I know this is a newbie type question, but do modern roof trusses require an intermediate support under the bottom cord or are they designed to require support only at the ends of the bottom cord ie. at the exterior walls?
Depends on materials used, size, location and what the engineering data states.
Most regular (isosceles) triangular truss systems usually carry loads to the outer bearing walls as in bugalows, full 2 storey homes without attic storage.
Most do not require any intermediate support, that’s why they installed trusses. Most trusses span the building from one end to the other.
This link will help you.
Every truss I ever installed or heard of over the years specified no bearing on the bottom chord. Simpson makes special clips to tie interior walls to the trusses. These clips have slots to allow for some movement.
Exactly what David, Brian, Barry, and Kent said…!!!
Trusses can be designed for any number of bearings. Sometimes the building designer requires intermediate support for various reasons. They might want to reduce loads on the window headers for example. They might be concerned about deflections of a long clear span.
Sometimes trusses have a paper tag installed at interior supports that is stapled to the webs. This practice is not common, but some manufacturers do it.
The bottom line is this. If the truss engineering drawing shows a bearing there then it is required. If not then it is supposed to be clear span. Most trusses under 60 ft spans do not typically require a support, but it’s really up to the person who designs the building do determine.