A wood product impregnated with chemical by a pressure process or other means during manufacture, which is tested in accordance with NFPA 255, has a listed flame spread of 25 or less, and shows no evidence of significant progressive combustion when the test is continued for an additional 20-minute period; nor does the flame front progress more than 10.5 ft (3.2 m) beyond the centerline of the burners at any time during the test.