Traditional Clay Tile Roofing: Investigation And Rehabilitation

1. INTRODUCTION
Traditional clay tile roofs combine both
modern and time-proven materials with traditional
craftsmanship to produce what can
be one of the most durable, aesthetically
pleasing, and architecturally distinct steepslope
roofing systems. On the downside,
designing and constructing clay tile roofs
present technical and aesthetic pitfalls that
can defeat the most durable materials. For
example, an inappropriately selected tile
color or finish can unacceptably alter the
appearance of a building, and a poorly
designed or constructed roofing system can
be quickly destroyed by material failure,
leakage, or wind uplift.
Achieving maximum durability requires
careful material selection, meticulous detailing,
and construction by knowledgeable
and diligent craftsmen. This paper presents
practical advice to help designers and builders
conceive, design, and construct durable
clay tile roofs. Several aspects of clay tile
roofing are covered in a restoration and
rehabilitation context, including investigation
of existing clay tile roofs to determine
causes of failure; selection of tile materials,
geometries and finishes; selection and
detailing of flashing and membrane underlayment;
and review of attachment methods.
Advice and recommendations based on
the authors’ experience are also included to
illustrate successful design and building
practices for clay tile roofs.
2. INVESTIGATION
Prior to beginning rehabilitation design,
a careful investigation of the clay tile roof is
required. The purpose of this step is to document
the condition and performance of
the roof and collect necessary product information
for the design. Field investigations
typically include an interior condition survey
to locate roof leaks; water testing to
track leakage paths into the building; and
exploratory openings in the roof to document
the configuration and condition of its
components. We discuss the critical steps
for a field investigation below.
2.1 Document Review and Preparation
for Field Work
In preparation for field work, the designer
should collect and review existing documentation,
such as original construction
documents, photographs, maintenance
logs, and repair histories pertaining to the
roof. Pre-planning for field work is a comprehensive
process involving multiple
tasks:
• identification and coordination of
appropriate access equipment;
• selection of exploratory opening
locations;
• selection of water test locations;
• coordination with a skilled roofing
contractor to assist with access and
make and repair openings;
• review of building plans, photographs,
and
• a pre-investigation visit.
Information shown on existing building
drawings (such as details, roof slope, tile
manufacturer, or prior repairs and modifications)
is helpful for preliminary planning,
but must be verified during the field investigation.
2.2 Field Investigation
Once the pre-planning work is complete,
the first step of the field investigation
is an interior building survey, starting with
interviews of building occupants to locate
roof leaks. The information collected during
this survey will serve as a “road map” to
help plan water tests and sample openings
of exterior roof features.
Exploratory openings are required to
document the roof’s as-built construction
and will make up the bulk of the field investigation.
Documentation of sample openings
must include necessary information to
design and detail roof repairs and should
include photographs, notes, and sketches.
The designer should make openings at all
typical roof locations, including roof eaves,
rakes, valleys, hips, ridges, roof penetrations,
and rising walls, as well as at unique
roofing features that will require special
detailing. Generally, the larger an exploratory
opening is, the more information it
reveals and the easier it is to view the detail
configuration. In areas that have been subject
to water leakage, sample openings will
reveal concealed damage, such as deteriorated
deck, that must be accounted for during
design.
During the investigation, clay tile specimens
should be removed for identification
of tile type and configuration, manufacturer,
exterior finish, and color range.
Preparation of custom tile finishes and
geometries to match existing tiles requires
considerable lead times, so starting this
process early in the rehabilitation design is
prudent. Tile specimens can also be tested
to determine relevant material properties
(see below). This step is necessary to evalu-
20 • I N T E R FA C E DE C E M B E R 2006
ate the tiles’ in-service performance and
their potential for reuse in repairs or reroofing.
Although a discussion of the
requirements for re-using tiles is beyond
the scope of this paper, we note that as a
result of the significant market for finding
matching replacement tiles and relative
ease of salvage, a large cottage industry has
arisen for salvaged, re-used clay tile.
Advertisements for used tile brokers can be
found in restoration magazines and Web
sites. We recommend laboratory testing of
any salvaged tile, prior to bulk purchase, to
assess its anticipated residual durability. If
the tile tests well, it may provide an economical
approach to finding durable, wellmatched
replacements to tile that are no
longer available.
In some cases, water tests that track
existing leakage paths to the interior are
required in order to determine the causes of
premature roofing failures and to pinpoint
roof details that must be reconfigured during
the rehabilitation design.
3. COMPONENTS OF CLAY TILE ROOFING SYSTEMS
Multiple individual components make
up clay tile roofing systems, including
slope, deck, underlayment, flashings, and
attachments. Each component can impact
the overall performance of the roofing system,
and it is important to understand the
role that each plays.
3.1 Roof Slope
All clay tile roofs must have sufficient
slope to shed rainwater off the assembly.
