Not sure what "an older house" is to everyone, but my 1964 house has a septic tank that suffered damage to the cast in place T that is just inside the tank (Terracotta fitting cast into the concrete tank). Someone slammed a snake into it and busted a piece out of it, allowing the scum layer to eventually back up into the house, clogging the pipe. Short of replacing the tank, which would require a new above ground drainfield, I have built a pipe to go inside the terracotta T and then another try to the outside of the terracotta T. Every time I did the 18 inches to the tank to "fix" this, I want to shoot the idiot who forced a snake so hard into the tank!
Make sure you know how long the snake is and how far to the tank!
David from jax