A Hausdorff topology τ on the bicyclic monoid with adjoined zero C 0 is called weak if it is contained in the coarsest inverse semigroup topology on C 0 . We show that the lattice W of all weak shift-continuous topologies on C 0 is isomorphic to the lattice SIF 1 ×SIF 1 where SIF 1 is a set of all shift-invariant filters on ω with an attached element 1 endowed with the following partial order: F ≤ G iff G = 1 or F ⊂ G. Also, we investigate cardinal characteristics of the lattice W. In particular, we proved that W contains an antichain of cardinality 2 c and a well-ordered chain of cardinality c. Moreover, there exists a well-ordered chain of first-countable weak topologies of order type t.