I provided the following answer a few days ago to a person with a similar problem. I hope it helps:
In general, the time to call a company like RotoRooter is when all of the drains in your house run equally slow. That is a very good indication that the problem is in the sewerage lines under your house or in the septic system. If just one drain is slow, the problem is probably isolated to just that one fixture (eg: hair clogging a bathroom sink drain). If the problem is systemic in nature, you will probably need the services of RotoRooter. Keep in mind that most plumbers also have the same equipment and get the same results, they just haven't bought into the RotoRooter franchise, and many times they are cheaper.
The basic business of Roto Rooter is to clear clogged sewerage drain lines. They do not usually service septic tanks since the only service usually needed on the tank is to have the accumulated solid sludge pumped out every few years. However, RotoRooter does service the sewerage lines anywhere in your homes waste system including those found in the "septic system". In most homes, there will be a 4" waste line that runs under your basement floor or crawlspace that branches out under your home to collect waste from various points. This line will exit the house underground and run to the septic tank. The septic tank simply breaks down the waste, and what comes in one side as a mix of solids and liquid, exits the other side as liquid only. This liquid only is distributed to the "leach field" where it is absorbed into the ground.
As the system has been described, it is RotoRooters' business to keep the lines both to and from the septic tank clear. Assuming that the septic tank does not need pumped out, a slow draining system can only have a few easily diagnosed problems.
The most common problem is tree or shrub roots growing into the drain line. Do you have trees or shrubs close to, or over, the drain lines leading from the house to the tank or in the proximity of the leach field? These roots grow like crazy in this wet and nutrient rich environment and unless the plants are removed, can easily clog a system in just a years time.
The second most common problem with clogged lines is foreign matter, other than human waste and toilet paper, flushed down the toilet. Any plumber can tell you stories about the things found clogging sewerage pipes in septic systems. The most common objects found are tampons, condoms, and even sanitary napkins. Even if these items make it all the way to the septic tank, and many times they don't, none of these items will decompose in the tank. At the very least, you will have to pay to have it pumped out more often. Usually, they simply get caught in the sewer line somewhere and quickly cause a stoppage.
The third most common problem is the most expensive to fix. If the system is old enough, it is probably made using short sections of "red clay" pipe. It is fairly common to discover that a section of this pipe is collapsed and it is necessary to dig up and replace the collapsed section. Also be aware that systems built many years ago often were constructed with leach fields that are woefully small by todays' standards and it could be that the leach field is just too small to handle the volume. You can sometimes diagnose this problem if the area of the leach field stays wet and emits an odor.
RotoRooter will clean out any drain line in your house using a "power augered snake" This device has an expandible cutter head that is inserted into the drain line at the cleanout in your basement or crawlspace. As it travels the length of the drain, the cutterhead spins against the walls of the drain line cutting up most obstacles in its path. If a section of drain line is collapsed, the operator will soon know it. If no collapse is present the cutterhead will eventually reach the septic tank, thereby clearing the line to that point. The same procedure is used to clean the leach lines on the other end of the septic tank. They can usually identify the cause of the stoppage by examining the debris that is cleaned out of the line.
I hope this information helps. Examine your system according to the information given before you call RotoRooter and you will have a good idea as to whether or not your system needs attention. You can bet that if you call them, they will recommend that the system be cleaned whether it needs it or not ... telling you it is a preventative measure. Now you are an educated consumer and can make the right decision.
Good luck.