The Rain Cap designed to prevent penetration of rain or snow inside the chimney, although it allows free flow of the smoke and fumes. This model feature side screening, which keeps most birds and small animals out of the chimney and also keeps larger sparks from exiting the chimney. Bolt down tops for easier cleaning. Designed for round double or triple wall chimney systems. Measure the inside diameter (ID) of the most inner pipe and outside diameter (OD) of the most outer pipe (not the old cap size).