Great question — answering as the UphillTech team:
The joists are likely on either 16 inch or 24 inch on center (OC) spacing, though rarely some are 19.2 inch oc. They will usually span in the direction of the shortest room dimension but not always. Since you have one located as you noted, I suggest measuring 16 inch and 24 inch in both directions and try drilling there and you should hit them with one of the holes. You can also 'thump' with your knuckle or object to get listen for the difference between the solid sound near a joist and the hollow sound between joists to get an idea of where they are located. And if you get very good, can even locate them fairly well. Your stud finder might have another setting for thick walls, try using...
Full answer by Ack: https://diy.stackexchange.com/a/189215 (CC BY-SA 4.0).
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