  Weak topological insulators have an even number of Dirac cones in their
surface spectrum and are thought to be unstable to disorder, which leads to an
insulating surface. Here we argue that the presence of disorder alone will not
localize the surface states, rather; the presence of a time-reversal symmetric
mass term is required for localization. Through numerical simulations, we show
that in the absence of the mass term the surface always flow to a stable
metallic phase and the conductivity obeys a one-parameter scaling relation,
just as in the case of a strong topological insulator surface. With the
inclusion of the mass, the transport properties of the surface of a weak
topological insulator follow a two-parameter scaling form.
