Slate roofing over cedar shakes

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I want to buy this 1901 brick house with a slate roof on it. But the slate roofing was installed over the cedar shakes, this let's the bats in the attic. With all the bat dung now inside the attic the insulation has to be removed. The inspector said that the roof needs to be replaced with new plywood then put the slate back on. Estimated ballpark cost 26K to 35K for replacement and install.
My question is with the new spray on insulation could this be applyed to the under side of the roofing to seal the cedar shakes and slate roofing so that the installation could be removed with the bat dung and to keep the bats out of the attic?