How Are Terra Cotta Roof Tiles Made?

Terra cotta roof tiles are a type of tile made from natural clay that has been used to cover rooftops for centuries. The term “terra cotta” comes from the Italian words for “baked earth,” referring to the process used to harden and fire the clay tiles.

The origin of terra cotta roof tiles can be traced back over 2000 years to ancient China and the Mediterranean region. These early tiles were handmade and sun-dried. Over time, the tile making process became more advanced with the use of kiln firing to harden the clay.

Terra cotta tiles offer many benefits as a roofing material. They are naturally fire and weather resistant, providing durability and longevity. The tiles come in a range of earthtone colors, providing an attractive roof. They also offer natural insulation properties. In hot climates, terra cotta’s breathability allows interior cooling. In cold climates, it provides warmth by retaining heat.

Today, terra cotta remains a popular roofing choice for residential and commercial buildings alike. The timeless beauty and protection of terra cotta tile roofs make them a smart, sustainable option.

Clay Preparation

The type of clay used for terra cotta tiles depends on the desired properties and look of the finished product. Terra cotta tiles are typically made from red clay or brown clay, which contain high levels of iron oxide that give terra cotta its reddish-orange color when fired. The clays used are natural clays that are mined from quarries and clay deposits.

At the manufacturing facility, the clay goes through an extensive preparation process to remove any impurities and ensure consistent physical properties. First, it is extracted from the earth through open pit mining or underground mining methods. Next, the clay is crushed and cleaned to remove rocks, twigs, pebbles, and other debris. It may go through a blunging process where water is added to create a slip that can be passed through screens to filter out remaining particles.

The clay also goes through weathering and aging processes which expose it to the air and elements. This improves plasticity and ensures consistent performance during extrusion and firing. The clay may be blended with other natural minerals to achieve the desired properties. Once prepared, the clay is stored until ready for the tile molding process.

Molding

The molding process is critical in giving terra cotta roof tiles their shape and texture. There are two main methods for molding terra cotta tiles: extrusion molds and press molds.

Extrusion molds use an auger to push clay through a die to create the tile shape. The die opening determines the profile of the tile, creating the distinctive S-shape or barrel shape of terra cotta roof tiles. Extrusion molds allow for high volume tile production. However, the texture and shapes are limited compared to press molded tiles.

Press molds use hydraulic presses to form terra cotta tiles within two-piece molds. The top and bottom mold pieces imprint decorative shapes and textures into the tile. More intricate details and custom profiles can be achieved with press molds. However, the production volume is lower compared to extrusion. Press molds work well for specialty tile shapes and historic roof tile replication.

The mold design is a crucial step in manufacturing roof tiles with the desired aesthetic qualities. Highly-skilled artisans often hand-carve and assemble the mold pieces. The mold creation process balances achieving artistic qualities with the practical aspects of press speed, tile release, and durability through repeated uses.

Drying

After the terra cotta tiles have been molded, they must go through a careful drying process before firing. There are a few different methods used for drying terra cotta tiles:

• Air drying: The molded tiles are left to air dry naturally. This is the most common drying method and involves placing the tiles on drying racks in a temperature and humidity controlled room. Air drying allows the tiles to dry slowly over 1-2 weeks.
• Kiln drying: The tiles are placed in large commercial kilns and dried at low temperatures over 24-48 hours. This speeds up drying time but can lead to increased shrinkage and cracking.
• Radio frequency drying: This uses radio frequency energy to very rapidly dry the tiles from the interior outwards. It reduces drying time to just a few hours but is not commonly used.

Slow air drying is generally preferred as it minimizes defects and allows the clay to dry evenly throughout the tile. Rushing the drying process can cause uneven shrinkage, cracks, and warping. During drying, moisture is carefully removed from the clay body in three stages:

1. Constant drying rate – Surface moisture evaporates steadily over the first few days.
2. Falling drying rate – As surface moisture reduces, the drying rate slows down.
3. Final equilibrium – Interior moisture migrates to the surface before evaporating. The tiles are left to stabilize at a slow pace for the final drying stage.

Well-dried tiles should have a moisture content of less than 1% before firing. The whole drying process usually takes 1-2 weeks depending on the climate, tile thickness, and drying method.

Glazing and Coloring

Glazing terra cotta roof tiles serves both decorative and functional purposes. The glaze is made from a mixture of mineral ingredients that create a glassy coating when fired at high temperatures. Common ingredients include:

• Silica – Provides the glassy texture
• Alumina – Lowers melting temperature
• Calcium, sodium, or potassium – Fluxes to facilitate melting
• Colorants – Metal oxides for desired colors

The glaze slurry is applied by spraying or brushing onto the dried terra cotta tiles before firing. Glazing provides a protective coating that is water-resistant and durable. It helps prevent staining, erosion, and absorption of moisture that could damage the clay tile over time. The glaze also enhances the color and makes cleaning easier.

In addition to glazing, terra cotta tiles can be colored using mineral pigments mixed directly into the clay. Common pigments include iron oxide for reds, chromium oxide for greens, cobalt oxide for blues, and manganese dioxide for browns. Coloring with pigments goes through the complete thickness of the tile, whereas glazed colors are surface-level only.

