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2.2 15-Day Deletion Timer for Not ā€˜Touched’ Structures.
After 15-days, a structure not ā€˜touched’(interacted with) will be deleted by the server. You or others must minimally ā€˜touch’ a structure every 15-days to avoid deletion.

touch means literally standing on top of the structure or walking on the blocks. HWS Structure Commander (via HWS Connect) displays the number of days since a structure was last touched. Using the Control Menu ( P ) is NOT considered touching.