and officially, you should not use partical board or luan plywood directly under ceramic tile floors. However, as a example, my house is 23 years old and in the laundry room (7'x13') there is inlaid placed directly over particle board. Now after these 23 years, you still cannot see any of the seams of the partical board coming through the inlaid! Go figure. I would suggest that if you have the time and money, and want the end result to last forever, remvove the luan and partical board. Put down 5/8 plywood followed with 1/4 inch cement board (such as Hardibacker) and then the tile.