I have a problem with the ceramic tile in my new house. The grout lines are cracking all over the place! Our contractor brought the tile subcontractor in and they got into an arguement over the cause of the cracks. The tile man says the underlayment is not sufficient and that the floor moves enough to cause grout cracking. The contractor claims that the underlayment is sufficient. After much arguing they arrived at this conclusion: the underlayment (which is one and an eighth inches...a three quarter plywood subfloor with a three eights layer of plywood) is suitable, however they conclude that the floor moves because it sits on 30'engineered floor trusses located on 16" centers. They say that the trusses flex enough to cause the tile grout to crack. They are asking us to allow them to place two posts under the center of the floor in order to prevent the trusses from flexing and therefore cracking the grout. The problem for us is that these posts would be placed in our basement and would block access to our basement garage door...where our boat stays. We wanted floor trusses so that the basement would be open and we could keep our boat there. Is this a logical conclusion or are they barking up the wrong tree? I hate to use the posts because it will make my basement unusable for storing my boat. I have been told that using a grout with an epoxy mixture might fix the problem. The current grout uses a latex mixture. Also...do floor trusses really move that much? If so, how does someone place ceramic tile over a floor that sits on trusses? I find it hard to believe that flexing floor trusses are causing the grout to crack. By the way, no tiles have broken...only the grout has cracked. I really need some help on this. Thanks.