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Historic Preservation - Technical Procedures

Spectitle:

Preservation Briefs: 30 The Preservation And Repair Of Historic Clay Tile Roofs

Procedure code:

0732101S

Source:

Preservation Briefs 30, National Park Service, Pad

Division:

Thermal And Moisture Protection

Section:

Clay Tiles

Last Modified:

09/05/2012

Details:

Preservation Briefs: 30 The Preservation And Repair Of Historic Clay Tile Roofs



PRESERVATION BRIEFS:  30

THE PRESERVATION AND REPAIR OF HISTORIC CLAY TILE ROOFS

 

The link immediately below connects to the latest version of Preservation Brief 30:

 

http://www.nps.gov/history/hps/tps/briefs/brief30.htm


Anne E. Grimmer and Paul K. Williams

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

              Superintendent of Documents
              P.O. Box 371954
              Pittsburgh, PA  15250-7954

              GPO #024-005-01027-1
              $1.00

Please call the Publication Order Information Desk at 202/783-3238
to verify price and availability.  


***INTRODUCTION***

Clay tiles are one of the most distinctive and decorative historic
roofing materials because of their great variety of shapes, colors,
profiles, patterns, and textures.  Traditionally, clay tiles were
formed by hand, and later by machine extrusion of natural clay,
textured or glazed with color, and fired in high-temperature kilns.
The unique visual qualities of a clay tile roof often make it a
prominent feature in defining the overall character of an historic
building.  The significance and inherently fragile nature of
historic tile roofs dictate that special care and precaution be
taken to preserve and repair them.

Clay tile has one of the longest life expectancies among historic
roofing materials - generally about 100 years, and often several
hundred.  Yet, a regularly scheduled maintenance program is
necessary to prolong the life of any roofing system.  A complete
internal and external inspection of the roof structure and the roof
covering is recommended to determine condition, potential causes of
failure, or source of leaks, and will help in developing a program
for the preservation and repair of the tile roof.  Before
initiating any repair work on historic clay tile roofs, it is
important to identify those qualities important in contributing to
the historic significance and character of the building.

This Brief will review the history of clay roofing tiles and will
include a description of the many types and shapes of historic
tiles, as well as their different methods of attachment.  It will
conclude with general guidance for the historic property owner or
building manager on how to plan and carry out a project involving
the repair and selected replacement of historic clay roofing tiles.
Repair of historic clay tile roofs is not a job for amateurs; it
should be undertaken only by professional roofers experienced in
working with clay tile roofs.


***HISTORICAL BACKGROUND***

The origin of clay roofing tile can be traced independently to two
different parts of the world: China, during the Neolithic Age,
beginning around 10,000 B.C.; and the Middle East, a short time
later.  From these regions, the use of clay tile spread throughout
Asia and Europe.  Not only the ancient Egyptians and Babylonians,
but also the Greeks and Romans roofed their buildings with clay
tiles, and adaptations of their practice continue in Europe to the
present.  European settlers brought this roofing tradition to
America where it was established in many places by the 17th
century.

Archaeologists have recovered specimens of clay roofing tiles from
the 1585 settlement of Roanoke Island in North Carolina.  Clay tile
was also used in the early English settlements in Jamestown,
Virginia, and nearby St. Mary's in Maryland.  Clay roofing tiles
were also used in the Spanish settlement of St. Augustine in
Florida, and by both the French and Spanish in New Orleans.
Dutch settlers on the east coast first imported clay tiles from
Holland.  By 1650, they had established their own full-scale
production of clay tiles in the upper Hudson River Valley, shipping
tiles south to New Amsterdam.  Several tile manufacturing
operations were in business around the time of the American
Revolution, offering both colored and glazed tile and unglazed
natural terra-cotta tile in the New York City area, and in
neighboring New Jersey.  A 1774 New York newspaper advertised the
availability of locally produced, glazed and unglazed pantiles for
sale that were guaranteed to "stand any weather."  On the west
coast, clay tile was first manufactured in wooden molds in 1780 at
Mission San Antonio de Padua in California by Indian neophytes
under the direction of Spanish missionaries.

