Dendrobium Spectabile - Rotten Roots?

[dianecty]

Hi everyone! I am a newbie at orchid growing but I love the more unusual flowers. I just bought a dendrobium spectabile.

When I received it, I depotted it right away and saw that most of the roots were dead - I think. My question is how can I tell if the roots are dead? They are beige or brown. None of them have green tips. When I pull on some of them, the velamen comes off and only the thread remains. That means they are dead, right? However, a lot of these roots are firm but I can still pull off the velamen.

The canes are very healthy, fat and firm.

What should I do? Should I just cut off all the rotten roots? Just pull out the velamen and leave the thread?

Can it survive if I remove the rotten roots? How can it be fat and firm if the roots are rotten?

Any help is appreciated!