The chimney could lean where it projects above the roof due to deteriorated mortar joints caused by wind-induced swaying or by sulfate attack from _____.

The correct answer is flue gases, which are the products of combustion that exit from the fireplace or furnace through the chimney. Over time, flue gases can lead to chemical reactions with the materials in and around the chimney, particularly when moisture is present. This process can result in sulfuric acid, which is a byproduct formed when sulfur dioxide from flue gases mingles with water vapor in the air. This acid can attack and deteriorate the mortar joints, weakening the structural integrity of the chimney and potentially causing it to lean as it projects above the roof.

Deteriorated mortar joints can lead to not only leaning but also other structural issues that can compromise the safety and usability of the chimney. Understanding the impact of flue gases is crucial for homeowners and inspectors alike since it highlights the importance of ensuring proper ventilation and maintenance to protect the integrity of chimney structures.