Minimum slope requirements vary by manufacturer
and tile geometry but generally
range from 3:12 to 5:12. While the 2003
edition of the International Building Code
(IBC 2003) requires a minimum slope of 2-
1/2:12, we recommend a minimum roof
slope of 4:12, consistent with National
Roofing Contractors Association (NRCA) tile
slope recommendations, to promote
drainage and improve the reliability of the
assembly in regions that are subject to
snow accumulation. At lower slopes, water
does not drain as promptly, more water is
prone to bypass the outermost surface of
the tile and reach the underlayment, and
thus, the performance of the roofing assembly
becomes more heavily reliant on the performance
and reliability of the underlayment,
particularly at fastener locations.
Consequently, where existing roof slopes
are 4:12 or less, providing self-adhered
membrane underlayment is especially
important to limit the risk of leakage; see
the discussion below.
Most tiles do not have maximum slope
limits. However, tiles installed over very
steep slopes – 18:12 or greater – are prone
to “chatter” (i.e., they rattle in windy conditions)
unless special attachment provisions,
such as wind clips and adhesives at the
nose of the tiles, are included in the design
to restrain movement. Chatter may also
occur due to local wind conditions on roofs
with lesser slopes.
3.2 Roof Deck
Proper selection of the deck is critical to
the installation and performance of a tile
roof assembly. The deck must support construction
loads along with code-required
dead and live loads, provide a continuous
substrate for the membrane underlayment,
supply adequate structural capacity for the
tile attachment, and meet code-required fire
resistance. Fire code considerations are not
discussed in this paper. In historic buildings,
we commonly encounter lightweight
cinder concrete decks, pre-cast concrete
planks with or without lightweight cinder
topping, continuous wood plank decks, discontinuous
wood “skip” sheathing, or discrete
metal or wood bars.
3.2.1 Concrete Decks
Poured-in-place lightweight concrete
decks and pre-cast concrete planks provide
both continuous support and a stable work
platform and may even add some insulation
value to the roof assembly. Unfortunately,
while lightweight concrete readily accepts
nails, it provides little resistance against
nail withdrawal and frequently cannot provide
code-specified wind uplift resistance
without more complex fastening arrangements,
such as screws or adhesive anchors.
We have seen other instances where the
concrete deck was too hard to accept roofing
nails or fractured during nail driving.
Because of these limitations, we prefer to
cover existing concrete decks with plywood
to facilitate better nail pull-out resistance.
3.2.2 Wood Decks
In older buildings, decks commonly
consist of tongue–and-groove wood decking.
For new design, plywood is typically used.
Both make for an excellent roof deck.
• Plywood must be rated for structural
use as roof sheathing by the
Engineered Wood Association, formerly
the American Plywood
Association (APA) and
conform to standard
PRP-108. Other panelized wood
products, such as oriented strand
board (OSB), are much less durable
than plywood and are not appropriate
for use in heritage buildings.
Even though designers intend
sheathing to remain dry in service,
we recommend kiln-dried, preservative-
treated plywood to provide protection
against unintended leakage
or exposure. Use of preservativetreated
wood requires additional fastener
considerations; see the attachment
section below. The plywood
attachment, span, and thickness
must be designed to withstand wind
uplift and accommodate service
loads. Traditional rules for plywood
installation still apply, including
providing slight gaps between adjacent
sheets to accommodate wood
expansion and avoid buckling of the
sheathing. Plywood also requires
special considerations when
installed below vapor impermeable
substrates, such as self-adhered
membrane underlayment, and
should be kiln-dried to avoid trapping
moisture within the roof
assembly. See also our discussion of
self-adhered membrane underlayment
below.
• Wood plank decking and/or tongueand-
groove wood decking must
accommodate many of the design
considerations discussed above for
plywood. Historic tongue-and-groove
decking ranges from 3/4-inch thick
on many residential applications, up
to between 2 and 3 inches thick on
some mills and other industrial
buildings. Tongue-and-groove sheathing
tends to have narrow, shallow
gaps between individual boards,
whereas plank or skip sheathing
often has wide gaps (often up to 1
inch wide) between boards. Large
gaps and “skip sheathing” are not
acceptable substrates for
new or rebuilt roof assemblies
because
the mem-
DE C E M B E R 2006 I N T E R FA C E • 2 1
brane underlayment sags into these
voids and causes membrane seams
to open, which leaves the membrane
vulnerable to leakage. Further, outof-
plane irregularities and sharp
edges of boards can cut or stress
and prematurely wear membrane
underlayments over time. In cases of
skip sheathing or plank sheathing
with wide gaps, we recommend covering
with plywood to provide a
smooth, clean, uniform substrate
for membrane underlayment. For
optimum adhesion of the membrane,
prime the plywood and seal
gaps between boards to prevent
bridging and provide for continuous
adhesion. Similarly, clean, prime,
and seal gaps in tongue-and-groove
sheathing prior to membrane installation
for optimum adhesion and
performance.
3.2.3 Other Decking Systems
In some historic buildings, clay tiles are
wired directly to horizontal metal or wood
bars that span continuously between roof
trusses or rafters. These traditional systems,
which have no continuous substrate
sheathing or underlayment membrane, are
inherently less reliable in their waterproofing
performance than similar systems with
continuous underlayment and sheathing.