By far the most significant factor in popularizing clay roofing
tiles during the Colonial period in America was the concern with
fire.  Devastating fires in London in 1666, and Boston in 1679,
prompted the establishment of building and fire codes in New York
and Boston.  These fire codes, which remained in effect for almost
two centuries, encouraged the use of tile for roofs, especially
in urban areas, because of its fireproof qualities.  Clay roofing
tile was also preferred because of its durability, ease of
maintenance, and lack of thermal conductivity.

Although more efficient production methods had lowered the cost of
clay tile, its use began to decline in much of the northeastern
United States during the second quarter of the 19th century.  In
most areas outside city-designated fire districts, wood shingles
were used widely; they were more affordable and much lighter, and
required less heavy and less expensive roof framing.  In addition,
new fire-resistant materials were becoming available that could be
used for roofing, including slate, and metals such as copper, iron,
tinplate, zinc, and galvanized iron.  Many of the metal roofing
materials could be installed at a fraction of the cost and weight
of clay tile.  Even the appearance of clay tile was no longer
fashionable, and by the 1830s clay roofing tiles had slipped
temporarily out of popularity in many parts of the country.


***REVIVAL STYLES RENEW INTEREST IN CLAY ROOFING TILES***

By the mid-19th century, the introduction of the Italianate Villa
style of architecture in the United States prompted a new interest
in clay tiles for roofing.  This had the effect of revitalizing the
clay tile manufacturing industry, and by the 1870s, new factories
were in business, including large operations in Akron, Ohio, and
Baltimore, Maryland.  Clay tiles were promoted by the Centennial
Exhibition in Philadelphia in 1876, which featured several
prominent buildings with tile roofs, including a pavilion for the
state of New Jersey roofed with clay tiles of local manufacture.
Tile-making machines were first patented in the 1870s, and although
much roofing tile continued to be made by hand, by the 1880s more
and more factories were beginning to use machines.  The development
of the Romanesque Revival style of architecture in the 1890s
further strengthened the role of clay roofing tiles as an American
building material.

Alternative substitutes for clay tiles were also needed to meet
this new demand.  By about 1855, sheet metal roofs designed to
replicate the patterns of clay tile were being produced.  Usually
painted a natural terra cotta color to emulate real clay tile,
these sheet metal roofs became popular because they were cheaper
and lighter, and easier to install than clay tile roofs.

Clay roofing tiles fell out of fashion again for a short time at
the end of the 19th century, but once more gained acceptance in the
20th century, due primarily to the popularity of the Romantic
Revival architectural styles, including Mission, Spanish,
Mediterranean, Georgian and Renaissance Revival in which clay tile
roofs featured prominently.  With the availability of machines
capable of extruding clay in a variety of forms in large
quantities, clay tiles became more readily available across the
nation.  More regional manufacturing plants were established in
areas with large natural deposits of clay, including Alfred, New
York; New Lexington, Ohio; Lincoln, California; and Atlanta,
Georgia; as well as Indiana, Illinois and Kansas.

The popularity of clay tile roofing, and look-alike substitute
roofing materials, continues in the 20th century, especially in
areas of the South and West - most notably Florida and California
- where Mediterranean and Spanish-influenced styles of architecture
still predominate.


