Can the tiles be installed over a timber frame or joist system?
Yes. Provided that the joists are 12″ off center and leveled with a laser level and the frame is sturdy, you can install the tiles over the joists. Your only limitation is the direction of the tiles; they must be installed perpendicular to the joists. Pedestal systems can be installed under the frame to even out any slanting or grade of the surface if working with a sloped roof deck.