concrete roof tile over asphalt shingles

Someone asked me a question today if it was ok to install concrete tile over asphalt. I told him no because concrete tile weighs about the same as 3 layers of asphalt shingles and he believes the house he is looking at already has 2 layers of asphalt shingle. In addition to making sure the truss/rafters (not sure, haven’t inspected it yet) are in good shape as is the plywood/osb sheathing, is there anything else to lookout for in a possible conversion from asphalt to concrete…structually speaking. thanks

Normally when a concrete tile is installed, and the roof previously had shingles, an engineer is supposed to provide proof that the roof is able to handle the load. That is part of the permit process down here in Floirda.