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This product does not find wood. It finds the metal head of a nail and/or screw. It is up to the DIY to know if wood or metal stud is beyond the drywall surface. I don't know to what depth it will "find" the metal head. Typical drywall depth is 1/2", sometimes 5/8". This product's magnet should find the nail or screw head to those depths. But, typically, the nail or screw head is just under the surface of the drywall. If the stud is metal, you will probably be able to determine the stud edges as long as the drywall thickness is as given above. If wood stud, you will not be able to determine the edges, only the "approximate center". Also, if wood stud, finding the edges is up to the DIY and his/her "sounding"/thumping for what is a void and what is solid beneath the drywall, Finding wood stud edges requires a more sophisticated product, which means higher cost.
Why not? A horizontal joist (ceiling) is a vertical stud (wall); but, different size and/or depth. This product has a magnet on the drywall side which plops onto the head of the nail or screw when you finally hover over the metal head. All that you are doing is "finding" the "approximate center" of the stud and/or joist.
Thank you for your recent inquiry with The Home Depot. Unfortunately, I need more information from you in order to properly address your question. For any other questions, please call us at HomeDepot.com Customer Service 800-430-3376. Thank you for shopping Home Depot.