When the tail has a dihedral angle in the range of 30 to 45 degrees, it is usually to combine the functions of the vertical and horizontal tails. There are at least two criteria for sizing such a tail. The tail can be sized to have the same pitch and yaw control authority as a conventional tail or it can be sized on the basis of stability considerations. A V-tail or an inverted V-tail can usually be built lighter for the same strength compared to a conventional tail. When a V- tail is used the tail cone of the fuselage has to be stiffer in torsion than for a conventional tail.

See:
http://www.charlesriverrc.org/articles_modeldesign.htm
for several articles on V-tails.