Chimney leaning above roof line

Heating · Masonry Chimneys · printed page 167 (PDF page 168)

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Leaning above Roof Line Leaning Chimneys

Text in the illustration

chimney lean above the roof line is most likely to be a problem in northern climates and on tall, unlined chimneys venting an oil-fired appliance
-the sulphur in the exhaust gases from oil burners combines with water when it condenses to form sulphuric acid that deteriorates the mortar
-the mortar is then more prone to swelling due to moisture from wind-driven rains
-when the chimney is tall, the effects are more pronounced
direction of
wind-driven rains
mortar on this side of the chimney swells
plumb bob
roof rafter
ceiling joist
attic
wall
cross section

Generated explanation

A cross section shows a brick masonry chimney passing through an attic and sloped roof, with its exposed upper portion leaning to the right. A plumb bob beside the chimney provides a vertical reference for the lean. An arrow indicates wind-driven rain striking the left side, where an annotation identifies swelling mortar. The explanatory text links this condition to sulphuric acid deterioration of mortar in tall, unlined chimneys venting oil-fired appliances, followed by moisture-related swelling, particularly in northern climates. Roof rafters, ceiling joists, and an exterior wall establish the chimney’s relationship to the house framing.