Have a plaster ceiling that will not hold a heavy 5kg lamp.There is a ceiling rose and a hole for wires. The only alternative i can think of is drilling 6 or so heavy duty screws (through the 4 inch hole) into the floor boards above the plaster ceiling. These floor boards are pine and about an inch thick. Will i get enough purchase to carry the weight?
The lamp must not be attached to the ceiling. It must be attached to an electrical junction box. That box, in turn, needs to be screwed or bolted to joists or other framing items in the ceiling. So far as I know, it's not only risky but violates code to mount either the lamp or electrical boxes to plaster/drywall, etc.