Glazed Roof Tile

A glazed roof tile is a clay tile whose exposed surface has been coated with a glaze, a thin layer of glass-forming minerals and colouring oxides that melts in the kiln into a glossy, hard skin. Glazed tiles offer strong, lasting colours such as green, yellow, blue and black, and a surface that sheds dirt and organic growth more easily than unglazed clay.

History

Coloured gloss glazes for roof tiles were developed to a high level in China, and the Song-dynasty building manual Yingzao Fashi standardised their manufacture. Imperial yellow glazed tiles became the mark of palaces such as the Forbidden City in Beijing. In Korea, the Goryeo dynasty invented celadon glazes, used on the roofs of upper-class buildings. In Europe, glazed tiles laid in coloured patterns are a feature of buildings in central Europe and Burgundy.

Glaze versus engobe

Many modern clay tiles use an engobe instead of a full glaze. An engobe is a thin coating of liquid clay, sometimes coloured, applied before firing. It gives a matte or satin finish and consistent colour. A glaze gives a glossy, glass-like finish. Both resist fading because the colour is fired in, unlike the surface paint or slurry used on many concrete tiles.

Glazing and water resistance

A common misconception is that glazing waterproofs a tile. In fact, glazing a porous clay tile does not make it watertight: moisture can still enter through unglazed backs and edges, and if the clay body itself is weak or porous, water trapped under the glaze can freeze and cause the glaze to flake. Durable glazed tiles therefore rely on a well-fired, low-absorption clay body; architectural ceramics intended for freezing climates are fired to or near vitrification.

Unglazed alternative

Unglazed (natural) clay tiles, often called terracotta, take their colour entirely from the clay and its firing. They weather and develop a patina over time, which many people value on traditional buildings.