Loft insulation

[JHJS]

I want to increase the depth of my loft insulation to 270mm, however, when I've looked at what's already there, it appears that the joists are 2 and a half inches deep with 2 inches of insulation laid between them. Everything I've read suggests that joists are always 4in deep? Do I need to raise the level of the joists, or can I put 4in insulation between the joists and then more at right angles across it? Would I be better leaving the original insulation or replacing it?

[welsh brickie]

2 inch deep joists ? anyway lay the insuation ontop and across the joists leave in whats there.