I would do some research to check out what the potential interaction might be between whitewash and the material your roof is composed of. I know that a problem has recently been observed around the use of copper flashing & wood shake roofs; because of the wood preservative being used leeches out & reacts with the copper.
You might also consider if it matters that the whitewash will change the Ph of the ground surrounding your house, potentially having an effect on whatever is growing there.
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