  We study the stability against disorder of surface states of weak
$\mathbb{Z}_2$ topological insulators (superconductors) which are stacks of
strong $\mathbb{Z}_2$ topological insulators (superconductors), considering
representative Dirac Hamiltonians in the Altland-Zirnbauer symmetry classes in
various spatial dimensions. We show that, in the absence of disorder, surface
Dirac fermions of weak $\mathbb{Z}_2$ topological insulators (superconductors)
can be gapped out by a Dirac mass term which couples surface Dirac cones and
leads to breaking of a translation symmetry (dimerization). The dimerization
mass is a unique Dirac mass term in the surface Dirac Hamiltonian, and the two
dimerized gapped phases which differ in the sign of the Dirac mass are
distinguished by a $\mathbb{Z}_2$ index. In other words the dimerized surfaces
can be regarded as a strong $\mathbb{Z}_2$ topological insulator
(superconductor). We argue that the surface states are not localized by
disorder when the ensemble average of the Dirac mass term vanishes.
