Slate, Clay and Fiber Cement Tiled Roofs

Many years ago, large uninterrupted sheets of roofing material (such as thin metal sheets or rubberized/vinyl membranes) had not been invented, were cost prohibitive, or difficult to install. As a result, many early roof coverings were made by building up a roof from smaller, overlapping elements. Shingles and roof tiles consist of either natural or man-made materials which are shaped into small individual rectangular components installed on pitched (sloping) roofs. To prevent water ingress through the joints where singles/tiles meet, the top and side edges of the tiles overlap. This allows water to flow over the tops of the tiles and off the bottom of the roof, to ground level or into gutters and downspouts.

Slate Singles, Clay Tile and Fiber Cement Tiles – What Are They and How Are They Installed?

Historically, natural products such as slate and clay tiles were used to manufacture roof tiles, although in more recent years fiber cement has been used as a low-cost man-made alternative. In the following sections, we will look over how these roof coverings are manufactured.

Slate

Slate is a naturally occurring metamorphic rock. Initially formed as a sedimentary rock, which was then subjected to extreme heat and pressure (under the earth’s surface). During this process, the rock metamorphizes and its chemical composition changes creating slate as we recognize it today. The extreme heat, pressure and chemical changes that occur during metamorphism create a rock that is built up in many fine layers and is extremely dense, hardwearing and water-resistant, making it perfect as a roofing product. Another advantage of the layer formation is that slate rock can be easily split to create flat sheets and rectangular shingles. Slate is mined from areas where the rock naturally occurs, before being cut, split and shaped using machines and hand tools into individual shingles. The quarry and area from where slate is mined affects the color, appearance and splitting characteristics of the slate. Typically, a standard slate is 3/16” – 1/4” thick.

When installed on sloped roofs, the National Roofing Contractor Association (NRCA) recommends that slates are installed over continuously or closely spaced wood decking. Decks usually consist of exterior grade plywood, and the NRCA recommends 5/8” thick exterior grade plywood is used. Decks manufactured from Oriented Strand Board (OSB) should not be used due to potential fastener instability and OSB’s movement tendencies when subjected to moisture. However, traditionally, slates have also been installed on wood battens attached to rafters. On this traditional installation, to prevent wind and water entry a lime or cement-based plaster known as “torching” is installed to the underside of the battens and shingles. Underlayment is also commonly installed under the shingles and can either consist of asphalt impregnated felt, or a lightweight synthetic membrane alternative. Where average January temperatures fall below 30° Fahrenheit, the National Roofing Contractors Association (NCRA) recommend the installation of an ice-dam membrane. This generally consists of a self-adhering modified bitumen membrane which is often polymer-based. This membrane runs from the eaves, up the line of the roof by 24”, or 36” if the roof has a pitch of less than 4:12 (18°). Slates are installed to the deck or batten systems with nails which are typically made from copper. Two fixings per slate should be included, which should be increased to four fixings per slate in high-wind areas.

Clay Tiles

Clay is a natural fine-grained rock or soil material which is moldable and flexible when removed from the ground. When dried or fired in a kiln, the chemical structure of clay changes and the material becomes non-flexible, hard and brittle. The change in material characteristic once fired makes clay an excellent roof covering material. Clay tiles are manufactured by extracting clay from the earth, processing into various shapes and sizes and firing the tiles. Typically, clay tiles are often shaped in ‘U’ profiles allowing convex (outward) and concave (inward) shapes to interlock across the roof. In addition to standard roof tile shapes, tiles are also manufactured in other profiles for installation at roof junction points (hips, ridges, intersections etc.).

When installed on sloped roofs, the National Roofing Contractor Association (NRCA) recommends that clay tiles are installed over continuous wood decking. Decks usually consist of exterior grade plywood, and the NRCA recommends 5/8” thick exterior grade plywood is used. Decks manufactured from Oriented Strand Board (OSB) should not be used due to potential fastener instability and OSB’s movement tendencies when subjected to moisture. As with slate roofs, some installations (especially older) rely on the use of a batten and counter-batten arrangements with battens spaced to suit the size of the tiles. Battens are typically fastened to the deck with galvanized nails at 12” centers. As with slate roofs, underlayment is recommended between the tile and roof deck, and in cold climates and ice dam may be necessary. The materials and specifications of underlayment and ice dams are the same as those detailed in the slate section above. Clay tiles are usually attached to the deck or battens using galvanized steel nails, although metal tie, strap and clip systems are also common. In warmer areas of the US, clay tiles can also be laid in a mortar bed; although this is not suitable for areas affected by freeze/thaw conditions, and this system still relies on additional support from traditional metal clips/fasteners already described.

Fiber Cement Tiles

Fiber cement tiles are manufactured in a factory by combining portland cement, sand and water to create a slurry. The mixture is poured into shaped molds, where it is then compressed under high pressure. Molds can be patterned or shaped to vary the surface texture of the shingles, but typically the tiles are left with a smooth or slightly textured finish and are manufactured in standard sizes. During mixing of the cement products, liquid or powdered tints can be applied to create a specifically colored tile, if required. In addition to the cement products, fiber materials can be added to bind the mix, creating a thin, lightweight and durable tile which is resistant to rain. Historically, asbestos fibers were used in the tiles, although these have now been replaced with alternative fibers such as wood pulp fibers. As with slate shingle and clay tile roofs, fiber cement tiles are usually installed over continuous decks, or on batten assemblies. Tiles are nailed to the decks/battens with metal nails through holes in the tile created during the manufacturing process. As with slate shingle and clay tile roofs, underlayment is recommended between the tile and roof deck, and in cold climates and ice dam may be necessary. The materials and specifications of underlayment and ice dams are the same as those detailed in the slate and clay tile sections above.

