We have tried a lot of different kinds of root killers. Some we put in
ourselves and one called Root-X had to be put in the pipes by a plumbing
contractor. Nothing has worked. Every 6-8 months the sewer needs to be
cleaned out by a snake. That is the only thing we can do.

I had a backup problem. Water was coming up from the washing machine
floor drain and drowning the kitchen floor. Plumber tried snakes. Didn't
work. Then he said he might have to dig up the line. This would involve
ripping out the front porch, and digging for maybe 30 feet.
Then I saw some root killer at a lumber company. Copper-based. Don't have
any empty container, so don't remember the name. Might be Roebic. It worked
fine. No more problems. Pour it in the toilet three times, and flush after
each application. But you have to remember not to run any water nor flush
the toilet for as long as possible - at least overnight. I might have used
it several nights in a row.
Roebic also makes a bacterial enzyme to unclog pipes. You are supposed to
mix it in warm water and pour it down all household drains once a month.
Again, don't run any water or flush anything.
I'm thinking it putting it in the washing machine in warm water, and
running the washing machine on rinse just before bedtime, because the floor
drain is now sealed. Not sure if this is a good idea.