Back of clay roof tiles wet - is it a problem?

[RufusStone]

My 1930s house has its original roof, which I have been told is made of Belgian clay tiles with no felt or membrane underneath. I recently had a look in the attic while it was raining heavily and I saw that the backs of several tiles had visible damp areas (not dripping).
Is this a problem, or is it how this type of roof is expected to perform?
Cheers.

 

[noseall]

Looks like condensation. Keep the loft hatch closed.

 

[^woody^]

Damp on the joints is normally seepage through the joint.

That's not to say it's an actual problem.

What's the external like? Any moss, any shaled tiles?