Clay tile and concrete tile
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Material description
Clay tile is produced by baking molded clay into tile. The density of the clay is
determined by the length of time and temperature at which it is heated. Tiles may
be glazed and also may have surface texture treatments applied. As a result, there
are a wide variety of tile profiles, styles, finishes and colors available. In addition,
each tile may have separate field, ridge, hip, gable and terminal tiles of various
shapes. Installation methods depend on the nature of the tile being installed; that
is, whether it is two piece, one piece, interlocking or flat.
Example of a clay tile roof
Concrete tiles are made of portland cement, sand and water in varying proportions.
The material is mixed and extruded on molds under high pressure. The exposed surface
of a tile may be finished with cementitious material colored with synthetic oxide
additives. The tiles are cured to reach the required strength. They generally have
lugs on their undersides for anchoring to batten strips. There are additional waterlocks
or interlocking ribs on the longitudinal edges that impede movement and prevent
water infiltration.
As with clay tile, there are a wide variety of profiles, styles, finishes and colors
available. Color may be added to the surface of a tile or dispersed throughout (color
through). Special texture may be added in surface treatment. In addition, each tile
type may have separate field, ridge, hip, gable and terminal tiles that are various
shapes.
Roof deck
NRCA recommends tile roof systems be applied over continuous wood decking. When
plywood is used, NRCA recommends the use of a minimum 5/8 thick nominal exterior-grade
plywood.
Caution should be exercised when roof decks are constructed out of the following
materials:
- Oriented strand board (OSB): NRCA is concerned with potential fastener-holding problems
and dimensional stability because of the effects of moisture where OSB and other
nonveneer products are used as roof decking.
- Preservative-treated wood: Many roofing material manufacturers recommend wood roof
decks be constructed with wood that has been treated with a nonoil preservative
pressure treatment or with nontreated air- or kiln-dried lumber.
-
Fire-retardant-treated wood: Because of the deterioration of some fire-retardant-treated
wood panels caused by chemical reaction, special care should be given to investigate
the use of fire-retardant-treated wood panel decks.
For some types of tile roofs, batten and counter-batten systems are used to hang
tile that has head lugs. Battens are spaced according to the length and exposure
of the specified tile. Battens should be fastened to the deck with 8d common, galvanized,
corrosion-resistant nails at approximately 12 inches on center.
Underlayment
Underlayment (or "felt paper" as it is frequently called) is installed over the
roof deck before the application of tile. An underlayment performs two primary functions:
it provides temporary weather protection until the tile is installed, and it provides
a secondary weatherproofing barrier if moisture infiltrates the tile roof covering.
Many tile roofs have outlived the underlayment felts over which they were installed.
Therefore, an underlayment's service life should be comparable to the design service
life of the tile roof covering.
Asphalt saturated, nonperforated organic felts are among the most common underlayments;
they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and
No. 30, which are reflective of a once used pound per square weight designation.
The terms Type I and Type II now are used within the industry in lieu of No. 15
or No. 30, respectively.
Another type of underlayment is a synthetic underlayment. It is characterized as
being lightweight, water-resistant and less likely to wrinkle; having high tear
strength; and being easy to walk on—even when wet. Theoretically, the product
may be left exposed to the elements for extended periods of time. Although synthetic
underlayments and their purported attributes seem appealing, there are significant
issues to consider before using them. To date, there are no applicable ASTM standards
for these products. Many synthetic underlayments do not meet current building code
requirements, so manufacturers need to obtain a code evaluation report for code
compliance.
NRCA recommends a minimum of one layer of No. 30 asphalt-saturated felt applied
horizontally in shingle fashion on roof decks having a slope of 10:12 (40 degrees)
or more. For roof decks having slopes of 4:12 (18 degrees) up to 10:12 (40 degrees),
a minimum of two layers of No. 30 asphalt-saturated underlayment should be applied
horizontally in shingle fashion. NRCA does not recommend installing tile roof systems
on roof slopes less than 4:12 (18 degrees).
