Roof Framing
has some answers, . . but if you have trusses, you don't have ceiling joists (not for the ceiling directly below your roof attic, that
is). You have the bottom chord of the truss.
Frankly, for vertical loading, the bottom chord is typically
the weakest member of the truss. When it fails, the web
members of the truss begin failing, along with the
connections, and . . . of course, finally, the the top chord
members. Meanwhile, your ceiling has become a mess.
You will notice, HVAC folks prefer to hang their air
handlers, etc., from the top chords rather than sit them on
top of the bottom chord. This has a secondary advantage of
further isolating the vibration from the ceiling membrane,
but the principal advantage is the better positioning of the
added weight load.
This is not to say that bottom chords can not be designed to
carry a floor load. They can. But in most housing, they
are not. Christmas trees and insualtion not yet installed
is about the best use for storage on the bottom chords.