Answer: There's really no way to tell whether or not you might damage the root systems until you start to dig. Damage of small roots probably won't have consequences but digging through large main roots could compromise the health of your arborvitaes. You might try a few pilot holes to see what's in the earth where you want to place the posts. I don't think there's any other way to tell. Best wishes with your landscape.
Q&A Library Searching Tips