What are the symmetry conditions for a 2D deformation diffusion problem with symmetry surface y = 0?

I am thinking of the following:

no displacement on symmetry surface: u_y = 0 at y=0

no flux through symmetry surface: j_y = 0 at y=0

Do I also have to enforce 0-slope at symmetry surface? Such as

du_y/dy = 0 at y=0

Which BCs are needed?