slab structural system.
Book tiles are relatively large structural
tile units that are supported by steel purlins
with the sides held in place with steel Tbracing,
as shown in Figure 3. Book tile was
primarily intended for use on steep roofs,
but they may be found on flat roofs, also.
The name “book tile” refers to the shape of
the tile, in that it resembles a closed book.
The strength of the tile unit resists tensile
and bending stresses.
On some structural clay tile roofs, a
mortar or concrete topping, typically 1 to 2
in thick, was often field-applied over the
tiles as a leveling and bedding layer. This
was done to provide a smooth, uniform, and
monolithic surface on which to install the
roof membrane. During demolition of a roofing
membrane from this deck, care needs to
be taken during chipping or sawing to avoid
potentially damaging the clay tile units and
compromising the combined tile/arch in –
tegrity.
LIGHTWEIGHT CONCRETE
Lightweight concrete for roof decks can
be characterized either by cast-in-place or
precast material that can be classified into
the following three groups:
• Cinder concrete
• Nailing concrete
Figure 1 – Typical assembly of flat and
segmental clay tile arch systems.
Figure 2 – Typical combination clay tile and
integral concrete topping roof-deck system.
A P R I L 2009 I N T E R FA C E • 5
View of 12½-in tile arch between 15-in I beams.
Weight, 45 lbs per sq ft.
• Sloped cinder fill and cementitious
topping
Cinder concrete is a low-quality, lightweight,
structural concrete that utilizes cinders
as the primary aggregate. A commonly
employed mix is one part cement, two parts
sand, and five parts cinder. Cinder aggregate
is a by-product of coal combustion and
it is highly porous and cellular in nature.
Cinder concretes have also been used as
sloped fills over normal-weight concrete.
Some cinder concretes have high sulfur
contents, which are deleterious to steel. A
nonstructural application for roofs utilizes
loose cinders graded in a sloped configuration
for drainage and then capped by a thin
concrete or mortar topping, which provides
a smooth surface for the roof membrane.
Proprietary lightweight concretes have
been available under the trade names of
“Federal nailing concrete,” “Haydite concrete,”
and “Porete slabs.” These systems
were either poured in place or precast, with
their chief benefits including speed of
installation and nail-holding ability for
attaching built-up membrane on low slopes
and slate and clay tile on steep slopes. In
precast systems,
joints were grouted,
and a thin cementitious
topping may
or may not have
been installed, de –
pending upon re –
quirements. The
pre cast systems
were made in a
channel configuration
and had un –
topped thicknesses
typically varying
from 2¾ to 3½ inches.
Sloped cinder fill
systems are nonstructural,
fieldmade
substrates for
flat roofs that were
placed over structural
concrete slabs
to provide slope for
drainage. Cinders
were loosely placed
and graded to the
required configurations
and then
topped by concrete
or mortar, usually
1½ to 2 in thick.
The topping would normally have a built-up
membrane applied directly to its surface. A
cinder-fill deck provides limited insulating
capacity for the roof system. A drawback
with sloped cinder fills is that water can collect
within the cinder fill layer. When making
an inspection opening, beware that the
thin, poured concrete or mortar layer over
the cinders may visually appear as though
the deck is structural concrete. If a cementitious
surface is observed at an inspection
opening, it may or may not be the actual
structural surface of the deck. Rather, it
may be a nonstructural concrete or mortar
topping. Chipping the cementitious surface
should be done because it may help reveal
if the surface onto which the membrane is
applied is structural or nonstructural.
MASONRY PARAPETS AND WALLS
Parapet walls were often built as multiwythe
masonry without cavities, frequently
three to four brick wythes thick. Stone
masonry and terra cotta were also used on
parapets. Typically, the interior surfaces
incorporated common brick masonry or
rubble fill. Stone coping units or terra cotta
tile were selected to cap the top of the parapet
walls. The interface of the horizontal
surface of the roof membrane with the vertical
flashing surfaces of parapet walls often
included a through-wall metal flashing system.
This system is intended to prevent
moisture within the wall system from entering
the roof system or the building and is a
good detail that is not utilized in today’s
construction as often as it should be.
Concealing or covering the through-wall
flashing with roofing material is a common
problem that leads to the early deterioration
of both the roof system and masonry. In
order to keep the roof system dry, the flashing
terminations must be below the line of
the through-wall flashing so that water
within the wall does not drain into the roof
system. Many older structures have the
through-wall flashing positioned such that
the minimum contemporary industry standard
flashing clearance of eight inches cannot
be achieved without terminating the
flashing above the line of the existing
through-wall flashing.
