In this work, we study the transmission properties of one-dimensional finite periodic systems with PT symmetry. A simple closed-form expression is obtained for the total transmittance from a lattice of N cells, that allows us to describe the transmission minima (maxima) when the system is in the PT-unbroken (broken) phase. Utilizing this expression, we provide the necessary conditions, independent of the number of cells, for the occurrence of a coherent perfect absorber and laser for any finite PT-symmetric periodic potential. Under these conditions, we provide a recipe for building finite periodic structures with near perfect absorption and extremely large amplification.