Masonry Chimney Flues

What is a chimney flue?


A chimney flue is the passageway inside the chimney that the smoke, heat and gases travel through before venting to the outside.

It is important that the chimney flue be well insulated with an adequate lining material since the flue can be exposed to very high temperatures–especially in the case of a chimney fire. Here in San Diego County, most masonry chimney flue linings are made of terracotta (clay) or pumice.

Unlined masonry chimney
Unlined masonry chimney

Most masonry chimneys are built with the thickness of a single layer of brick. Before the 1940’s, these bricks were covered with limestone mortar to coat the inside of the bricks of the chimney flue.  With time, weather exposure, age, and usage, this material mortar coating has worn away and is now nothing more than the naked bricks of the chimney structure.  This is what is referred to as an “unlined chimney,” meaning that it has no lining.

In addition, at that time of the construction of the chimney, masons used a lot of sand in their mortar.  After many decades, the mortar joints have become nothing more than dust, which can add to the danger of using an unlined chimney.

In this condition, the chimney can no longer hold back the high heat of a chimney fire since heat moves through the brick chimney walls very quickly.

According to the Chimney Safety Institute of America: “Never use a chimney that is unlined, or has a damaged or improper lining!”

Masonry chimney with a clay lining
Masonry chimney with a clay lining

Here in San Diego, terracotta (clay) is the most common flue lining we find for a masonry chimney.

The terracotta flue lining does an excellent job of protection, but if the flue tiles become cracked, the chimney is a MAJOR potential hazard to use.

A cracked flue lining needs to be repaired by a qualified chimney mason. We cannot emphasize enough how important it is for the fireplace NOT TO BE USED if the flue lining is cracked!

During a chimney inspection, our primary concern and focus is the condition of the flue lining.

Masonry chimney with pumice flue lining
Masonry chimney with pumice flue lining

Also found in San Diego is a pumice flue lining, which is generally what is now being installed in a masonry chimney.

Pumice flue linings also do an excellent job of protection.

The same guidelines do apply to pumice flue linings as terracotta flue linings.