the strength of the loft floor is a function of what exists in the roof space and a loft is only as strong as the supporting structure.
The central binder usually sits on a spine wall and just ties the loft together over the spine wall.
If this is the case you can use this member to support new joists, your next point is supporting your new joist at the perimeter wall plate.
In order to do this you will need to profile the end of your new joist.
To avoid this you can span a flitch beam between the party walls (you will need to serve a party wall notice on your neighbours, but this should not put you off).
The flitch beam can then carry the new joists, allowing you to increase the load carrying capacity of your floor.
