Flue cap serves for preventing infiltration 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 square or round chimneys. Measure the outside length (L) and outside width (W) of the chimney (not the old cap size).