I want to thank everyone that has offered an answer to this problem. However, I made one mistake in my description the trusses are not on 16" centers....they are on 20" centers. From what you all have told me, this may not be a good sign. Keep those suggestions coming. By the way we talked to the tile distributor and he said that we needed a minimum of 1 and a quarter inches of underlayment and that trusses on 20" centers were not good.