Fireline board to underside of new second floor joists

[sunshineyday]

I have had new second floor joists built to take a future loft conversion while refurbing my two-storey (currently) house. They are supported on a steel beam which reduces the span and will support the purlin (in the future).

Do I need to use fireline plasterboard to the underside to comply with building regs application when I get the loft conversion done?

There are two thicknesses. Would 12.5mm or 15mm be required or more than one layer of either?

Want to get this right now as redoing it later would be awful.

 

[freddiemercurystwin]

1 layer of 15mm fireboard beneath and 22mm t&g flooring above will achieve 1/2 hours fire resistance. Technically speaking the joists should be at least 195mm high. Or use 1 layer of British Gypsum 12.5mm SoundBloc board instead.

 

[DevilDamo]

Or if you have any spare plasterboard, 2 layers of 12.5mm will achieve the min. 30 minute fire resistance required.