Block wall cap

I have a painted block wall around a garden, top course is grouted. The wall now stays damp enough from rain so that mildew grows. I want to cap the wall by mortaring on clay barrel tiles. The tiles certainly aren't waterproof but by nature of their shape they should decrease the amount ow water that now penetrates the wall.
My questions: will the clay tiles keep the wall dry enough internally so that mildew isn't encouraged? If not, what suggestion can you offer? I had thought of installing them over a membrane of 90# roofing material (as was the house roof) but that would mean imbedding the roofing material in hot tar...not something I care to do. Any alternatives to that?
Thanks,
dadiOH
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