Hi, would anyone know what you can put down your floor drain to kill tree roots

Copper sulfate bad part it could kill the whole tree

Tim got this correct. However, if you have a drain line clogging with roots copper will kill the root but not remove it. You will still need to rod the line with a 3" cutter and a sewer rodder. Copper sulfate would then be applied maybe twice per year to keep the roots from getting back inisde, the tree actually could survive, but you rod first and then the copper in 6 months.

In addition: You never remove roots from sewer line. Once they are there - there are there to stay and prosper. You cut them, they'll grow back. Roots in the sewer line require periodical drain cleaning to cut new growth down to minimum.

FYI: I understand it may not be option in your case - but the best way of getting rid of roots is to replace the sewer with plastic pipe with glued / welded joints.

Going in, let me say that sooner or later you're going to have to bite the bullet and replace that old sewer line with PVC. If you wish to control roots RootX or Robics Foaming Root Killer that contain Dichlobrnic are two products you could try.
A less expensive way would be to call around to garden supply stores and ask for fine grain Copper Sulphate. Put a 1/2 pound in your toilet and flush it down. Repeat in 6 months.
Hope this helps and thank you for rating my answer, Tom