I know this is a late reply, but I have a few things to add. If you
have a chronic rat problem, chances are pretty good you'll need a
professioanl. The most important thing you need to know is that there
are differnt kinds of professionals.

We first called an exterminator/pest control company (Terminix). All
they did was put out traps and pick up the carcasses. The problem
didn't go away until they finally referred us to a company called
Proof It, also in the "Pest Control" area of the yellow pages, that
specializes in "animal exclusion, cleansing, and sanitation." These
guys were not cheap and must be making a bundle an hour, but the guy
that came out found every hole in the roof, attic, and crawl space,
and sealed it up nice and neat--nothing ugly or even obvious. They
also removed all the NASTY insulation that had been soaked with rat
urine and a whole bunch of carcasses that were the result of
poisoning. (Besides the pet/child health risk, I don't recommend
poisoning--the previous owners of our house did that and there were
putrid smells coming from our bedroom wall where the rats scurried
back and forth.)

The problem's been away for well over a year. I didn't want to buy our
house because it smelled so bad when we looked at it, but now that
we've conquered the rat problem, it's in great shape. Good luck!

Mary