In some cases, depending upon the interlocking
geometry and watertightness of the
particular tile, they can provide sufficient
waterproofing performance over unoccupied
and unfinished areas, such as attics,
which have some tolerance for water leakage.
However, they are vulnerable to winddriven
rain, particularly at flashing transitions.
In rehabilitation work, it is advisable
to add continuous sheathing and membrane
underlayment to improve the reliability
of the system; however, the additional
weight of the sheathing and underlayment
must be considered during the design.
3.3 Underlayment
Clay tile roofs are water-shedding systems.
The tiles interlock and/or overlap to
intercept and shed most water off of the roof
assembly. However, most clay tile installations
alone are far from watertight and
inevitably allow some water to pass through
the tile joints, particularly at roof perimeter
conditions, such as hips, valleys, ridges,
and eaves, and at roof penetrations. Membrane
underlayment is typically needed to
collect this water and conduct it to the exterior
along the roof eaves and valleys.
When evaluating roof underlayment,
durability of the membrane
is an important
design consideration.
Good quality clay tiles
have a typical service
life of 75 years or longer;
ideally, the quality of the
membrane underlayment
must match this
life expectancy. We have
seen instances where
non-durable or poorly
detailed and installed
membrane underlayment
resulted in leakage
and required reconstruction
of the entire
roof well before the clay
tile reached the end of
its service life.
3.3.1 Asphalt-saturated
Felt
Asphalt-saturated
felt is a traditional
choice for clay tile roof
underlayment and has a
long track record of performance.
Felt is
straightforward to roll out, install in shingle-
lap fashion, and attach to the roof deck.
For attachment, we prefer button cap nails
over staples because staples penetrate in
two places and frequently tear the felt at
fastener locations, which reduces the reliability
of the membrane underlayment.
IBC 2003 requires that underlayment
conform with ASTM D 226, Type II (No. 30
asphalt-saturated felt), ASTM D 2626, or
ASTM D 249 Type I mineral-surfaced roll
roofing. Furthermore, IBC 2003 requires
two layers of underlayment for low-slope
applications. Low slope is defined as
between 2-1/2:12 and 4:12. We prefer two
layers of ASTM D 226, Type II membrane
underlayment for increased durability and
redundancy against leakage. Asphalt-saturated
felts will embrittle with age, particularly
if they have been exposed for an
extended time prior to clay tile installation,
and are prone to tearing, especially at ridge,
hips, and other details where the membrane
is creased; see Photo 1.
For roof slopes of 3:12 and greater, IBC
2003 also requires an additional layer of 36-
in.-wide [ASTM D 226] Type I membrane
underlayment at valley locations. This additional
layer is intended to provide added
protection from leakage and membrane erosion.
We prefer self-adhered membrane
22 • I N T E R FA C E DE C E M B E R 2006
Photo 1 – Torn felt membrane underlayment. Asphalt-saturated felt embrittles with age and becomes prone to
tearing.
underlayment, regardless of the roof slope,
below roof perimeter conditions, and areas
that conduct large volumes of water, such
as valley flashings. Self-adhered membrane
underlayment, when properly lapped and
installed, provides a more reliable waterproofing
layer than loose-laid felt, and
resists erosion of asphalt oils from frequent
runoff or standing water that contribute to
premature, asphalt-saturated, felt degradation.
See below.
3.3.2 Self-Adhered Membrane
Self-adhered membrane underlayments
(typically rubberized asphalt on a polyethylene
or fiberglass carrier) for roofing applications
may include features such as slip- or
abrasion-resistant surfaces, reinforcement
to resist tearing, or formulations to resist
ultraviolet degradation. Due to the wide
variety of available self-adhered membranes,
we limit our discussion to general
principles applicable to most self-adhered
membranes.
Self-adhered membranes provide improved
weather resistance over felt underlayments,
mainly because they provide
some self-sealing capabilities at fastener
penetrations, have reasonably watertight
membrane-to-membrane seams, and fully
adhere to the deck (creating isolation of any
small leak). Together, these characteristics
provide greater protection against leakage
in low-slope applications in all climates,
and from water ponding behind ice dams in
cold climates, than does shingle-lapped felt
underlayment.
In cold climates, self-adhered membrane
underlayments should be used and
may be required by code, for ice dam protection.
Felt membrane underlayment must
overlap the self-adhered membrane underlayment
if self-adhered membrane underlayment
is used for ice dam protection. A
full discussion of ice dams and ice dam protection
is beyond the scope of this paper.
For maximum waterproofing performance,
self-adhered membrane underlayment
must be installed without gaps, wrinkles, or
fishmouths (small, tunnel-like openings in
lap seams at wrinkled membranes).
Most membrane underlayments, and
particularly self-adhered membranes with
polyethylene carrier sheets and their
release paper, are slippery to walk on. Wet
membrane underlayment frequently occurs
due to rain, dew, or frost. Under slippery
conditions, foot traffic should be prohibited,
and release paper should always be
promptly removed from the roof during
installation. Many self-adhered membrane
manufacturers also make products with
textured facers to improve slip resistance.
