Roof truss design is a science in itself. I’ve done a bit of design and build roof trusses using green oak, and also familiar with cutting and pitching, but I am by no means an expert on pre-fab stick frame trusses, which could be what you have in your garage.
Much will depend upon their design as to how they give and take the loads and forces, so pictures will be helpful and probably necessary before anyone with any knowledge could say yay or nay.
In general terms, if you are wanting to support a floor by the roof, your trusses will want one central or two vertical posts, which will transfer the resistance of the side walls wanting to spread due to the weight of the roof, upwards through the King (1 post) or Queen (2 posts) post/s.
It is probable that the truss design in your garage was designed to resist external loading (e.g. snow) as well as the weight of the tiles.
Also, bear in mind that a little knowledge is a dangerous thing and my knowledge is close to dangerous 🙂