Since I didn't get an answer to this question in my thread about salt
damaging my shower base, I'll try my question here.

What is the prescribed dosage of rock salt to control roots in a
drain?

Some folks in this group have said they dissolve 25#, but they don't
say how often.

One site, supposedly quoting a Mr. Rooter contractor, had this:

"Periodic (every 5-6 months) treatment with 1 cup of rock salt poured
into the clean-out, followed by running roughly 1 gallon of water
through the system and then allowing it all to "soak" overnight"

eHow says: "Rock salt will kill any plant or root it touches. Flushing
it into the sewer system several times daily should penetrate and kill
the roots."

Other sites simply say "use rock salt to kill roots in drains" or some
variation on that theme.

Nobody seems to have a consistant answer as to a specific amount and
how often - except for Mr. Rooter - but that suggestion is so far off
from the "25#" suggestion that one or the other doesn't seem valid.

Does anyone have a source for a firm answer? I don't mind using too
much salt, but I'd really like to know *how often* I should be
treating my drains.

I usually have to snake my drain once every couple of years but I'd
rather do preventative maintenance.