In hot conditions, we have seen instances
where the modified asphalt melted and
caused the carrier sheet to slip when
stepped on.
Self-adhered membrane must also be
protected from exposure to sunlight and
must not be used in permanently exposed
locations since most membranes will
degrade in sunlight. Allowable exposure is
generally limited to about 30 days, although
limitations vary by manufacturer and membrane
type.
Additionally, self-adhered membranes
form an effective vapor retarder and may
cause condensation unless the roof assembly
is properly vented. Although a discussion
of vapor retarders in roof assemblies is
beyond the scope of this paper, we note that
potential applications where a significant
proportion of the roof deck is covered with
self-adhered membrane should be analyzed
during the design phase to assess their condensation
potential.
DE C E M B E R 2006 I N T E R FA C E • 2 3
3.3.3 Hybrid
Underlayment
Where the potential
for condensation does
not preclude the installation
over the entire deck, we recommend
providing a layer of selfadhered
membrane installed over the
roof deck and covering it with a layer of
asphalt-saturated felt. This arrangement
provides the advantages of both systems,
including tighter seals at fasteners and icedam
protection provided by the self-adhered
membrane.
The felt, on the other hand, has a longer
allowable exposure time and protects the
self-adhered membrane from UV exposure.
To a limited degree, it also protects it from
wear and tear during roofing installation
(e.g., protection from dropped nails and
tools), can provide a more slip-resistant
work surface, and provides waterproofing
redundancy.
3.4 Flashings
Flashings provide durable waterproofing
in exposed locations and avenues that conduct
high volumes of water such as open
valley flashings, eave flashings, exposed
counterflashings, and roof transitions.
Flashings are typically exposed and must
be UV-resistant. Flashings must also be
carefully integrated with the underlayment
to provide continuous waterproofing while
resisting corrosion and premature wear.
Flashings are made almost exclusively from
metals and must be carefully selected and
detailed for durability.
While IBC 2003 allows a minimum
flashing thickness of 0.019 inches, we find
that more robust flashings are required to
match the expected service life of a clay tile
roof. Typically, 16- or 20-oz. copper is sufficient
for most flashing areas that are not
subjected to concentrated water run-off.
High-flow areas, such as valleys, require
thicker flashings to provide a reasonable
service life.
To be durable and reliable over the longterm,
metal flashings should be solderable
and non-corroding. Aluminum is not solderable,
and galvanized steel flashings
readily corrode at cut edges, and thus, do
not meet these criteria. Copper (including
coated coppers such as zinc-tin, alloy-coated
copper) or stainless steel are two metals
that meet these criteria and that we recommend
and use for flashings.
• Copper is the traditional flashing
material of choice for its exceptional
workability, durability, and in-service
performance record. Unlike aluminum,
copper can be soldered to
provide watertight and durable
flashing connections. “Red” or uncoated
copper will turn brown, then
greenish blue, and eventually green
with patina due to oxidation and
natural weathering. (Refer to Revere
Copper’s Copper and Common Sense
for a more complete discussion of
the patination process.) This process
varies by region and exposure and
occurs at different rates for different
copper surfaces. The green patina
can also bleed onto walls or other
building components and stain
them. Effective management of copper
runoff is essential to prevent
such staining. Lead- or zinc-coated
copper resists patina formation and
staining as long as the coating
24 • I N T E R FA C E DE C E M B E R 2006
remains intact. Environmental and
health concerns about lead runoff
preclude the use of lead-coated copper
in most applications. Zinc-coated
copper does not have the lengthy
performance history of lead-coated
copper but provides a similar
appearance, supposedly without
environmental concerns.
• Stainless steel is another durable
flashing material because of its
exceptional corrosion resistance,
including resistance to the more corrosive
wood preservative treatments
used today. Unlike galvanized steel
flashings, stainless steel is equally
durable at all cut edges and at
scratches. Stainless steel can be soldered
watertight, but is tougher to
work with than copper, and some
consider its shiny appearance aesthetically
objectionable. However, it
may be aesthetically appropriate
and desirable for modern/contemporary
buildings.
IBC 2003 and the NRCA Roofing and
Waterproofing Manual permit other metals,
such as aluminum and galvanized steel, but
these lack the durability of copper or stainless
steel, and we consider them inappropriate
for use in monumental buildings.
We recommend that flashing design
include provisions to facilitate maintenance
and eventual roof replacement, such as
removable skirt components on throughwall
flashings.
3.5 Attachment
Clay tile attachment is critical to the
overall roof performance and must account
for wind loads while maintaining a durable
and weathertight roofing system. IBC 2003
offers a methodology to determine the aerodynamic
uplift moment acting to raise the
nose of the tile and describes limitations for
its use, which may be used to determine
adequate attachment provisions. Two common
methods of clay-tile attachment permitted
by IBC 2003 are adhesives and
mechanical fasteners. In some traditional
applications, clay tiles were simply hung
from horizontal battens; in other installations,
tiles were fastened or wired to battens.
