Your roof is way superior than any roof made with asphalt tiles Lise. If you put asphalt tile on your roof, you will lower the value of your house and will have to change your tile many decades earlier.
I think it probably is as well Michel....although I've never understood how a porous material can be effective in a cold wet climate. Their intended use is for warm arid places. If they behave anything like terracotta pots, then I don't get how they can work here at all. They hold the moisture, then as soon as there is a frost, the expansion upon freezing causes them to crack.