Brick chimneys in older properties with oil heat sometimes curve above the roof line. What is a possible cause of this curving?

The curving of brick chimneys above the roof line in older properties that utilize oil heat can often be attributed to the expansion of the mortar used in the chimney's construction. Over time, temperature fluctuations and the effects of the heating system can cause the mortar to expand and contract. This movement can lead to structural stress, ultimately resulting in the chimney bending or curving.

Mortar is designed to be flexible to some extent, but excessive heat and age-related degradation can compromise its integrity. When the mortar expands due to prolonged exposure to high temperatures from the oil heating system, it may not be able to accommodate the resulting pressure, leading to the visible curving of the chimney.

While other factors like settling of the chimney footing, absence of proper drainage features such as cricket flashing, or the lack of a rain cap can contribute to chimney issues, they are less directly related to the specific phenomenon of curving caused by the thermal properties of the materials involved. In this case, the expansion of the mortar stands out as the primary cause of the observed curvature in brick chimneys.