The tiles, while being largely waterproof, do not prevent all water from reaching the roof deck. Without an additional waterproofing layer your roof would have many leaks. There are many areas on the tile roof where water typically can find its way beneath the tile. At the ridges, hips, penetrations for your water system venting and gable ends and also sometimes between the tiles themselves.
There are two main types of tile. Concrete and Clay. There are several profiles available with each type of tile and many colors to suit everyone’s taste.
The choice is mainly a matter of taste. Concrete is very durable, is available in 3 main profiles and is more common that clay. Clay tile tends to be more brittle and is difficult to walk on them without breakage. There are also fewer choices of colors. Clay tile has the perception of being more valuable or higher end.
A roofing expert can evaluate the roof. The following are some of the things they look for. Discolored underlayment which indicates the underlayment is drying out, the asphalt based underlayment should be a deep black color. Splits and tears in the underlayment due to dryness. The underlayment should be flexible to allow the membrane to expand and contract during the structures heating and cooling cycles. As the underlayment dries out over time it loses its flexibility and tears or splits where it is attached to the roof deck. Each tear is a potential leak.