Eventually weight will be properly implemented and even those precious ceilings won't be enough to save you.
After three months of preparation, the dwarves have finally prepared. The distant blue horizon has grown bigger every day, and now it looks to be only a few dozen miles off. Children and gatherers are hurried behind the stern steel doors and microcline walls, not a single crack or crevice left intact by the masterful masons.
After only an hour or two, the rumbling becomes overwhelming, forcing all but the most fearless dwarves back into their subterranean homes, leaving the mayor and a few strong soldiers to bear witness to the structural integrity of the only defence they have.
Eventually the wave crashes down upon the door and walls, each bending or moaning in protest, the guardsmen throw their weight against the three inch thick door but the hinges snap and tear across the small covered space, pushed by a wall of azure water. It takes a few moments for the children and workers below to discover what has been happening up above, as the tunnel fills. A few dwarves hide in their room, hoping to hold back the tide and swim out when the worst has passed, but, trapped deep in the bowels of the mountains, even the passing of the flood does not save them. The lucky ones venture out and drown quickly. The most stubborn just... fade away.