We present a theoretical study of electron-phonon scattering effects in thin films made of a strong topological insulator. Phonons are modelled by isotropic elastic continuum theory with stress-free boundary conditions, and the interaction with the helical surface Dirac fermions is mediated by the deformation potential. We determine the temperature-dependent electrical resistivity $ρ(T)$ and the quasiparticle decay rate $Γ(T)$ observable in photoemission. The low- and high-temperature power laws for both quantities are obtained analytically. Detailed estimates covering the full temperature range are provided for Bi$_2$Se$_3$. ; Comment: 11 pages, 6 figures