In this article, we study the scattering problem of a truncated grounded slab illuminated by an arbitrarily incident <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E_{\text{z}}$ </tex-math></inline-formula> -polarized plane wave. We present a solution using the novel semianalytical spectral method based on an original extension of the Wiener–Hopf (WH) technique that uses the concept of characteristic Green’s function and the Fredholm factorization technique. The combination of these mathematical tools allows to extend the capabilities of classical WH method to a new set of problems. One of the main benefits of the proposed semianalytical solution is that it allows the computation of field components similar to what is done with closed-form spectral solutions when available. Physical phenomena, such as the reflection, diffraction, and excitation of surface/leaky waves, are reported. Numerical results validate the proposed methodology.