In locations where the average temperature for January is 30º F or less, NRCA suggests
installation of an ice-dam protection membrane. An ice-dam protection membrane generally
is a self-adhering polymer-modified bitumen membrane.
An ice dam protection membrane should be applied starting at a roof's eaves and
extending upslope a minimum of 24 inches from the exterior wall line of a building.
For slopes less than 4:12 (18 degrees), NRCA recommends a minimum of 36 inches.
See Figure 1.
Figure 1 - Example of ice damming
Securement methods
Many different types and combinations of securement methods are used for the various
types of roof tile. To select a method of securement, many conditions need to be
considered: wind, deck type, seismic considerations, slope, building codes, local
practices and manufacturer recommendations. Fasteners should be made of noncorrosive
materials that will remain serviceable in the intended environment for the roof's
design life.
- Nails—Nailing is the most common method. NRCA recommends that roofing nails
be 11- gauge or 12-gauge galvanized steel or the equivalent corrosion-resistant
roofing nails. Nail heads should be low-profile, smooth and flat. Shanks should
be barbed or otherwise deformed for added pull-out strength. Nails should be long
enough to penetrate through all layers of roofing materials and extend through the
underside of the roof deck or penetrate at least ¾ inch into wood plank or board
decks.
- Wire tie and strapping systems—Hanging tile with wire is used on non-nailable
decks, insulated decks or where fastening through metal flashings needs to be avoided.
- Clips—Nose or butt clips sometimes are used in conjunction with other attachment
methods in high-wind or seismic areas.
- Lug-hung-tile—Many types of tile have formed undersides near their heads that
may be hung over a batten. Lug hanging tile usually is used in combination with
other securement methods.
-
Bedding Tile—Laying tile in a bed of mortar of foam adhesive is common in some
areas of North America where freeze/thaw conditions are not encountered. Bedding
often is used in combination with other securement methods.
Flashings
Flashings for tile roofs fall into four categories: perimeter edge metal, penetrations,
valleys and vertical surfaces. See Figure 2.
- Perimeter edge metal-depending on the severity of climate, anticipated rainfall
and freeze-thaw cycling, the use of perimeter edge metal should be considered.
- Penetrations-plumbing soil stacks, exhaust vents and pipes are flashed into tile
roof systems with some type of flat flange that extends around a penetration and
is installed under shingles on the upslope of a flange.
- Valleys-valleys that are called "open valleys" are typically lined with sheet metal.
-
Vertical surfaces-when a roof system abuts a vertical surface, there are four types
of flashing commonly used: apron, step, cricket (or backer) and counterflashing.
Figure 2 - Basic sheet metal flashing components
Apron, step and cricket flashings require some form of counterflashing to cover
and protect the top edges from water intrusion. In many cases, the wall covering
or cladding material acts as counterflashing. When this does not occur, a metal
counterflashing mounted to the vertical surface should be installed. See Figures
3,4 and 5 for examples.
Figure 3 - Example of metal counterflashing inset in masonry mortar joint
Figure 4 - Example of through-wall metal counterflashing embedded in masonry mortar
joint
Figure 5 - Example of surface-mount metal counterflashing
Material standards
NRCA does not make any recommendations about which tile or manufacturer to use;
however, NRCA does recommend clay tile roof coverings meet American Society for
Testing and Materials (ASTM) Standard C1167, "Standard Specification for Clay Roof
Tiles."
Currently, there are no ASTM standards for concrete tile.
Warranties
When purchasing a new roof system, there will be two warranties to consider. First,
there will be the manufacturer's warranty. In general, these warranties cover defects
in the manufacture of the roof covering. Please read
NRCA's consumer advisory bulletin addressing roofing warranties
for more information. Once the project is complete, be sure the contractor provides
you with a certificate for your records.
Second, the roofing contractor will provide you with a warranty covering his workmanship.
Typically, this will cover installation and related issues. The warranty should
contain what items are covered and what will void them. Many companies offer one
year or two years of coverage; however, there is no industry standard.
Construction details
Below are links to some common tile construction details.