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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?

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    there aren't joists that you can attach to? Nov 18, 2016 at 14:56
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    Those screws are called lag screws... But attaching the electrical ceiling box to the bottom of the upstairs floor boards is the wrong way of doing this.
    – Netduke
    Nov 18, 2016 at 15:05
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    Definitely don't go into the underside of floor boards; future homeowners may decide to rip up that floor and get an unpleasant surprise.
    – SqlACID
    Nov 18, 2016 at 16:57

2 Answers 2

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Nik, you need to install a piece of lumber (or some such support member) that is securely attached to joists, that spans the location where you want to attach the fixture. They make some ready-to-install gadgets for this also:

enter image description here

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    It's worth noting that this type of device expands mechanically and punctures the joist to create a suspended system. That's not apparent from the answer or the drawing.
    – isherwood
    Nov 18, 2016 at 19:32
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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.

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    You can attach electrical boxes directly to plaster/drywall if they are designed and approved for that use... not to say that would be appropriate in this scenario though. Nov 18, 2016 at 18:12

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