It sounds to me like you are confused about how your septic system works. There is a sewer line running from your house, typically your basement, to the septic tank. The line is often 4 inch diameter PVC, however it could be cast iron, and if your system is really old, it could be something else like fiber cement or asbestos.
The line enters the septic tank, which is designed to allow solids to settle to the bottom, and has an overflow for (relatively) clear effluent to run to the septic field. There is a pipe, typically 2 inch diameter, that runs from the septic tank to a distribution box, usually made of concrete, located at the head of the septic field. On really old systems, there may be no distribution box.
There are typically at least two lines, often 4 or more, that leave the distribution box. These lines are often 2 inch perforated PVC, and they run in gravel filled trenches. The trenches are the leach field, although there are alternatives, like a field with no trenches, or specialized plastic housings that serve in lieu of the trenches.
If you have a pumped system, there is pump chamber after the septic tank, with a pressure line leading to the distribution box. The rest is the same.
So, you need to look up the plans for your system, which are typically filed at the local Board of Health, unless of course your system was built without a permit, in which case you may have a hard time finding the plans.
Now to your problem. You say the septic tank fills up, and you think this is due to roots. The most common cause of septic tank failure is not the tank, but the system clogging up downstream, causing a backup into the tank. Systems fail for a wide variety of reasons, including high groundwater, poor construction, chemicals entering the system, too many solids entering the system due to failure to pump the septic tank, undersized system, and some less common causes.
Putting root killer into your system is a VERY BAD IDEA, as it may well kill the bacteria that live in your septic tank, or the bacteria that break down sewage in the field. Either way, this is big trouble.
Step one, you need to get your system inspected. I do not recommend this as a DIY, since you need to get your tank pumped, and the tank must then be inspected. Inspecting a septic tank is VERY DANGEROUS, since the tank may be filled with toxic gases, and even after pumping entering the tank requires special training, and special equipment. Once you pump the tank, and pop the cover on the tank, it may be possible to see with a flashlight if roots have in fact entered your septic tank through joints or otherwise. Most probably, you problem is in the field, which is best inspected by finding the distribution box, and determing the elevation of liquid in the box. There should not be any liquid above the invert of the pipes leaving the box.
If the distribution box is filled with liquid, the system has failed, and you will need to get it replaced. That is also not a DIY job, and will almost certainly require a permit from the Board of Health, and may well require an engineer to design the replacement system.
