try pointing your cursor along the edge of the floor you're building on. that will have the new piece snap to that edge. don't hold shift, it shuts off snapping (allowing you to place pieces more freely).
if you can't see the edge of the floor you want to snap to, (for instance building a bridge) you can place the second floor on top of the first, so it's on top and over hanging the first some. then place a 3rd. if you do it right, the 3rd should snap to the 1st correctly and you can delete the 2nd.