  We explore the impact of modified gravity on B-modes, identifying two main
separate effects: lensing and propagation of tensor modes. The location of the
inflationary peak of the BB spectrum depends on the speed of gravitational
waves; the amplitude of the lensing contribution depends on the anisotropic
stress. We single out these effects using the quasi-static regime and
considering models for which the background and the growth of matter
perturbations are standard. Using available data we obtain that the
gravitational wave speed is compatible with the speed of light and constrained
to within about 10%.