3.5.1 Battens
Some clay tiles, such as pan and cover
tiles, require wood battens (i.e., strips of
wood set on or over the structural deck
used to elevate and/or attach the tile roof
covering) to attach the cover tiles. Wood battens
are typically outboard of the roof
underlayment, i.e., they are expected to
endure some moisture exposure and must
be preservative-treated to resist deterioration
in service. Today’s commonly available
wood preservatives are more corrosive than
their predecessors and require special fastener
considerations; see the attachment
discussion below.
Toe-nailing to the deck is the traditional
way to secure battens, but we have found
that metal angle brackets provide for more
reliable attachment because they avoid
splitting the wood batten, a common problem
where toe nails are installed with little
edge distance.
3.5.2 Adhesive Attachment
We have seen mortar, aphaltic mastics,
and spray foam used as clay tile adhesives.
One drawback of all adhesives is that they
inhibit drainage by reducing or eliminating
the “drainage plane” free space between the
underside of the tile and the underlayment.
Spray foams can provide tenacious adhesion
to many substrates;
many are “Dade County Approved”
for use in Florida’s
high wind hurricane zones.
Some foam manufacturers
have training programs that
address typical challenges of
spray-foam adhesive, including
a “blind” installation procedure
that does not allow a
visual review of the adhesive
after tile installation, code limitations
on the permitted
adhesive contact area, unpredictable
expansion patterns,
and proper clean-up of overspray.
Spray foams have a limited
performance history and are
not yet proven to match the
expected service life of clay tile
roofs. Additionally, most spray
foams cannot be installed in
cold temperatures, which limits
their use to a short construction
season in cold climates,
and they have a limited
track record in freeze/thaw
exposure. They do not adhere
to some underlayments, especially
membranes with a polyethylene
carrier sheet.
Because of their limited track
record and the aforementioned
difficulties, we recommend using mechanical
fasteners to provide adequate tile
restraint, and using spray foam only as a
supplemental measure to reduce “chatter”
of the tiles in strong winds.
On the other hand, mortars and mastics
are traditional materials. Their traditional
use was typically not as a primary means of
attachment, but rather as a “hole filler” for
edge tiles at rakes and valleys; to provide a
closed, finished appearance; and to reduce
nesting of insects beneath open tile edges.
Mortars tend to have very limited adhesion
to clay tiles. Mortar should not be used as
primary clay tile attachment but can be
used as a supplement to mechanical fasteners
to reduce chatter and reduce nesting of
nuisance insects (e.g. hornets, wasps) in
edge voids. If left exposed or subject to
water run-off, mortar may also produce
unsightly white efflorescence staining.
Mortar is also susceptible to freeze/thaw
damage under these conditions and must
be evaluated for durability when exposed to
such conditions in service.
Photo 2 – Fastening eave and valley tiles (not shown)
with wire ties can avoid penetrations through metal
flashings, which would result in less reliable
waterproofing performance.
DE C E M B E R 2006 I N T E R FA C E • 2 5
3.5.3 Mechanical
Attachment
Mechanical fasteners,
such as nails, screws, wire
ties, and nose clips, have a
long performance history
with clay tile roofing and they
remain the most reliable attachment
method. Mechanical
fasteners must be installed
to achieve sufficient
deck penetration but should
not strain the tile, which can
cause breakage. Clay tiles
should “hang” from the fasteners
to allow some movement.
Two fastener materials
meet the corrosion resistance
and durability required with
clay tile roofs: copper and
stainless steel. Galvanized
fasteners typically do not provide
reliable corrosion resistance over the
anticipated life of the tile (e.g., 75 years),
especially when in contact with commonly
available, corrosive wood preservative treatments.
A full discussion of corrosive wood
preservative treatments is beyond the scope
of this paper.
Traditionally, clay tiles are held in place
with one or two fasteners at the head or
along the edge of rake tiles. IBC 2003 has
numerous prescriptive fastener requirements,
including using a minimum of 11-
gauge fasteners, 5/16-inch diameter heads,
and sufficient length to penetrate the deck a
minimum of 3/4-inch or through the thickness
of the deck. IBC 2003 also provides a
table to describe the minimum fastener
requirements to simplify design efforts at
some basic wind speeds and roof heights.
3.5.3.1 Nails and Screws
Nails are generally the most straightforward
type of fastener to install. We recommend
the use of ring-shank or spiral-shank
nails, as they provide additional withdrawal
(“pull-out”) resistance, at minimal increase
in material cost. Copper nails have long
been a traditional fastener choice due to
their corrosion resistance and ease of
installation. Copper nails are also easy to
Photo 3 – Nose clips are used to resist uplift forces on tiles. The photo
shows both a wire nose clip in the field of the roof (top) and a custommanufactured
nose clip along the roof eave (bottom). Custom-manufactured
nose clips may be required to provide adequate stiffness and length to
fasten above, rather than through, metal flashing along roof eaves.
