I'm finishing my basement. Before starting, the joists supporting the first
floor were very visible. I want to put in a ceiling in part of the basement
and I can use either drywall or a drop ceiling. I understand that if I use
drywall I must use 5/8" type for fire resistance purposes. If I install a
drop ceiling this will be the kind where the 2' x 2' tiles are supported by
plastic extrusions (CeilingMax/GridMax system) which are attached directly
to the joists. This gives a drop which is only 1.5" below the joists. I have
two questions.

1). If the joists are currently unprotected why is it necessary to use 5/8"
drywall for the ceilings in the finished areas?

2). If I use ceiling tiles supported by plastic extrusions, the plastic
would melt in a fire. How can the drop ceiling be a satisfactory alternative
to a 5/8" drywall ceiling?

I prefer the drop ceiling because there are several electrical junction
boxes attached to the joists and I assume reasonable access to these should
be provided.

Peter.