An evaluation of the condition of the
parapet walls is critical when designing a
roof-replacement project. Good roofing
practice dictates that any reroof flashing
should never be terminated above the line
of a masonry through-wall flashing. A new
through-wall flashing assembly could be
installed at a higher position in the wall by
reconstructing the wall. Another problem is
encapsulating the entire inside surface of a
tall parapet with roofing material. Covering
the entire surface of the masonry with roof
membrane flashing is contrary to good
masonry practices and, in northern climates,
can accelerate deterioration of the
masonry. Over time, water infiltration,
cyclic freeze-thaw damage, and efflorescence
cause corrosion of embedded steel.
(See Figure 4.)
Stair stepping the flashing along the
parapet keeps the height within recommended
industry standards, while allowing
the masonry above the flashing to breathe.
It may be necessary to locally remove sections
of the masonry parapet and install a
stair step design to maintain sufficient vertical
height for membrane base flashings.
TYPICAL PROBLEMS ASSOCIATED WITH SUBSTRATES
Deterioration of the structural deck and
parapet wall or corrosion of embedded steel
components is usually attributable to moisture
infiltration. Other causes may include
building movement from either expansion/
contraction or settling over time, or
interior conditions that may contribute to
Figure 3 – Typical steep-slope application of clay book tiles.
6 • IN T E R FA C E A P R I L 2009
condensation. Often, because of the type of
construction, water leakage into the building
may go unnoticed for an extended period
of time. Figure 5 highlights the many
paths water can travel before it leaks into
the interior spaces.
Deterioration of the mortar joints is a
common problem in masonry parapets.
Identifying the quantity and location where
repointing work or brick replacement is
needed will often avoid potentially costly
change orders. If a through-wall flashing
system is deteriorated or if flashing height is
insufficient, some parapet repairs should be
anticipated. Through-wall flashing in a twopiece
configuration allows the counterflashing
to be removed to allow maintenance of
the membrane flashing, as well as future
reuse of the counterflashing when reroofing
occurs.
Stone parapet walls may be very porous.
The porosity of the stone may allow water to
travel through the wall, thus bypassing any
surface-mounted roof flashings. A good
detail is to provide a through-wall flashing
to manage water that will eventually infiltrate
the wall.
The bearing conditions of the structural
framing members may be affected by moisture
infiltration. This is a serious problem
that should be examined thoroughly.
Temporary shoring of the framing while
masonry re pairs are undertaken may be
needed if deterioration
is advanced.
Deterioration
of the top shell in a
clay tile unit within
an arch system
may reduce structural
capacity and
compromise the
in tegrity of the
arch. Similarly,
concrete or mortar
toppings may have
delaminated and
may conceal damaged
top shells. A
structural engineer
fa miliar with
clay-tile arch systems
should be
consulted to as –
sess these issues.
Several attachment
options must
be evaluated to
determine how the
new roof system
will be attached to the substrate. For flat
roofs, the attachment options include fully
or partially adhered, mechanically fastened,
and ballasted systems. The condition of the
deck will influence decisions for roofreplacement
systems.
Wood saddles and cants should be
inspected to determine their condition.
Replacement should probably be anticipated
unless their condition is exceptionally
good. If the roof was re-covered in the past,
perhaps new cants and saddles were
installed, but no deck
repairs were performed
be neath the saddles.
During construction,
damage can occur
to the roof substrate
from the equipment
used to remove the old
system. Storage of
materials and construction
traffic across
the deck while removing
the old roof and
installing the new one
may also weaken it and
cause further damage.
Stockpiling roofing
material may overload
already weakened
areas of structural
deck.
All of the above conditions and hypotheses
should be evaluated prior to the start of
the repair work to determine if the substrate
for the new roof system is capable of
providing continued safe performance.
DESIGN CONSIDERATIONS FOR MATCHING THE
ROOF SYSTEM TO THE SUBSTRATE
Following are some of the design issues
that should be considered and incorporated
in a roof-replacement plan.
Consider Existing Conversions and
Additions
Additions or conversions of space may
have resulted in additional mechanical
equipment on the roof and offsets between
areas that create snowdrifting issues where
none existed prior to the revision. A significant
change in interior humidity and/or
temperature may require a different
amount and type of insulation as well as
vapor retarder location. Expansion joints
separating building additions need to be
incorporated into the roof design. Removal
and replacement of mechanical equipment
may be necessary, which will add complexity
to the overall renovation plan.
Provide Required Fire Protection
Clay tile provides an excellent source of
fire protection for steel framing. Figure 6
shows how the clay tile typically protected
structural steel members. If there is significant
deterioration of the existing clay tile
deck and selected removal and replacement
of tile units is necessary, the replacement
materials need to provide the same or better
fire protection of structural steel, should
Figure 4 – A typical lightweight concrete topping course
applied on a concrete structural slab.
