Lemons or Lemonade

A few months ago our sons reported that "something strange" was happening to the wall behind their toilet. I sent hubby downstairs to investigate. He discovered what can only be described as "blisters" on the walls, and being the manly man that he is, he popped one and found that it was full of water. Blistered, sagging walls meant only one thing, we had a leaky pipe somewhere above or behind