Firing

Firing is one of the most crucial steps in terra cotta roof tile manufacturing. This process hardens the clay tiles and sets their final color and durability. Firing is done in special high-temperature ovens called kilns.

There are two main types of kilns used for firing terra cotta tiles:

• Tunnel kilns – These are long, elongated kilns that tiles pass through on a conveyor belt. The tiles enter at one end and gradually move through different temperature zones inside the tunnel.

• Intermittent kilns – These are chamber kilns where tiles remain stationary inside the chamber during firing. The temperature is brought up to a peak and then allowed to cool in cycles.

Firing temperatures for terra cotta tiles range from around 1,800°F to 2,200°F. Higher temperatures result in harder, more durable tiles. The tiles may go through multiple firing cycles to achieve the desired characteristics.

Some key benefits of multiple firings include:

• Improved strength and weather resistance

• Enhanced color uniformity

• Reduced risk of breakage

Skillful control over the firing process helps bring out the best qualities in terra cotta roof tiles.

Final Shaping

After the terra cotta roof tiles have been fired in the kiln, they undergo final shaping and finishing processes. This is done to smooth any rough edges, create a uniform shape, and finalize the aesthetic design of the tile.

One common technique for final shaping is grinding. Workers will use rotating abrasive wheels or belts to gently grind down any protruding clay and create smooth, even edges along the tile. This evens out the overall shape and creates clean lines along the tile’s profile.

Tiles may also undergo chiseling or hand-cutting to refine their shape. Skilled workers will use metal chisels or cutting tools to delicately remove excess clay and cut precise angles or curves on the tile. This kind of hand-worked finishing creates an organic, handmade look.

For a more weathered or antique appearance, tiles can be tumbled in large rotating drums. The tiles gently knock against each other, naturally wearing down sharp corners and creating a textured, nuanced surface. Tumbling gives the tiles more character and visual interest.

These methods allow manufacturers to transform the raw terra cotta tiles into their final desired shape, with decorative and functional features that will endure on rooftops for decades.

Testing and Quality Control

Ensuring the quality and durability of terra cotta roof tiles is critical. After firing, every tile undergoes rigorous testing and inspection before being approved for use.

The main tests conducted are:

• Cracks – Tiles are visually examined and tapped with a small hammer to detect any cracks or flaws that could compromise their strength or weather resistance.
• Strength – Sample tiles are subject to load testing to determine if they can withstand the required weight and pressure when installed on a roof.
• Water absorption – The porosity and water absorption rates of tiles are measured by submerging them in water and comparing their dry and soaked weights.
• Color consistency – Batches of tiles are evaluated to ensure uniform and consistent coloring across all tiles, with no noticeable variation between pieces.

Any tiles that fail these quality control tests are rejected and will not be used. Only tiles that pass inspection can be approved for packaging and sale.

Strict quality control testing ensures that the terra cotta roof tiles that make it to the customer are durable, weather-resistant, and uniform in appearance.

Packaging and Shipping

After terra cotta roof tiles are made, fired, and shaped, they need to be carefully prepared for shipping to installation sites. Because terra cotta tiles are brittle, they can easily break or chip if not properly packaged.

Terra cotta tiles are first inspected for any defects or cracks that could make them prone to breaking in transit. Any cracked or defective tiles are removed. The tiles are then sorted by size, color, and style based on what the customer ordered.

The tiles are placed on wooden pallets surrounded by a cardboard or styrofoam perimeter to cushion them during shipping. The pallets keep the bottom layer of tiles elevated so they don’t experience too much pressure. Within each pallet, tiles are separated by cardboard or styrofoam spacers.

The palletized tiles are then shrink-wrapped in plastic wrap to protect them from weather and keep everything contained together. The pallets are strategically stacked and secured inside trucks or shipping containers using straps, air bags, or other restraints.

Care is taken not to overstack the tiles or subject them to shifting that could cause breakage in transit. The pallets are lined up so the weight is distributed evenly across the truck or container.

With proper packaging and restraint, the terra cotta roof tiles can be shipped safely over long distances to arrive intact at building sites for installation.

Installation

Installing terra cotta roof tiles requires careful preparation and technique to ensure they are securely fastened and properly placed. Proper installation will provide decades of weather protection and enhance the aesthetics of a home.

Before installation, the roof deck and framing must be assessed to confirm it can structurally support the weight of the terra cotta tiles. Any repairs or reinforcements should be made at this stage. The roof surface itself should be waterproofed with an underlayment or other barrier. This protects against any moisture infiltration from wind-driven rain passing between tiles.

Mortar is commonly used to adhere terra cotta roof tiles. A wet mortar mixture is applied to the back of each tile before pressing it into place. The mortar hardens to permanently affix the tiles. In some cases, metal clips or wiring may be used instead of or in addition to mortar. Adhesives are another option, offering strong bonding power. When working in cold weather, adhesives may be preferable over mortar.

During installation, terra cotta tiles must be properly aligned and spaced. Tiles are laid in overlapping rows, working from the bottom edge of the roof up. Placement techniques aim to produce smooth, even courses without gaps. Staggering and blending tile colors can create an attractive, natural look on the roof. Skilled installers take great care in handling and positioning each tile.

With meticulous preparation, high-quality materials, and proper installation techniques, terra cotta roof tiles will provide lasting beauty and protection.