26 • I N T E R FA C E DE C E M B E R 2006
Photo 4 – Delamination in clay tile as a result of freezethaw
damage. Testing of clay tiles prior to selection and
installation can help avoid such damage.
cut to accommodate later tile removal and
replacement. Unfortunately, thin copper
nails may bend during installation into
hard substrates, such as concrete and plywood
decks. Stainless steel nails are a
stiffer and more corrosion-resistant alternative
to copper, and they rarely bend.
However, stainless steel fasteners are tough
to remove to allow later piecemeal tile
replacement; they usually must be cut off
with a hacksaw. Screw fasteners typically
provide better pull-out resistance than nails
but take longer to install and make later
repair or replacement much more difficult
and time consuming.
3.5.3.2 Wire Ties
Eave tiles and tiles adjacent to valley
flashings are frequently wired into place;
i.e., corrosion-resistant wire is fastened to
the deck beyond the flashing and the tiles
are hung into position (Photo 2). This configuration
avoids fastening through flashings,
which compromises their watertightness.
3.5.3.3 Nose Clips
Supplemental clips to hold down the
front edge of the tile, commonly called nose
clips, are particularly important in high
wind regions because they provide two
points of tile attachment – one at the head
and one at the nose – to resist uplift force.
IBC 2003 requires nose clips at eave tiles
under some circumstances. While not
required by IBC 2003, we recommend the
use of nose clips in the field of the roof to
provide greater resistance to tile uplift in
high-wind regions. Numerous proprietary
nose clip products are available. Designers
must select nose clips with adequate stiffness
to hold the tile nose in place; long, thin
wires or sheet-metal clips are often too flexible
for this purpose.
Nose clips along eaves frequently present
additional problems because adequate
clip stiffness and the desire to fasten the
clip away from the eave flashing generally
oppose each other. In these cases, we have
successfully used custom-manufactured,
heavy-gauge stainless steel or copper nose
straps to provide tile restraint along eaves
(Photo 3).
3.6 Clay Tiles
Durability is the most important quality
for clay tile selection. The tiles must be
appropriate for their intended geographical
location and use. Proven performance in
similar climates and applications is one way
to gauge tile quality and designers should
visit such installations if possible. In addition
to a review of in-service performance,
ASTM C 1167, the Standard Specification
for Clay Tile Roofs, includes tests to help
gauge the expected durability of tiles. We
discuss some of these tests below. (Photo 4)
3.6.1 Testing
ASTM C 1167 classifies clay tiles into
grades for durability, based on their resistance
to frost action. Tile grades range from
Grade 1, representing significant resistance
to severe frost action; to Grade 3, with only
negligible resistance to any frost action. A
map in ASTM C 1167 recommends tile
Photo 5 – Some common tile shapes include Pan and Cover or Straight Barrel Mission Tile (left), Spanish or “S” Tile (center) and Flat
Interlocking Tile (right). Mission tiles are also available with tapered profiles. Numerous other tile shapes are available. Contact tile
manufacturers for information on available shapes.
DE C E M B E R 2006 I N T E R FA C E • 2 7
grade by region. At
a minimum, designers
should adhere
to the grade recommendations
of
ASTM C 1167. We
recommend only
specifying and using
Grade 1 tile in
northern climates,
and whenever possible,
we prefer to
use Grade 1 tiles for
increased durability
in less severe climates.
The “gateway”
grade requirements
of ASTM C 1167 are
based on the tile’s
cold-water absorption
and saturation
coefficient, which
compares coldwater
absorption to
boiling-water absorption
and is
used to gauge the
freeze/thaw resistance
of the tiles.
Gateway requirements
for the performance grades were
established to limit the need for freeze/thaw
tests, which may take ten weeks or longer to
complete and are often too long to allow
testing of the actual batch of tiles intended
for the project, while still meeting construction
schedules. We have found that these
gateway absorption and saturation coefficients
provide a good indicator of long-term
tile durability under freeze/thaw weathering
and general exposure. We have conducted
laboratory tests on existing tiles that
have failed as well as existing clay tiles that
have performed well for over 100 years and
found the absorption results to correlate
well with tile durability.
While not required by ASTM C 1167,
freeze/thaw testing according to ASTM C
67, Standard Test Methods for Sampling
and Testing Brick and Structural Clay Tile,
as modified per ASTM C 1167, subjects tiles
to freeze/thaw cycles similar to those
expected in service in northern climates.
Freeze/thaw testing is allowed by the standard
as a means to “prove” tile resistance to
severe weathering conditions if they fail to
meet the specified gateway absorption grade
requirements described above. However, the
50 freeze/thaw cycles may represent only 1-
2 years of actual exposure in many northern
climates in the U.S.; thus, the passing of
this test is not “proof” of long-term durability.
Transverse breaking strength of tiles
(i.e., bending strength) offers the best indicator
of resistance of the tile to breaking
due to impact (e.g., tree branches or ice
28 • I N T E R FA C E DE C E M B E R 2006
Photo 7 – Accessory tiles, such as the rounded hip closure tile shown above (or tiles for ridges, rakes, and starter
courses) are often available from manufacturers for various tile shapes and styles. In some cases, these require
special order and have considerably longer lead times than typical field tile shapes. Identify and order accessory
or custom tile shapes early to avoid delays during construction.
