It is best to NEVER install conduit directly under the roof decking. Run your conduit and cable on the bottom part of the truss, and secure it in place with tye-wire if a ceiling is to be installed, or strut if it will be exposed.
Be careful here. To be in complete compliance with some building codes you must have a written statement from the engineer of record to support anything off the bottom chord of a roof truss. The engineer of record must be the one who stamped the roof structure design.
