This is taken from the Potomac registration form:
"ROLL BARS - All open top cars (e.g., cabriolets) and all cars that exceed the modifications permitted for the
“Stock“ and “Prepared” classes per PCA club race rules (i.e., would be classified GT-1 through GT-6) must
have roll bars that meet club race specs for materials, design, installation, and height. Top of the main hoop
must be at least two inches above the driver’s/passenger’s helmet when seated in the normal driving position.
(Details in Appendix A of Club Race rules.) Integral roll bar in the 986 and pop-up roll bar in the 996 cabriolet are
acceptable, subject to two inch height requirement specified above. 914’s, Targas, and 986 w/ hard top in
place do not require roll bars. (Note: Roll bars in cars not requiring roll bars are not subject to the above requirements, but must be
sufficiently secure to not become missiles in an accident.)"
The entire form can be found at
http://www.pcapotomac.org/Drivers_Education/2002PotomacTechForm.pdf
hope this helps!