Loft conv RSJ's v Joist doubling

[Chri5]

Whats the criteria for choosing RSJ's over doubling up of existing ceiling joists.

Reason being that I thought pretty well much any new loft conversion had to be done via steel with a clearance between steels and the existing ceiling joists they sit over (25mm gap between).

I have friends in a different area of the UK who have a similar (with planning at moment) loft conversion design and they are saying that doubling up of the joists will be okay as the base for the loft floor.

Thanks for any help

 

[Bowness72]

Steel beams are not necessarily required, but your existing ceiling joists probably wont be strong enough to span the distance between the walls carrying the increased floor load. The design loading for a floor is six times as much as a roof void with light storage.
There used to be guide for sizing of joists in Part A of the building regs but that has now been removed but I think it can still be found at the "trada" website based on the spacing and load being carried (i.e. - floorboards).