***EARLY TILES***

During the 17th and 18th centuries the most common type of clay
roofing tiles used in America were flat and rectangular.  They
measured approximately 10" x 6" x 1/2" (25cm x 15cm x 1.25cm), and
had two nail or peg holes at one end through which they were
anchored to the roofing laths.  Sometimes a strip of mortar was
placed between the overlapping rows of tile to prevent the tiles
from lifting in high winds.  In addition to flat tiles,
interlocking S-shaped pantiles were also used in the 18th century.
These were formed by molding clay over tapered sections of logs,
and were generally quite large.  Alternately termed pan, crooked,
or Flemish tiles, and measuring approximately 141/2" x 9 1/2" (37cm
x 24cm), these interlocking tiles were hung on roofing lath by
means of a ridge or lug located on the upper part of the underside
of each tile.  Both plain (flat) tile and pantile (S-shaped or
curved) roofs were capped at the ridge with semicircular ridge
tiles.  Clay roofing tiles on buildings in mid-18th century
Moravian settlements in Pennsylvania closely resembled those used
in Germany at the time.  These tiles were about 14"-15" long x
6"-7" wide (36cm-38cm x 15cm-18cm) with a curved butt, and with
vertical grooves to help drainage.  They were also designed with a
lug or nib on the back so that the tiles could hang on lath without
nails or pegs.

The accurate dating of early roofing tiles is difficult and often
impossible.  Fragments of tile found at archeological sites may
indicate the existence of clay tile roofs, but the same type of
tile was also sometimes used for other purposes such as paving, and
in bake ovens.  To further complicate dating, since clay tile
frequently outlasted many of the earliest, less permanent
structures, it was often reused on later buildings.


***CLAY TILE SUBSTITUTES***

In addition to sheet metal "tile" roofs introduced in the middle of
the 19th century, concrete roofing tile was developed as another
substitute for clay tile in the latter part of the 19th century. It
became quite popular by the beginning of the 20th century. Concrete
tile is composed of a dense mixture of portland cement blended with
aggregates, including sand, and pigment, and  extruded from
high-pressure machines.  Although it tends to lack the color
permanence and the subtle color variations inherent in natural clay
tile, concrete tile continues to be a popular roofing material
today because it reproduces the general look of clay tile, if not
always the exact profile or proportions of historic clay tile, at
a somewhat lower cost and weight.  Another modern, slightly cheaper
and lighter substitute for clay tile more recently developed
consists of a mixture of mineral fiber and cement with pigments
added to supply color.  While these aggregate tiles also replicate
the shape and appearance of clay roofing tiles, they have many of
the same dissimilarities to clay tiles that are found in concrete
tiles.  Thus, like concrete tiles, they are seldom appropriate
substitutes for clay tiles.


***TRADITIONAL TILE SHAPES AND COLORS***

There are two types of clay roofing tiles: interlocking and
overlapping.  Interlocking tiles are designed in pairs so that an
extrusion or "lip" on one of the tiles "hooks" over the other tile
thereby "locking" or securing the two together; they are also
usually nailed to the roof structure.  Overlapping tiles, which can
also function in pairs, generally do not have any sort of "lip" and
must be nailed in place.  There is a wide range of shapes of
historic clay roofing tiles, and many, sometimes with slight
variations, are still produced today.  There are many variations,
and the country of origin of some of them may be revealed in their
names, but there are essentially only two kinds of shapes: pantiles
and flat tiles.  Both pantiles and flat tiles may be either
interlocking or overlapping.

PANTILES:

The shape most commonly associated with historic clay roofing tiles
is probably that of convex or rounded tiles, often grouped together
generically as "pan tiles" or "pantiles." These include Spanish
tiles - sometimes called "S" tiles, or the similarly shaped Mission
tiles, also known as Barrel or Barrel Mission tiles, straight or
tapered, as well as Roman tiles, and their Greek variation.

FLAT TILES:

Flat, shingle tiles are another type of historic clay roofing
tiles.  Flat tiles can be completely plain and flat, and, like
roofing slates, overlap one another, attached with nails to the
roof sheathing.  Or they may interlock at the top and on one side.
Although the "interlock" holds them together, most interlocking
shingle tiles also have one or more holes, usually near the top,
for nailing to the roof sheathing.  Flat tiles are mostly
variations of English or Shingle tiles, and include English
Shingle, Closed Shingle, Flat, Shingle or Slab Shingle, as well as
French tiles which have a slightly higher and more contoured
profile.

