Re: Sagging tongue and groove ceiling tiles
I'm thinking those tiles are 12 x12 inches wood fiber tongue & groove..
Ideally they should either be attached to the ceiling with adheasive applied to the backs of each tile.
there should be a series of strapping spaced at 12 inch centers on the existing ceiling which would be running perpendicular to the direction of the ceiling rafters .... the tiles attached to the strapping. along one side commonly with staples in the groove.
It's unknown as to how things were done for those tiles you currently have .... the fact they are sagging is clear that whatever method used is failing.
At this point it's going to be difficult to say what the best way to reattach them short of possibly trying to remove one of two tiles to see what's behind.
The problem may be they will fall like dominos which means you might end up taking them down before they fall down then put them back up properly.
Hope this helps.
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