Photo 6 – The devolution of tile surface features: Three
generations of tile from the same manufacturer show loss of
surface articulation from a circa 1951 roof tile (left) to its
later (center) and contemporary (right) counterparts. On the
earliest tile (left), note the raised hub at fastener hole to
protect fastener hole from leakage and deeper drainage
channel (indicated by longer shadow from back leg of top
channel). These features are muted or eliminated on later
tiles to right.
falling onto the tile) and point loading (e.g.,
foot loads from workers walking on the
roof). The importance of breaking strength
data in selecting and specifying tile for a
roof should be assessed by the designer on
a case-by-case basis. On roofs with overhanging
trees, large roofs above that shed
ice to lower roofs, and rooftop elements
(e.g., painted dormers, chimneys, etc.) that
require frequent maintenance, the roof will
be subjected to more frequent and severe
field conditions that promote breakage than
roofs not having any of these characteristics.
While breaking strength is not a direct
indicator of durability due to freeze/thaw
weathering, on some tests of existing clay
tile we have noted an indirect correlation
between the two. Tile that are dense and
well-fired (highly vitrified) tend to test better
in terms of both their breaking strength
(creating greater resistance to impact), and
their absorption (creating greater resistance
to freeze/thaw deterioration).
3.6.2 Tile Profile
The shape and surface profile of an individual
tile also affect its durability and waterproofing
performance. Many different
stock tile shapes are commonly available,
some of which are shown in Photo 5. Consult
tile manufacturers’ literature for descriptions
and illustrations of available shapes.
Traditionally, many tiles included intricate
features to channel water out of the
roof assembly and limit fastener exposure
to the weather. Such features include ridges
to shield fastener holes and drainage channels
with weep slots to collect water and
direct it downward. Many modern tile
shapes either lack these traditional surface
features altogether or have muted versions
that are less effective at protecting tiles and
fasteners than their traditional counterparts.
Photo 6 shows several generations of
clay tiles that illustrate the devolution of
these surface features.
3.6.3. Reproduction Tiles
Reproduction of traditional tiles to
match specific existing tiles for restoration
projects frequently presents challenges.
Traditionally, clay tile manufacturers often
used labor-intensive fabrication methods to
produce intricate and irregular tile shapes
and finishes. While these techniques can
still be replicated, high initial set-up costs
and labor costs make large-scale production
of custom tiles too costly for most jobs.
Today, custom, hand-formed tiles are considerably
more expensive and take much
longer to procure than mass-produced production
line tiles, which may be stockpiled
for rapid distribution. For commonly available
field tiles, some manufacturers offer
“accessory” pieces, specially shaped for
common non-field conditions such as hips,
ridges, rake closures, etc. The rounded hip
closure tile in Photo 7 is one example of an
accessory tile shape. Where custom-shaped
tiles are required, plan ahead and allow
considerable lead time and additional cost
compared to standard tiles to allow for their
production and delivery.
Tiles are often available in many color
options and with numerous finishes. Some
variations in the finish texture are even
available, such as the depth and degree of
scored surface textures. Unfortunately,
these modern variations do not approach
the almost limitless variations available to
craftsmen of hand-formed and pressed tiles
of the past. As a result, matching traditional
tile finishes, while keeping to reasonable
production costs and schedules, is often
difficult. Existing clay tiles may include
formed edges, tile color variations due to
antiquated firing techniques, or surface finishes
that are no longer commonly used.
Manufacturers are often willing to vary
surface textures within the limits of their
production equipment, blend different tile
colors, or use special firing techniques that
introduce trace gasses or mists to produce
variation in finishes in an attempt to mimic
existing tile roofs. Some other features of
existing tiles, such as “battered” edges and
custom surface finishes, are more difficult
to procure in a cost-effective manner.
Generally, appearance options are limited
by budget, schedule, and the willingness
and capability of the manufacturer to modify
its production equipment and process to
replicate traditional tile features with reproduction
tiles.
4. OTHER CONSIDERATIONS
4.1 Hazardous Materials
Older clay tile installations frequently
include roofing cements or mastics containing
asbestos fibers. Susceptible locations
include hips, ridges, and eaves where
roofers relied on cement to adhere tiles or to
provide a “finished” appearance to the roof.
Similarly, some roofing felts may include
asbestos. A discussion of hazardous materials
is beyond the scope of this paper. The
roof designer should consult a qualified
industrial hygienist and the building official
for the jurisdiction to specify appropriate
precautions and abatement procedures.
Test your knowledge of roof drainage with the
following questions, developed by Donald E.
Bush Sr., RRC, FRCI, PE, chairman of the RRC
Examination Development Subcommittee.
1. Which factors must
be considered in the
design of roof
drainage systems for
steep-slope roofing
systems?
2. In sizing downspouts,
which considerations
apply?
3. In sizing gutters,
which considerations
apply for typical
section lengths of 8 –
10 feet?
