Suspended Ceiling under a flat roof : how best?

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I need to install a suspended (dropped, false)  ceiling under a flat roof.


The primary aim,  is to provide a carrier for  a vapour barrier and 25mm of insulation under our POSI joists.


At the same time, I'd like to create a generous service gap to carry water, electricity cable,  MVHR piping and I suspect there'll be a bit of data cabling too. 


The span to bridge is less than 2 meters. The load on top of the joists is very small : just some insulation and a waterproof membrane. I have checked with the joist manufacturers and they say that what I propose is well within the design limits, and the BCO, when told, didn't even bat an eyelid.


I've read these posts;


The POSI designer sucked his teeth a bit when I talked about screwing stuff to the bottom of the joists: I explained that SWMBO had vetoed me suspending my engine hoist from the joists.... 


My instinct is to KISS

  • 3 by 2 wallplates
  • 2 by 2 cross-pieces
  • vapour barrier
  • batten the 25mm insulation (under the vapour barrier) to the 2 by 2 cross-pieces
  • plasterboard
  • skim


  • Screw some sort of simple support system to the bottom chord of the POSIs  


I'm at the ' its keeping me awake at night stage '   Help me untangle my brain about this one please!

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What do you really want to achieve? 


More specifically if you don't drop the ceiling at least the diameter of your mvhr pipes you are going to have penetrations in it. Technically it would be that diameter +25mm so you don't need to channel the celotex. 


Just to throw it out there. Why not utilise the fact you have posi joists. Run everything in the joist space. (through ducts will make sealing the penetrations easier) 


Then insulate. Vcl. 25mm batten and screw up into the posis. 


25mm is plenty of space for flat panel down lights and associated cables. Nice short screws. 


Done. Onto the next job 😊

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can we have  drawing of what you have please ?

are posi not deep enough to make holes in for services?

where is the insulation for the flat roof ?

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