I recently repotted four phals and encountered roots in different
conditions:

1) Greenish white, aerial roots, sticking out of the pot. I was careful
to mentally earmark these so that, in the new pot, they are not put
into the bark put left aerial--outside the potting material.

2) Healthy light and dark green, thick roots that were in the potting
material. Actually, some of them aren't round but, adapting to gaps in
the potting material, have an oval or almost flat cross-section but at
some points are more roundish. Also, they show some branching. I potted
these back into the new bark in the pot.

3) Dark brown, rotten roots with dark material oozing and coming off:
gross! I cut these off with a sterilized scissors, and washed away the
residues.

4) Questionable dark greenish brown roots with some sections sloughing
off, leaving the wire core you describe. Comparing to the greener
healthy roots (2) and the darker rotten roots (4), I postulated that
these roots were in the process of decaying and decided to cut them
off. Note, however, that some of them were quite long and looked
healthy up to some point. In those cases, I only cut off the unhealthy
portions.

During potting, I found that two of the phals had mostly healthy roots,
one had one-half of its roots rotten, and the last had only two viable
roots! But after potting, they're each doing well. I expected the one
with 99% rotten roots to die (since it had so few roots left after
surgery), but instead it's been growing new leaves and roots and
apparently thriving in the new potting material. In both of the
high-rot cases, the orchids had been potted--by the vendor--in a thick
peat moss that retained too much moisture when placed indoors. I
replaced that with a bark-perlite mixture in transparent, ventilated
pots.

-Munir