 With the Song-style forty-percent overlap, it was not possible to have triple tile overlap, as there was a twenty-percent gap between the first plate tile and the third plate tile.Hence, if a crack developed in the second tile, water leakage was inevitable.On the other hand, with the Qing dynasty style seventy-percent overlapping, the first plate tile was overlapped seventy percent, forty percent, and ten percent by the second, third and fourth tiles, respectively; thus even if the second and the third tiles developed cracks, there would be no leakage.