Any of the standard tile shapes may be known by a different name in
another region of the country, or in different parts of the world.
For example, what are known as Spanish or "S" tiles in the United
States, may be called Single Roman tiles in England.  Sometimes
Spanish and Mission tiles are equated despite the fact that the
former are usually 1-piece interlocking tiles and the latter are
single 1/2 cylinders that overlap.  Since missions and the Mission
style are associated with the Americas, Mission tiles in the United
States are more commonly referred to as Spanish tiles in England
and Europe.  In a similar vein, Spanish or "S" tiles, or Barrel
tiles, might seem to be more typical of some tiles used in France
than what are marketed as French tiles by American manufacturers.

Today some tile manufacturers have given their own trademark name
to historic tile shapes.  Other companies market uniquely shaped
"S" tiles that are more in the shape of a true, but rather low
profile "s" without the customary flat portion of traditional
American "S" tiles.

FIELD AND SPECIALTY TILE:

The tiles that cover the majority of the flat surface of the roof
are called field tile.  Some roof shapes, particularly conical
towers or turrets, require tiles of graduated sizes, and some
shapes or patterns of field tile also require specially shaped
finish tiles to complete the roof covering package.  Other
uniquely-shaped tiles were made to fit odd-shaped spaces and places
including dormers and valleys, roof hips, rakes, ridges and
corners.

There are also finish tiles that fulfill certain needs, such as
eave closures or clay plugs called "birdstops."  These are intended
to keep out snow and rain, and birds from nesting in the voids
under the bottom row of curved tiles.  Different patterns and
designs can also be created by combining, or mixing and matching
flat tiles with dimensional tiles.

TILE COLORS:

A terra cotta red is the color most commonly associated with
historic clay roofing tiles.  The reddish color comes from clay
with a large percentage of iron oxide, and there are many
variations of this natural color to be found in tiles ranging from
deep reddish browns to softer and paler oranges and pinks.  Lighter
buff and beige colors, as well as black, also appear on traditional
tile-roofed buildings.  Buff-colored tiles were made from nearly
pure fire clay, and pouring manganese dissolved in water over the
tile before firing resulted in smoke brown or black glazed tiles.
Toward the end of the 19th century the popularity of colored glazes
for roofing tiles increased, and their use and the range of colors
continues to expand today.  Most historic glazed roofing tiles are
in fairly natural hues that range from reds and browns and buffs,
to blacks and purples, blues (often created with smalt, or powdered
blue glass), and a wide variety of greens (usually created with
copper slag).  There could be a considerable range in the colors of
tiles that were baked over a wood fire because the temperature
within the kiln was so uneven; tiles closest to the fire cooked all
the way through and turned a darker red, while tiles farthest from
the flames were likely to be smoke-stained, and lighter orange in
color.


***HOW TILES ARE ATTACHED***

The method used to attach clay roofing tiles varies according to
the shape, size and style of the particular tile.  For the most
part, traditional and modern methods of installing clay roofing
tiles are very similar, except that modern practice always includes
the use of wood sheathing and roofing felt.  But most of the
earliest clay roofing tiles were laid without benefit of wood
sheathing and hung directly on roofing laths and battens that were
nailed to the roof rafters; this practice continued up into the
mid-19th century in some regions.  While this method of attachment
allowed for plenty of ventilation, and made it easy to find leaks
and make repairs, it also meant that the overall watertightness of
the roof depended entirely on the tiles themselves.

Gradually, the practice evolved of nailing roofing tiles directly
onto continuous wood sheathing, or hanging them from "nibs" on
horizontal lath that was attached to roof rafters or sheathing.
Some kinds of tile, especially the later Mission or Barrel tiles
were laid over vertical strips or battens nailed to the sheathing,
or the tiles were fastened to wood purlins with copper wire.