Figure 5 – Some of the paths water can follow before
leakage is discovered in the interior spaces.
A P R I L 2009 I N T E R FA C E • 7
any be uncovered. The building code, local
officials, and insurance company representatives
should be consulted to make sure
the repair design satisfies local fire-resistance
protection requirements.
Consider Dead- and Live-Load Limita –
tions, New Code Requirements, Uplift
Design
New roofs should be designed to conform
to the current building code requirements
for dead, live, and wind loading.
Particular attention should be paid to the
condition of the structural slab for the loadcarrying
capacity and uplift resistance of
the new roof covering. If the uplift resistance
of the roof system over an historic
deck substrate is not strong enough to overcome
the imposed wind loading, the roof
will not last long and can potentially blow
off.
Achieve Proper Slope to Drain
Saddles built between drains may have
been originally constructed of wood or mortar.
The condition of these saddles needs to
be evaluated, and they may need to be
replaced or repaired. If a sloped cinder-fill
system on sloped concrete topping exists and
it is then removed, a new system should be
provided that achieves adequate drainage
slope.
Verify Plumbing Code Requirements
It is wise to add overflow drains or scuppers
for roofs surrounded by parapet walls,
and this is likely to be a building code
requirement. Drains can become clogged
and allow water to accumulate on the roof
surface. Emergency overflows are designed
and installed to prevent collapse from
occurring. Often, the condition of the drainheads
and drain leader lines is poor, necessitating
replacement of a portion of the
plumbing system itself. Existing drain lines
may be potentially undersized and may
require plumbing repairs in order to bring
the drainage system up to current code.
Address Electrical Repairs
Electrical conduits may be buried in
concrete toppings or insulation, and their
existence can influence the repair approach
to be taken. Conduits can be identified
using a metal detector or by careful observation
from the underside of the deck.
Structural Engineering
If the structural deck requires extensive
repair, a qualified structural engineer
should be consulted to evaluate the need for
shoring and to recommend repair options.
Provide for Special Removal and
Disposal Procedures for Asbestos Felts
and Flashings
Testing of the existing roofing membrane
and flashings should include a check
to verify whether either of these materials
contains asbestos. Special removal techniques
and waste disposal procedures are
regulated by government agencies.
Evaluate the Existing Bituminous
Membrane if Well Adhered to the
Structural Deck
If an existing bituminous membrane is
present, is found to be tenaciously adhered
to the deck, and is in good condition, one
option may be to retain it rather than
remove it. This membrane may provide a
reasonable temporary protection, but it
needs to be evaluated in terms of its interface
with a new roof assembly. If a welladhered
bituminous membrane needs to be
removed for any reason, major deck repair
is almost certain to be required. In all likelihood,
the condition of the bituminous
membrane relates directly to the condition
of the structural deck.
Consider Code and Insurance
Requirements
Building code provisions for roofs need
to be carefully reviewed on older buildings.
The codes have established wind-resistance
and fire-rating classifications. Applicable
specifications should be followed based
upon code- and insurance-prescribed ratings.
Roof replacement normally deals with
external fire exposure, but if the deck is
included in the repairs, then the overall
ceiling, deck, and membrane systems need
to be considered together.
Consider Warranty Issues:
What Is Not Warranted?
Roofing manufacturers will warranty
the performance of products they manufacture
and supply. However, they normally do
not warranty the condition of the existing
structural deck or how roofing materials are
adhered to the deck. The design professional
is required to verify the condition of the
deck and method of attachment of the roof
assembly.
Consider Insulation Requirements
The addition of insulation to the roof
assembly may be desired but it may not be
necessary if an attic space exists. Then, the
choice to insulate the attic rather than the
roof system is a more viable option in order
to meet energy code requirements. Insu –
lation can also be used in the roof assembly
Figure 6 – How clay tile was used to cover structural steel members to provide fire
protection.
8 • IN T E R FA C E A P R I L 2009
where the substrate is uneven and irregular
to provide a smooth, uniform surface for the
roof membrane.
Perform a Dew Point Analysis of the
Roof Assembly
A dew point and thermal analysis should
be performed to determine if condensation
would form within or on the underside of the
structural deck, as is possible with a roof
membrane in a cold climate.
Consider Mechanical Fastener Types
Carefully
Withdrawal or pullout tests are useful to
verify the holding strength and load capacity
of mechanical fasteners. These tests
assist in evaluating potential problems with
anchorage of components such as wood
blocking or prefabricated curbs to the substrate.
