An electrical contractor told me about plastic 4" round ceiling boxes. He said that in order to be rated for 50 lbs. you had to install a screw in the box and attach it to the ceiling joist. I asked him, OK where does it say that? and he said he the last AHJ told him. Now I don't know if it was true or not, and I was trying to find something on either Slater or Carlon website, but all I found is that those boxes are rated for 50lbs. I tried to find installation instructions, but nothing was written there either.Anyone heard this before?