Clay Tiles + Moss = Disaster
Have you ever had a clay or terracotta plant pot in your garden and used "feet" to raise it off the ground? You will probably know that if you leave your pot in direct contact with the ground it will crack during the cold winter months.
That is exactly what can happen to clay roof tiles
Moisture is the problem here, when it freezes during winter months it will expand and create a crack. That is not so much of a problem with clay pots as they are not too expensive. Clay roof tiles however are.