Years ago my dad got a convertable extension/step ladder at sears I use, but it won't go in storage, I made some major mods to carry it....
Ripped off the factory excuse for a ladder that could not even come close to holding my 300 pounds let alone a tool box, Put a couple of heavy duty "J" Hooks at the roof line, These can hold my weight,, In fact just one of them can hold my weight. Put a stainless steel "Grab bar" at the bottom where the ladder "Hits" the rear cap (only it does not hit, actually the bar is about a foot up from that).
Extended it puts me on the roof
Step mode handy for wash and wax
Holds my 300 pounds real nice
I do not, however recommend it.
There is the "Quickee" ladder (I think that is what it is called) this is the one that can be folded in segments and can be used as a scaffold, or step or extension ladder. And tehre are some folding ladders,, I have never used any of them.
I also have a telescoping.. It is not long enough for the roof (normally) but it too holds my weight no tribble and is great for working up aroudnd the roof line (Say on the awnings or some such)