We propose a theory of ripples in suspended graphene sheets based on two-dimensional elasticity equations that are made discrete on the honeycomb lattice and then periodized. At each point carbon atoms are coupled to Ising spins whose values indicate the atoms' local trend to move vertically off-plane. The Ising spins are in contact with a thermal bath and evolve according to Glauber dynamics. In the limit of slow spin flip compared to membrane vibrations, ripples with no preferred orientation appear as long-lived metastable states for any temperature. Numerical solutions confirm this picture.