Best way to texture roof?

I'm having problems texturing roof shingles.

For tiling wise, I uses different textures and apply them randomly between.
However, what about applying dirt/rust on them? I always couldn't get a nice dirt onto these roof.

Can you guys share some of your secrets on texturing them?

nDo does a very good job of adding dirt and rust maps. Buggy to use at times but it does what it was made for very quickly and to the highest standard.

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