In calculating free area as required, consideration shall be given to the blocking effects of louvers, grilles, or screens protecting such opening. Screen mesh used shall not be smaller than 1/4 inch. (NFPA 54, 188.8.131.52). If the free area through the design of louver or grille is known, it shall be used in calculating the size opening required to provide the free area specified. If the design and free area is not known, it may be assumed that wood louvers will have 25% free area, and metal louvers and grilles will have 75% free area. (NFPA 54, 184.108.40.206).