Usually, conventional expansion
anchors do not work well in cinder concrete
or older lightweight concrete be cause of
marginal concrete strength. Drill ing into
clay tile often results in spalling or cracking
of the interior face of the top shell. The
material the fastener is made from, along
with the coating, if present, can greatly
affect the long-term performance of the fastener.
Including a fastener manufacturer
early in the process is recommended to help
identify which type of anchors will work.
Not knowing in advance of construction
whether special fasteners are needed can
easily increase costs significantly.
CONCLUSIONS AND SUMMARY
The substrates of older structures need
to be carefully examined in order to formulate
a successful reroofing design. This plan
will be significantly influenced by the type
and condition of the roof deck and by the
nature and condition of adjoining parapet
wall systems. The roof of every building is
unique, and a project-specific, holistic
assessment of conditions is necessary to
predict the manner in which to implement
repairs as well as to make an informed
selection of roofing system type.
Repair concepts on historic buildings
often have structural performance implications.
This necessitates involvement of a
qualified structural engineer during the
design phase to check the feasibility and
constructability of those repairs. This step
is important because the means and methods
of repair are variable and are often limited
by site and time constraints and by
potential variability in workmanship. Trial
repairs or mock-ups should be initiated, as
this will help reveal problems and conditions
that can only be identified during the
physical act of construction and will also
permit evaluation and refinement of proposed
repair details.
The reroofing design plan should be
guided by investigative findings and proper
evaluation of all of the required design considerations.
Economic constraints are often
imposed on the designer. However, those
constraints should not compromise professional
opinions or technically appropriate
decisions reached when proper analysis has
been performed. The success of a new roofing
system on an older historic building will
ultimately be determined by the thoroughness
of the investigation and care taken in
selection and implementation of an appropriate
repair or reroofing solution.
EDITOR’S NOTE: The original version of
this article was published in the Roofing
Handbook for Historic Buildings (Wash –
ington, DC: Historic Preservation Edu –
cation Foundation, 1999).
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A P R I L 2009 I N T E R FA C E • 9
REFERENCES
George A. Hool and Nathan C. Johnson,
Handbook of Building (Construction
Data for Architects, Designing and
Construction Engineers, and Con –
tract ors), New York, New York:
McGraw-Hill Book Company, Inc.,
1920.
John Mulligan, Handbook of Brick Ma –
sonry Construction, York, Pennsyl –
vania: The Maple Press, McGraw-
Hill Book Company, 1920.
Harry Parker and Frank E. Kidder,
Kidder-Parker Architects and Build –
ers Handbook, New York, New York:
John Wiley and Son, 18th edition,
1950.
Harry C. Plummer and Edwin E. Wan –
ner, Principles of Tile Engineering,
Washington, DC: Structural Clay
Products Institute, 1947.
Charles George Ramsey and Harold
Reeve Sleeper, Architectural Graphic
Standards, 1932 edition, New York,
New York: John Wiley & Sons, Inc.,
1932.
“Time Saver Standards, a Manual of
Essential Architectural Datam,”
Archi tectural Record, New York, New
York: E.W. Dodge Corp., 1946.
10 • I N T E R FA C E A P R I L 2009
Richard S. Koziol is a licensed architect and principal with
Wiss, Janney, Elstner Associates, Inc. (WJE), in Northbrook,
Illinois. He specializes in the investigation and repair of
water-infiltration problems in buildings. He has been
involved with many roofing and waterproofing projects in
both historic and contemporary structures. Notable projects
include the Arizona Science Center, Hawken School
Natatorium, Parkside Elementary School, and the Lowe’s
Corporate Headquarters Building. Koziol has provided peer
review of roofing and waterproofing systems for architects, contractors, and owners. He
has also authored several articles and papers on roofing systems and has made numerous
technical presentations on roofing and plaza system waterproofing technology to
various technical and professional societies.
Richard S. Koziol, AIA, NCARB
You’re traveling and realize an important file you need is back at the office on your
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Christopher W. Giffin is a licensed architect who specializes
in the diagnosis and repair of building envelope problems. He
has been involved with many roofing and waterproofing projects
relating to both historic and contemporary structures.
Notable projects include the Candler Building, The Grove
Park Inn Resort & Spa, Chicago Public Schools, and the U.S.
Cellular Field. Mr. Giffin has performed numerous building
envelope condition assessments and investigations, including
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of several new or renovated roofing and waterproofing systems.
Christopher W. Giffin, AIA, NCARB
Gregg Marshall, CPMR, CSP, is a speaker, author, and consultant. He can be reached by e-mail at
gmarshall@repconnection.com, or you may visit his Web site at http://www.repconnection.com.
FORGET THAT FILE AT WORK?