It is often said that clay tiles have a limited life of up to 60 years or thereabouts. However, walking around the countryside you will often come across peg-tiled roofs that are several hundreds of years old, so this is clearly not a reliable guide.
The failure of the tile itself will depend on many different factors, including the original manufacture, the make-up of the material within the tile and its firing in the kiln.
One of the most common problems of a tiled roof is slippage due to rusting metal fixings and decaying battens. The best solution is to re-lay the tiles, salvaging and reusing as many of the originals as possible. The alternatives (see below) can be disastrous.
Because tiles are much thinner in section than brick, they are less susceptible to variations in firing. Nonetheless, there will always be some tiles that are from the cooler parts of the kiln and therefore more vulnerable to early failure, particularly as a result of frost damage. That said, as a rule handmade tiles tend to have great durability and, if well-fired, tend not to be particularly vulnerable to frost damage. Only after many years will the best examples eventually weather, exposing the softer and more porous clay body below to frost damage.
Other factors which can influence the longevity of tiles (and, in fact, any roof covering) will be the orientation of a building, the steepness of the roof and indeed the microclimate around the building. Clay tiles are best used on roof pitches of 40° but some single lap tiles can be used down to 25° pitches.
Other more controllable factors include such matters as tree branches brushing up against the roof covering and dislodging or breaking tiles, climbing plants being allowed to grow over and into tiling to dislodge and damage it, and clumsy workmen treading on the tiles.
Due to the rough texture of a clay tile surface it is likely to harbour lichens and mosses. These plants should not necessarily be regarded as harmful. Although lichens produce acidic secretions and moss can hold moisture and lead to frost damage, they are unlikely to cause much damage. Indeed moss can provide a protective layer and lichens contribute to the characteristic colouring of tiled roofs. However, significant moss growth can increase the weight on the roof structure generally, and when it dies and rolls into the gutter it can cause quite serious gutter blockages.
If moss is to be removed, care should be taken. Simply pulling moss from the roof surface is more likely to cause damage than by letting it die naturally or by appropriate chemical removal means (biocidal treatment). However, care should be taken with chemical removal methods to ensure that the chemicals do not run down to the gutter and into the surface water system.
The defects that most often affect tiled roof coverings are in fact the sort of defects that affect all roof coverings: failure of battens (rot, woodworm etc); failure of the batten fixings (nail corrosion); deterioration of the tile fixings (rotting pegs, corroding nails or crumbling torching); failure of or defects to the roof frame; defects to perimeter details (soakers, flashings, etc); defects to roof details (valleys, verges, eaves, etc); and wind uplift.