Partitions walls are taken as live (imposed) loads, not dead loads. This is because they are structurally redundant and can be moved around as the use of the building changes over time. For this reason, partition loads are modeled as area loads, rather than line loads, because their positions may change.

On the other hand, a load-bearing masonry wall that is structurally important would be modelled in its position and as a dead line load.

The rafters carry the dead load of the sheathing and roofing material, and the live loads above

Imposed loads - or live loads, movable loads that act on the structure when it is in use

Imposed load = Live load

;)

In England its is 'generally' 400mm (centres) but it depends on the live loads (wind) and dead loads (tiles etc. on the truss).

Consult your local building regulations department or a structural engineer, architect etc.