Question
I live in Southern California. Recently I drained the water from my swimming pool onto the lawn next to the pool. A day later, I noticed a hole in the fiberglass pool.  The repair man said it was due to a tree root under the pool. I have 2 Italian Cypress trees about 15 and 20 feet away from that hole.  The trees are about 20 feet tall. I want to know  whether these trees' roots could have coused that problem, and thus if I need to cut them down to prevent the problem in the future. The only other tree that is suspect is my neighbor's oak tree, which is probably ovr 100 years old and is about 70 feet away. I don't want to cut the trees, but I need to know if their roots grow straight down or spread out. Thank you.

Answer
First you need to determine how far out the roots will be.
This is called the Protected Root Zone, PRZ. This is typically about 40% larger than then dripline (reach of outer branches) area.


To approximate a tree抯 Protected Root Zone:
1. Measure the tree抯 circumference (distance around the tree) in inches at 4.5 feet high.
2. Divide the number by 2
3. This is the radius (distance out from the tree), in feet, of the Protected Root Zone.

Example:
1. Circumference = 30 inches
2. 30 / 2 = 15
3. PRZ = 15 feet out from the tree

Now if the pool is within this area there is the possiblty of the roots getting to the pool. Do this measurement and you should be able to determine which if any tree maybe the problem. One way the combat this without cutting the trees is to dug a trench near the pool and cut the roots. You will have to dig down 12 -18 inches and then you can either construct a barrier or fill the trench with a lighter soil maybe sand. This trench will need to be dug out again in several years to insure the roots are kept away. A lighter soil (like sand) will ease the digging. A barrier would be premanent something like a cement or brick wall.   
I hope this gives you some insight.