Partly because they do not always fit together very closely, some
tile shapes, including Spanish, Barrel or Mission as well as other
types of interlocking tiles, are not themselves completely
water-repellent when used on very low-pitched roofs.  These have
always required some form of sub-roofing, or an additional
waterproof underlayer, such as felting, a bituminous or a
cementitious coating.  In some traditional English applications, a
treatment called "torching," involved using a simple kind of mortar
most commonly consisting of straw, mud, and moss.  The tapered
Mission tiles of the old Spanish missions in California were also
laid in a bed of mud mortar mixed with grass or straw which was
their only means of attachment to the very low-pitched reed or twig
sheathing (latia) that supported the tiles.

More recent and contemporary roofing practices require that the
tiles be laid on solid 1'' (2.5cm) wood sheathing felted with
coated base sheets of at least 30 lbs., or built-up membranes or
single-ply roof membranes.  This substantially increases the
watertightness of the roof by adding a second layer of
waterproofing.  Horizontal and vertical chalk lines are drawn to
serve as a guide in laying the tile and to indicate its patterning.
Most tiles are designed with one or two holes so they can be
attached by copper nails or hangers, and/or with projecting nibs,
to interlock or hang on battens or lath attached to the base
sheathing.

Before laying the tiles, the copper or lead gutters, flashings and
valleys must be installed, preferably using at least #26 gauge
(20-24 ounce) corrosion-resistant metal extending a minimum of 12"
(30.5cm) under the tile from the edge, or in accordance with the
manufacturer's specifications.  The long life and expected
durability of clay tiles require that, as with the roofing nails,
only the best quality metal be selected for the flashing and
guttering.

"Field tile" is usually ordered by the number of "squares" - that
is, a flat section 10' x 10' (25cm x 25cm) - needed to cover a roof
section.  The tile company or roofing contractor should calculate
the number of tiles needed according to the type of roof, and based
on architect's drawings to ensure accuracy.  This should include
specialty ridge and eave tiles, decorative trim, partial "squares"
approximately 10-20 per cent allowance for breakage, and extra
tiles to store for repairing incidental damage later on.  Once at
the site, the tile is evenly distributed in piles on the roof,
within easy reach for the roofers.

The tiles are laid beginning with the first course at the lower
edge of the roof at the eaves.  The method by which roofing tiles
are laid and attached varies, depending on the type and design of
the tiles and roof shape, as well as on regional practice and local
weather conditions.  A raised fascia, a cant strip, a double or
triple layer of tiles, or special "birdstop" tiles for under the
eaves, may be used to raise the first row of tiles to the requisite
height and angle necessary for the best functioning of the roof.
The tile is positioned to overhang the previously installed gutter
system by at least 1-1/2" (4cm) to ensure that rainwater discharges
into the central portion of the gutter.  Once this first course is
carefully fitted and examined from the ground level for
straightness and color nuances, and adjusted accordingly,
successive courses are lapped over the ones below as the roofer
works diagonally up the roof toward the ridge.  Positioning and
laying tiles in a 10'x 10' (25cm x 25cm) square may take on the
average of 16 1/2 man hours.



***FLAT TILES***

Most flat clay tiles have one or two holes located at the top, or
on a "nib" or "lug" that projects vertically either from the face
or the underside of the tiles, for nailing the tile to the
sheathing, battens, or furring strips beneath.  As successive rows
of tile are installed, these holes will be covered by the next
course of tiles above.  Traditionally, clay tiles on the oldest
tile roofs were hung on roofing laths with oak wooden pegs.  As
these wood pegs rotted, they were commonly replaced with nails.
Today, copper nails, 1 3/4" (4.5cm) slaters' nails, are preferred
for attaching the tiles because they are the longest lasting,
although other corrosion-resistant nails can also be used.  Less
durable nails reduce the longevity of a clay tile roof which
depends on the fastening agents and the other roofing components,
as much as on the tiles themselves.  Clay roofing tiles, like
roofing slates, are intended to hang on the nails, and nailheads
should always be left to protrude slightly above the surface of the
tile.  Nails should not be driven too deeply into the furring
strips because too much pressure on the tile can cause it to break
during freeze/thaw cycles, or when someone walks on the roof.

