From ASTM C 1283 ....
8. Chimney Caps
8.1 All masonry chimneys shall have a chimney cap that slopes a minimum of 10° downward from the flue liner to the edge of the chimney cap (see Fig. 1).
8.2 Chimney caps shall overhang the chimney wall by a minimum of 2 in. (51 mm).
FIG. 1 Chimney Cap
8.3 Chimney caps are of the following types and construction:
8.3.2 Metal chimney caps must lap down the chimney wall at least 4 in. (100 mm) and a noncombustible resilient sealant shall be used between the metal cap and the flue liner and between the base of the metal cap and the chimney. 8.3.3 Stone caps shall be a minimum of 2 in. (51 mm) in thickness and shall have a drip slot on the underside at least 11/2 in. (38 mm) away from the chimney wall.
8.5 Clay flue linings shall extend above the chimney cap by not more than 4 in. (100 mm).
8.6 Where a clay chimney pot is installed on a masonry chimney, the chimney pot shall comply with the requirements of Specification C315.
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