Typical Flashing Details

Flashing details for slate and fiber cement tiles are largely similar. Flashings typically consist of metal flashings (usually made from soft metals such as lead or copper) which consist of steps, aprons and crickets. Where penetrations though the roof occur (such as around chimney stacks), stepped metal counter flashings are often provided. Penetrations from pipework/soil stacks etc. usually consist of metal flashings around the pipe(s) with a flange that extends under the shingle/tile surfaces. Valleys are usually lined with metal.

Flashings at clay roofs are typically the same as listed above; however, metal counter flashings and channels are often provided. Because of the typical profiled tile shape, at the junction between tiles and channels/counter, the final tile typically is convex (outward) facing, allowing water to drain from the top of the tile into the metal channel(s) beneath.

Pros, Cons and Common Defects

Slate shingle, clay tile and fiber cement tiles have earned a reputation as popular roofing options for domestic buildings. However, like all roof systems they have their pros and cons. Table 1-1 summarizes the typical pros and cons of slate shingle, clay tile and fiber cement tile roof systems.

Table 1-1 – Pros and Cons of Tiled Roofing Systems

Material Type Pros Cons
Slate
  • Extremely hardwearing
  • Architecturally attractive
  • Natural product
  • Inherently fire resisting
  • More expensive than modern alternatives
  • Requires skilled installers
  • Heavy
  • Can crack/break if walked on during maintenance
Clay Tile
  • Hardwearing
  • Architecturally attractive
  • Natural insulating and heat-retaining properties
  • Heavy
  • Can crack/break if walked on during maintenance
  • Require careful and precise installation, due to the interlocking design
Cement Fiber Tile
  • Low cost
  • Available in various colors and designs
  • Fire resistant
  • Insect and rot resistant
  • Lightweight
  • Can crack/break if walked on during maintenance
  • Weather exposure can erode tiles in layers, causing them to delaminate and deteriorate
  • Textured surface can promote moss/lichen growth
  • Older tiles are likely to contain asbestos and will need to be removed by an abatement specialist

Typical Maintenance

Slate Shingle and Clay Tile

An advantage of slate shingle and clay tile roofs are that they are hard wearing and require little to no maintenance. Typical maintenance activities consist of replacement of isolated cracked/damaged tiles, and repairs to damaged flashing. Moss and lichen growth can occur, particularly on clay tiles due to their porosity. In especially bad instances, moss and lichen should be pressure washed off the roof, taking care not to damage the exposed shingle/tile finish. Gutters and downspouts should be regularly cleared to ensure that water is able to freely flow off the roof.

Fiber Cement Tiles

Fiber cement tiles are relatively hardy and do not require excessive maintenance. As with slate shingle and clay tile roofs, typical maintenance activities consist of replacement of isolated cracked/damaged tiles, and repairs to damaged flashing. Moss and lichen growth can occur which should be pressure washed off the roof, taking care not to damage the finish to the fiber cement tile, as removal of the top layer of a fiber cement tile can actually make the tile more porous to water. Gutters and downspouts should be regularly cleared to ensure that water is able to freely flow off the roof. Finally, care should be taken with older fiber cement tile roofs as historically asbestos fibers were used as a filer material in these tiles – see “STOP – Safety Moment” below.

STOP – Safety Moment

Historically asbestos fibers have been used in fiber cement tiles. Although encapsulated within the cement composite, if tiles are cut, removed, break down or are pressure washed, asbestos fibers can be released which are harmful to human health. The presence of asbestos in fiber cement roofing tiles can only be determined through sampling and testing of the material in a laboratory.

Roofing works and maintenance/cleaning activities for all roofing systems are completed at heights where injury or death from falling from height can occur. We therefore recommend that roofing systems should generally be inspected, tested, removed, installed and maintained by competent trained roofing professionals.

Roof Warranties

Two main warranties exist relating to roofs – a material warranty which covers the roofing product, and a contractor warranty which covers the installation by a roofing contractor/business. Usually, both warranties are provided on completion of a roofing project. Typically, roofing materials (the product which is installed) are guaranteed by a roofing manufacturer for a substantial period (say 20 to 25 years) against defects caused during the manufacturing process. A contractor warranty is typically much shorter (usually 1-2 years) and covers installation-related defects. The contractor warranty should detail what items are covered, and activities that will void them. It is important to ensure that copies of the warranties are obtained, and that building inspections and maintenance activities do not invalidate any warranties in place.

Related Websites

http://www.nrca.net/ – National Roofing Contractors Association

http://www.everybodyneedsaroof.com/clay-tile-and-concrete-tile – Clay and Concrete Roof Tiles

http://www.everybodyneedsaroof.com/slate – Slate Roof Tiles

1 Comment

  1. A lot of great information there, I currently at the beginning of building a house and was looking into roof slates and which to get. I was told natural slates are good but the house style I am going with, they may not suit unlike fibre cement slates that have a range of designs. The article shows me the benefits and pros of a few and will be using this in my decision making.

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