Plain flat tiles, like roofing slates, are attached to the roof
sheathing only with nails.  They are laid in a pattern overlapping
one another in order to provide the degree of impermeability
necessary for the roof covering.  Because plain flat tiles overlap
in most cases almost as much of one half of the tile, this type of
tile roof covering results in a considerably heavier roof than does
an interlocking tile roof which does not require that the tiles
overlap to such an extent.  Interlocking flat tiles form a single
layer, and an unbroken roof covering.  Although most interlocking
tiles on all but the steepest roofs can technically be expected to
remain in place because they hang on protruding nibs from the
roofing laths or battens, in contemporary roofing practices they
are often likely to be nailed for added security.  In most cases it
is usually a good idea to nail at least every other tile.


***PANTILES***

With Mission or Barrel tiles, where one half-cylinder overlaps
another inverted half-cylinder to form a cover and pan (cap and
trough) arrangement, the fastening is more complicated.  While the
pantiles that rest directly on the sheathing are simply nailed in
place, there are two ways of attaching the cover tiles that rest on
the pantiles.  They can be secured by a copper wire nailed to the
sheathing or tied to vertical copper strips running behind the
tiles.  Another method requires the installation of vertical
battens or nailing strips on the roof to which the cover tiles are
nailed, or the use of tile nails or hooks, which are hooked to the
pantile below and secured with twisted copper wire.

Sometimes cement mortar, or another underlayer such as grass, moss
or straw, or hair-reinforced mortar was added under the tiles.
Before the use of felting this was a particularly common practice
on some of the plain flat tile or Spanish tile roofs with low rises
that were themselves not especially waterproof.  Mortar also helped
to keep driving rain from getting under the pantiles, and it is
still customary in contemporary roofing to add a dab of cement
mortar to help secure them.


***RIDGE OR HIP TILES***

At the roof ridge or hip, clay tile is usually attached to a raised
stringer with nails and a small amount of mortar, elastic cement or
mastic.  The joint is sealed with a flexible flashing such as
copper or lead.  Ridge tiles are often somewhat larger and more
decorative than the field tile utilized on the broad sections of
the roof.


***ROOF PITCH AND WEATHER ARE FACTORS IN TILE ATTACHMENT***

The means by which clay tile is attached to the sheathing is also
partly determined by the roof pitch.  Generally the fastening
requirements increase with an increase of roof pitch.  For
low-pitched rises of 4"-6" (lOcm-15cm) in a 12" (30.5cm) run the
weight of the tiles is usually sufficient to hold them in place on
the lath by the ridge or "lug" on the underside of the tile, with
only the perimeter tiles requiring metal clips to secure them to
the sheathing.  But the tiles on even these low-pitched roofs are
usually nailed for added security, and additional fastening
measures are necessary on roofs with a higher pitch, or in areas
subject to high winds or earthquakes.  For steeper pitched roofs,
such as towers, 7"-11" (18cm-28cm), or 12"-15" (30.5cm-38cm) in a
12" (30.5cm) run, the tiles are nailed and a band of perimeter
tiles three to four tiles thick is secured with clips.  For roof
rises over 16" (41cm) in a 12" (30.5cm) run, and in areas prone to
earthquakes or hurricanes, every tile may be secured with both a
nail and a copper or non-corrosive metal clip, and often also with
a dab of roofing mastic or mortar.

The installation of clay roofing tiles in areas with significant
amounts of snowfall - over 24"(61cm) per year - also varies
somewhat from the normal guidelines.  Larger battens may be
necessary, as well as additional clipping or tying of the tile to
securely attach it to the sheathing.  The roof structure itself may
also need added bracing, as well as the insertion of small snow
clips or snow birds that protrude above the surface of the tile to
prevent snow and ice from sliding off the roof and damaging the
tile.


