I'm detailing a secondary ceiling to separate two flats in an old Victorian town house.

[MrBird]

The proposed method is to use clear span secondary joists to support two layers of 12.5mm plasterboard with batts between the joists. I am trying to optimise joist spacing and depth. Joists to be support on hangers off new wall-plate under the existing sandtex-overpainted high-relief coving (by deceased vandal). Any suggestions, please - I assume deflection mitigates against C24 timber.

 

[bernardgreen]

Not sure that that would provide adequate fire resistance.

Local Building Control would need to accept the design as being suitable for compartmentising the two flats to prevent spread of fire from one flat to the other.

 

[tony1851]

What's the span?

 

[MrBird]

Thank you both.
I've discussed with local Building Control who are content with the proposal, provided the wall joint is fire-sealed. Specifically, they have accepted existing lath & plaster ceiling without additional plasterboard, which would be difficult given the extent of debased mouldings.
Spans are 5.1m into the alcoves or 4.75 to the chimney breast. I could put a jack-beam across each of the alcoves to reduce the span.

I want to avoid using suspension from floor above to reduce sag of beams -especially long term.

Has anyone used cold-rolled Z-sections for the work?

 

[tony1851]

You would be looking at something like 125 x 50 C24 @ 300 centres to keep deflection within resonable limits.

 

[Leofric]

Have Building Control said it will be subject to sound testing when completed ?

 

[big-all]

Have Building Control said it will be subject to sound testing when completed ?

exactly my thoughts
are they aware its not a single dwelling ??

 

[bobasd]

this seems to be a ground floor room with first floor joists above - so where's the issue in fixing boards to the existing ceiling and joists?
Plasterboard can easily and neatly be pinned up leaving a definite shadow line about 5mm to 10mm short of the moulded coving.

More important is that BCO allowed such a fire risk dropped ceiling in a flat below a flat?
Do your multiple occupancy insurers know about this design?
If a Fire Service inspection is necessary they can impose powerful conditions or fail it.

Maybe nobody cares but i know the fire service loathe dropped ceilings of any kind according to their inspector.