A wood product impregnated with chemical by a pressure process or other means during manufacture, which is tested in accordance with ASTM E 84 or UL 723; has a listed flame spread index 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 (3200 mm) beyond the centerline of the burners at any time during the test.