  We explore the topological behavior of the binary Zintl phase of the alkaline
earth metals based compounds Sr$_2$Pb and Sr$_2$Sn using both standard and
hybrid density functional theory. It is found that Sr$_2$Pb lies on the verge
of a topological instability which can be suitably tuned through the
application of a small uniaxial expansion strain ($>$ 3%). The resulting
non-trivial topologically insulating state display well-defined metallic states
in the Sr$_{2}$Pb(010) surface, whose evolution is studied as a function of the
film thickness.
