Double boarding a ceiling?

Just as I take delivery of 350m of CLS and 40 sheets of PB a friend turns up and gets roped into helping get in indoors quick before it rains.
He happended to mention when he'd refurbed a few flats he'd had to either double board or use fire resistant board on the ceilings and questioned what I was doing.
I just plan to stickup sheets of 12.5mm PB, the original ceiling (lathe & plaster) is about 600mm above the new one so is there any problem?
Tony
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TonyK wrote:

The building regs have a section on this. It does depend on what is above it, but for fire and sound transmission between floors and especially between properties, two sheets of staggered 12.5mm boards are recommended, with all the joints & edges taped and stopped. Lathe & plaster is quite highly regarded by the building regs if it is intact, so a single sheet will do when directly covering it.
Ultimately it depends on the building inspector.
--
Toby.

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up
what
It's a dropped ceiling with fire retardant material above, so if the original ceiling and walls in the void don't have huge big holes in it, then you are OK with just a single layer of 12.5mm Plasterboard.
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