Plain tiles have been used for hundreds of years with the earliest examples being hand-made. Traditionally, they were made from clay, although concrete has also been used since the 1950s.
They are a simple rectangular shape and fairly small in size, commonly 265 x 165 mm. Plain tiles have to be laid double lapped, which means there have to be two layers of tile throughout the roof and at the end laps of the tiles the top tile must overlap the tile two below it. Due to the double lapping and the small size of these tiles, they can be relatively expensive, slow to lay and roofs can be heavy.
Interlocking tiles overlap and interlock at their sides and so can be laid single lap. Traditional examples include clay pantiles and roman tiles, although since the 1970’s the majority have been manufactured from concrete. They are commonly around 420 x 300 mm and can be laid to angles as low as 15°. Due to their large size and single lap, they are relatively inexpensive, quick to lay and weigh around 50-60% that of a plain tile covering.