Designers and builders of residential chimneys sometimes fail to realize the severity of the conditions to which a chimney is exposed. As a result, efflorescence, staining, water penetration, and deterioration of masonry chimneys may occur, unless precautions are taken. CHIMNEY EXTERIOR The materials used to construct exterior chimney walls are subject to the most severe exposure. If brick is used, it should conform to the requirements of ASTM C 216 Specification for Facing Brick for Grade SW (severe weathering). Mortar used on the exterior of a chimney can be Type N or S mortar made from portland cement and lime or from masonry cement. CHIMNEY INTERIOR The interior of a masonry chimney typically is constructed with clay flue liners conforming to the requirements of ASTM C 315 Specification for Clay Flue Linings. Only non-water-soluble refractory mortars should be used for laying the flue, as specified by the National Fire Protection Association standard NFPA 211. CROWN Better chimney design always includes a crown to protect the chimney walls and interior from rain and snow. A portland cement concrete crown is recommended. Don't use a mortar wash in lieu of a chimney crown. FLASHING Flashing should be placed at three locations in a masonry chimney--directly beneath the crown, at the intersection of the chimney and the roof, and at the base of the chimney. CRICKET A cricket should be used when the chimney penetrates the roof below the ridge line and the face of the intersecting chimney wall parallel to the roof line is over 30 inches long. Framed of wood and covered with roofing material, crickets are used to divert water from the roof around the chimney.