you will never find joists in a cieling joists are in floors while rafters or trusses are found in cielings start at an outside wall measure in 24 inch icrements rafters an trusses are set every 24 inches by code unless your house was built by an inexperianced wanna-be carpenter they will fall every 24 inches

some are 16 " & some are 24"

should be 16 inches to center

I have tried to tap my knuckles-did not work, tried a top of the line stud finder & it did not work. I have tapped several holes and found some joist but have noticed that half the room run the length of the room and the other half run the width of the room. is this possible? any other ways to find ceiling joist in a bedroom? the room is downstairs with another room above it.

Thanks for your help