Ceiling - enough strength?

A month ago I bought a ceiling fan, and replaced the light above the bed in the master bedroom with it. This worked fine.
I purchased a second fan today and planned to replace the light in the second bedboom.
However, after taking the little plastic fitting for the traditional bulb and lampshade type fitting (dunno the name!), I realised that there's no proper wood for me to screw the header of the fan into, as there is in the master bedroom.
It appears the plaster is held against some thing slats, which cannot be screwed into... well they can kind of, I tried it, put the base up, which immediately pulled down, along with some plaster.
How should I affix this five or so kilogram ceiling fan? SAFELY?
So far all I can think of is laying a plank of wood across the ceiling (in the attic!) and then screwing through the plaster again and affixing the mounting to the plank. This sounds the easiest so far.
What is a better idea?
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Basically you want to put a baton between the joists. Then screw into the new baton
--

SantaUK
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Don't think those are adequate for a 5kg wobbling load into a lath and plaster ceiling.
It either has to be fixed to a joist if one is convenient, or a noggin or two added from above if not.
--
*The severity of the itch is proportional to the reach *

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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thanks folks
i'll put some strong wood across the joists :-)
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