We have 3 trees that are about 30 years old and maybe 30 feet tall. We are installing new patios and want to cut the roots back so they don't buckle the patios. The trees are about 10 feet away from the edge of the patios. We are hoping to keep them because they provide shade. We are in Santa Barbara, CA.
Most sources recommend following this rule of thumb when trimming tree roots:
Generally, you can safely prune roots that are 3-5 times the diameter away from your tree. So, if your tree has a diameter of 3 feet, only cut tree roots 9-15 feet away from the tree. (source)
There is a more conservative rule recommended by some:
[Don't cut] roots closer than 6 to 8 inches from the trunk for each inch in trunk diameter. That means stay at least 10 feet away from a 20 inch tree. (source)
To calculate your tree trunk diameter, wrap a measuring tape around the trunk "at breast height," about 4 feet off the ground. Divide the circumference by pi (3.14) to get the diameter. (Make a mental note to remember this moment for when your kid complains, "When am I ever going to use this?" about their geometry lessons.)
There are some other factors to consider. Depending on which factors your tree has, you may decide to not prune the roots, or to err on the side of the more conservative rule.
- number of cut roots: more roots cut means more tree stress
- proximity of cuts to the trunk: the closer cuts are to the trunk the bigger the impact
- species: some species tolerate it better than others tree
- age: old trees are more likely to stress and die tree
- condition: trees in poor health should not be root pruned tree
- lean: leaning trees should not be root pruned
- soil type and site drainage: shallow soils mean stay farther from the trunk (source)
Ficus trees are known for growing lots of roots relatively quickly, so I would expect them to recover more quickly from root trimming than other tree species. The important thing is not to destabilize the tree so much that it falls over. On the other hand, they will probably grow new roots exactly where you don't want them. You might want to install a root barrier to protect your patio from future root growth, as explained here.
If you add some photos of your trees, the community can make some specific recommendations about your trees, eg whether they look in good enough condition to tolerate root trimming.