  Rotation and reflection symmetries impose that out-of-plane (flexural)
phonons of free-standing graphene membranes have a quadratic dispersion at long
wavelength and can be excited by charge carriers in pairs only. As a result, we
find that flexural phonons dominate the phonon contribution to the resistivity
$\rho$ below a crossover temperature T_x where we obtain an anomalous
temperature dependence $\rho\propto T^{5/2}_{}\ln T$. The logarithmic factor
arises from renormalizations of the flexural phonon dispersion due to coupling
between bending and stretching degrees of freedom of the membrane.
