Abstract: The present invention discloses a cutting guide that permits the use of a power saw in trimming roofing materials in the installation of those roofing materials. The cutting guide may be constructed as an accessory or as a unitary roofing material cutting apparatus and generally comprises a mounting plate for coupling a power saw to the cutting guide, a post extending downwardly from the mounting plate in general alignment with a blade of the power saw, and a base that supports the power saw and cutting guide and which has at least one lateral edge that is arranged substantially parallel with the blade of the power saw at a predetermined offset distance therefrom. The cutting guide of the present invention may be readily configured to trim roofing materials such as shingles, membranes, shakes, or tiles in roof valleys, at ridges, and at the gable end of a roof.