The TM electromagnetic wave propagation along a strip grating above an impedance plane is studied. The problem is formulated in terms of a modified Riemann-Hilbert problem. The modal equation of the structure is derived and solved numerically. Surface wave and leaky wave modes are identified. The phase velocity and attenuation constant of the dominant leaky wave modes are presented, and their dependence on frequency and geometrical parameters is discussed. The possibility of applying this structure to the design of a leaky-wave antenna is suggested.