The question as to whether one needs a header bar installed across the top of the glass panels in a frameless shower is purely one of safety and function.
Unfortunately, our industry as a whole struggles with this issue and has failed to create a universal code or standard as to when a header is absolutely required.
Most experienced glass companies (20+ years) will tell you to always have a header because we truly know the dangers of glass and because we’ve been building custom shower enclosures for a long time. More and more less experienced companies are popping up all the time that will sell you a heavy glass shower door at a cheaper price and will install it however you ask. Even though the glass is tempered safety glass, if not installed correctly it can cause serious damage to you and your bathroom.
Some configurations absolutely require a header while others may not. Every year, new hardware pieces are developed to better support glass panels without the need for a header. It’s best to leave it to the experts to help you determine whether a header is needed for your particular situation. Silicon should never be considered as a sole means of support.
By-pass or standard sliding type doors will have a header 99% of the time as it is the only support for the glass panels.