this truss thing on a 24 wide building and all the concern of roof failure is nearly babble, there are a lot of old buildings that are 24 foot wide, that only have 2x4 Douglas fir rafters no trusses at all, and the building roof has not failed, now the normal 2x4 of today is not strong enough but a 2x6 is, so it would not take that much to modify or rebuild the truss or rafters,
I will agree a truss should not be modified as there a designed structure, but if the alternate is designed to do the same thing what the problem? if he sisters up existing top cord of the truss with a rafter capable of handling the load there would not be a problem, before the rest of the truss is removed,
If this was a 30' wide or wider building there would be a need for totally engineered, design, and would most likely need to be a replacement truss of some form.
and going back to the quote, that is in the above post, tell me how many houses that have trusses, also have interior walls that touch the truss bottoms, most likely 98% do in some place of the house, thus what would equal to an additional support,
I am not saying that an additional support may or may not make a truss fail, but there is a large number of buildings that have been built that would equal to additional supports in them,
and the post also said snow was a factor, (was the truss designed for the proper snow load?)
I have designed a few truss, in the past (many years ago), so I have a reasonable Idea on what it takes to make and what the stresses are, (and I can see how a block in the wrong place on a truss could be damaging),
but like I said my guess is 98% of all homes with trusses most like have a wall some where that touches the truss bottom cord, and in most homes truss do not fail even with snow loads, and if the snow dose cause a failure then most like it was over loaded, additional under support or no support,