Tree roots can effectively block water flow inside sewer pipes. So, this is probably something you need to get on a schedule of doing. We have religiously done this for more than 30 years, and have never been troubled by root stoppage in all that time. (We only bother with doing the flush method..... once a month.)
Before that, it was backed up about once or twice a year... and the pros just charged us... never telling us about copper sulfate crystals.
The inside of a sewer pipe contains all the nutrients a tree needs, and trees are adept at sending their roots through small cracks and fissures to get them, thus causing plumbing problems. One way to prevent this is to treat the soil and insides of the pipes with copper sulfate.
Clearing Roots with Copper Sulphate
Copper is poisonous to roots, but it usually isn't fatal to trees, because the roots absorb it for only a short distance. Consequently, copper sulphate is a recommended treatment for sewer pipes, whether they are already blocked or not. The chemical is available in crystalline form, and a typical treatment consists of flushing the crystals down the toilet. It's more effective against small feeder roots than it is against larger, better-established ones. The latter may die within a week or so of treatment, but they take much longer to wither and wash away.
Preventative Maintenance
Tree roots avoid copper sulphate, so one way to keep your sewer lines clear is to regularly pour a small amount -- less than 1/2 cup -- of copper sulphate into the toilet. If you know where the sewer lines are, you can also treat the ground around them. An effective method is to dig a hole to the top of the sewer with an auger, sink a pipe and fill it periodically with copper sulphate crystals and hot water. The chemical leaches into the soil, and roots will avoid growing there.