Fireproofing old 96-inch ceiling plasterboards

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Hi,

I've just bought a house which has a 1960s bedroom extension above the original 1930s garage. The ceiling panels are standard 9.5mm-thick plasterboard, but there is no fireproofing, which is now required - naturally I want to protect the bedroom above as best as possible.

I'm considering replacing the plasterboards with Gyproc fire resistant boards, but the existing boards are 'old money' 96-inch lengths = 243cm, rather than the modern 240cm. Currently the garage is 3 panels long, but clearly this isn't going to work, if I'm losing 3cm x 3 panels then I'll be 9cm short at the back.

More to the point, none of panels will reach a joist exactly. Is this a problem - do plasterboard have to always terminate at a joist, or does it not matter as long as they're screwed to *some* joists?

The alternative is to paint the existing boards with Thermoguard type products, and fill the gaps between the board, and round the edges, with intumescent mastic. Has anyone got any experience of this? How do you fill in relatively large holes for lighting cables, and a couple of old unused service holes (diameter approx 50mm)?

One final point, there is zero insulation between the (existing, or proposed new) ceiling plasterboards and the bedroom above, making the bedroom floor very cold. I'd like to install some insulation, is this difficult to do as a DIYer?

Thanks very much for any advice!
 
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take the ceiling down and add insulation from below . screw 4x2s or cls timbers alongside the existing joists and reboard.
Making sure the mating joints of the boards land on and are screwed to the same joist.
otherwise it’ll 100% crack.
yes all joins have to land on timbers.
 

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