Bestvina [1] introduced the notion of a (weak) Z-structure and (weak) Z-boundary for a torsion-free group, motivated by the notion of boundary for hyperbolic and CAT(0) groups. Since then, some classes of groups have been shown to admit a (weak) Z-structure (see [5,20,22] for example); in fact, in all cases these groups are semistable at infinity and happen to have a pro-(finitely generated free) fundamental pro-group. The question whether or not every type F group admits such a structure remains open. In [33] it was shown that the property of admitting such a structure is closed under direct products and free products. Our main results are as follows.THEOREM: Let G be a torsion-free and semistable at infinity finitely presented group with a pro-(finitely generated free) fundamental pro-group at each end. If G has a finite graph of groups decomposition in which all the groups involved are of type F and inward tame (in particular, if they all admit a weak Z-structure) then G admits a weak Z-structure.COROLLARY: The class of those 1-ended and semistable at infinity torsion-free finitely presented groups which admit a weak Z-structure and have a pro-(finitely generated free) fundamental pro-group is closed under amalgamated products (resp. HNN-extensions) over finitely generated free groups.On the other hand, given a finitely presented group G and a monomorphism φ:G⟶G, we may consider the ascending HNN-extension G⁎φ=〈G,t;t−1gt=φ(g),g∈G〉. The results in [26] together with the Theorem above yield the following:PROPOSITION: If a finitely presented torsion-free group G is of type F and inward tame, then any (1-ended) ascending HNN-extension G⁎φ admits a weak Z-structure.In the particular case φ∈Aut(G), this ascending HNN-extension corresponds to a semidirect product G⋊φZ, and it has been shown in [18] that if G admits a Z-structure then so does G⋊φZ.