If your twenty foot extension ladder wont reach it should probably have a roof access. Unfortunately that isnt always the case. There is a code in place with regards to it but im sure its different everywhere.
There's gotta be a code for it, in most places anyway.. ask Payson about that one lol.
Around here I've seen buildings over 30' tall with no access anywhere. But I've seen buildings I could get on with an 8' ladder with a fixed ladder on them.
If I were an engineer I'd have a stairway (the kinda steep kind) access to every roof for a building I designed. I know how much it sucks going up a verticle ladder with tools, or up an extension ladder that barely reaches.
Hummmmm... perhaps an elevator to the roof instead?
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NFPA 54 chapter 5.4.3 requires roofs that are over 15 and have gas equipment shall be equipped with inside access to the roof. The section describes the type of access required. So if you have a new building being permitted, you can have your access but if it is an existing - good luck on the code enforcement. Why do I still buy a ton of extension ladders???