We give an overview of the theory of functional relations for zeta-functions of root systems, and show some new results on functional relations involving zeta-functions of root systems of types Br, Dr, A3 and C2. To show those new results, we use two different methods. The first method, for Br, Dr, A3, is via generating functions, which is based on the symmetry with respect to Weyl groups, or more generally, on our theory of lattice sums of certain hyperplane arrangements. The second method for C2 is more elementary, using partial fraction decompositions.