***PRESERVATION AND REPAIR***

IDENTIFYING COMMON PROBLEMS AND FAILURES:

While clay roofing tiles themselves are most likely to deteriorate
because of frost damage, a clay tile roof system most commonly
fails due to the breakdown of the fastening system.  As the wooden
pegs that fastened the early tiles to hand-riven battens rotted,
they were often replaced with iron nails which are themselves
easily corroded by tannic acid from oak battens or sheathing.  The
deterioration of metal flashing, valleys, and gutters can also lead
to the failure of a clay tile roof.

Another area of potential failure of a historic clay tile roof is
the support system.  Clay tiles are heavy and it is important that
the roof structure be sound.  If gutters and downspouts are allowed
to fill with debris, water can back up and seep under roofing
tiles, causing the eventual deterioration of roofing battens, the
sheathing and fastening system, or even the roof's structural
members.  During freezing weather, ice can build up under tiles and
cause breakage during the freeze/thaw cycle.  Thus, as with any
type of roof, water and improperly maintained rainwater removal and
drainage systems are also chief causes for the failure of historic
clay tile roofs.

Clay tiles may be either handcrafted or machine-made.  In general,
roofs installed before the end of the 19th century consist of
hand-formed tiles, with machine-made tiles becoming more dominant
as technology improved during the 20th century. Clay tile itself,
whether made by hand or made by machine, can vary in quality from
tile to tile.  Efflorescence of soluble salts on the surface may
indicate that a tile has excessive porosity which results from
underburning during its manufacture.  Poor quality porous tiles are
particularly susceptible to breaking and exterior surface spalling
during freeze-thaw cycles.  By letting in moisture, porous tiles
can permit the roof battens and roof structure to rot.  The problem
may be compounded by waterproof building paper or building felt
laid underneath which can, in some instances, prevent adequate
ventilation.

Clay roofing tiles can also be damaged by roofers walking
carelessly on an unprotected roof while making repairs, or by
overhanging tree branches, falling tree limbs, or heavy hail.
Broken tiles may no longer provide a continuous waterproof surface,
thereby allowing water to penetrate the roofing structure, and may
eventually result in its deterioration if the broken tiles are not
replaced in a timely manner.

Although modern, machine-made clay tiles are more uniform in
appearance than their hand-made counterparts, they also have the
potential for failure.  Occasionally, entire batches of
mass-produced tile can be defective.

REGULAR INSPECTION AND MAINTENANCE:  

Broken or missing tiles, or leaks on the interior of the building,
are obvious clues that a historic clay tile roof needs repair.
Even though it may be clear that the roof is leaking, finding the
source of the leak may not be so easy.  It may require thorough
investigation in the attic, as well as going up on the roof and
removing tiles selectively in the approximate area of the roof
leak.  The source of the leak may not actually be located where it
appears to be.  Water may come in one place and travel along a
roofing member some distance from the actual leak before revealing
itself by a water stain, plaster damage, or rotted wooden
structural members.

TEMPORARY PROTECTION DURING REPAIR:

In some instances temporary protection and stabilization may be
necessary to prevent further damage or deterioration of a historic
clay tile roof.  Plywood sheets, plastic, roll roofing, or roofing
felt can provide short-term protection until repair or replacement
materials can be purchased. Another option may be to erect a
temporary scaffold that is encased or covered with clear or
semitransparent polyethylene sheeting over the entire roof.  This
will not only protect the exposed roofing members during repair or
until repairs can be made, but also lets in enough natural light to
enable the re-roofing work to take place while sheltering workmen
from cold or wet weather.

GENERAL REPAIR GUIDANCE:

Once the source and cause of a leak has been identified,
appropriate repairs must be made to structural roofing members,
wood sheathing, felt or roofing paper if it is part of the roofing
membrane, or possibly to vertical roof battens to which the tiles
may be attached.  If the problem appears limited to gutters and
flashing in disrepair, repair or replacement will probably require
temporary removal of some of the adjacent tiles to gain access to
them.  If the roofing tiles are extremely fragile and cannot be
walked on even with adequate protection (see below), it may also be
roof.








 


Preservation Brief 30, clay tile roof, repair of clay roof tiles, preservation of clay tile roof