I've been "off line" with respect to the FML for a few months and just
getting back to reading.  The suggestion about tacking down chimney flashing
sounds effective.  I used a similar approach but with a metal door threshold
that can be purchased at a hardware store.  Its a bit more decorative.  They
did eventually start digging at the edge of the threshold, however.  I
finally managed a permanent fix about three years ago.  Threw out the carpet
and put in hardwood floors ;-) They are also easier to clean up than carpet
and don't hold the smell!
[Posted in FML issue 2349]