4. The size of
rectangular gutters
depends on which
factors?
5. Which procedures are
used for scupper
sizing?
Reference: Architectural Sheet Metal
Manual — Fifth Edition
Answers on page 30
DE C E M B E R 2006 I N T E R FA C E • 2 9
4.2 Insect Infestation
Clay tile roofs, which typically include
large dry and concealed spaces, are notorious
as ideal habitats for several insect
species, including yellow jackets and hornets.
If the re-roofing project is undertaken
in the spring and summer when wasps are
most active, large swarms can injure workers.
If necessary, tile roof restoration projects
should include procedures for handling
wasp infestations. One proactive management
strategy entails baiting for wasps
in the early spring to limit the number of
“foundress queens,” wasps that go on to
establish nests and large colonies. Another
strategy is to seek out the “clusters” of displaced
wasps after roof removal during periods
of inactivity between evening and morning
to spray and eliminate them. Workers
should be prepared to encounter wasps
throughout the work and must make appropriate
preparations for personal protection.
EDITOR’S NOTE: This article was originally presented
as part of the Proceedings of the RCI 2006
Symposium on Building Envelope Technology on
October 31, 2006 in Washington, DC.
USEFUL REFERENCES
The following references are useful
when designing and installing clay tile
roofs:
Copper and Common Sense, published
by Revere Copper Products, Inc. This reference
provides descriptions and illustrations
for many metal flashing conditions, including
eave flashings, expansion joints, valley
flashings, and gutters.
The Clay Tile Installation Manual,
published by Ludowici Roof Tile. This reference
provides descriptions and illustrations
of clay tile installations for many common
tile types and includes recommendations
related to decking, underlayments, and fastening.
The NRCA Roofing and Waterproofing
Manual, published by the National
Roofing Contractors Association. This reference
provides descriptions of tile roofing
components, including clay tiles, with
installation recommendations and illustrative
figures.
Concrete and Clay Tile Installation
Manual for Moderate Climate Regions
and Concrete and Clay Tile Design Criteria
for Cold and Snow Regions, both
published by the Tile Roofing Institute
(www.tileroofing.org).
Nicholas A. Piteo is a senior engineer with Simpson Gumpertz
& Heger Inc. (SGH), a national design and consulting engineering
firm that designs, investigates, and rehabilitates
structures and building enclosures. Mr. Piteo has a breadth
of experience in the investigation, repair, design, and rehabilitation
of building envelopes, with a specialization in masonry.
He has performed numerous façade and roof investigations,
including clay tile roofing, for which he also provided
new designs. He received his Masters of Architectural
Engineering degree from Pennsylvania State University, where he won an award for best
structural thesis for his presentation on blast-resistant design of buildings.
Nicholas A. Piteo
Niklas W. Vigener, PE, is a principal with Simpson Gumpertz
& Heger Inc. He received his B.S. in civil engineering from
Clarkson University and his M.S. in structural engineering
from the University of California at Berkeley. Vigener is the
project manager on many of SGH’s most notable building
technology and historic preservation projects. He worked on
the rehabilitation of Yale University’s Sterling Memorial
Library and led the master plan condition assessment, historic
consultation, and design of comprehensive envelope
rehabilitation at the New York State Capitol Building. Mr. Vigener is also a lecturer in
the Department of Civil Engineering at Johns Hopkins University.
Niklas W. Vigener, PE
Answers to questions from page 29:
1. • Area to be drained
• Size of gutters
• Size of downspouts
• Size of outlets
• Slope of roof
• Type of building
• Appearance
2. • Downspouts of less than 7.00 sq. in.
cross section should not be used
except for small areas such as
porches and canopies.
• The size of the downspout should be
constant throughout its length.
• Downspouts should be constructed
with conductor heads every 40 feet
to admit air and prevent vacuum.
• Offset of more than 10 feet can
affect drainage capacity.
• The gutter outlet capacity should
suit the downspout capacity.
• The downspout size must suit the
bottom dimension of the gutter.
• Gutter capacity and length.
• Capacity of the inlet tube.
• Potential for water freezing.
• Appearance of downspout system.
• The greater runoff rate of a pitched
gutter.
• The downspout discharge location.
• The risk of gutter overflow from
insufficient drainage capacity.
• A scupper serving a designated roof
area.
3. • Spacing and size of outlet opening.
• Slope of the roof.
• Style of gutters to be used.
• Maximum length of gutters.
• Gutter support capability.
4. • Area to be drained.
• Rainfall intensity per hour.
• Length of gutter in feet.
• Ratio of depth to width of gutter.
5. • Determine the head of inches of
water (typically 1″ minimum by code)
at a point six feet back from the
scupper opening.
• Determine the roof drainage area in
square feet.
• Using rainfall intensity in inches per
hour (IPH), determine the discharge
capacity in gallons per minute (GPM).
GPM equals S.F of roof area time IPH
times 0.0104. The constant is 7.48
gallons per cubic foot divided by 12
inches per foot divided by 60
minutes per hour.
30 • I N T E R FA C E DE C E M B E R 2006