What type of mortar is used in chimneys?

The type of mortar used in brick construction can have a dramatic impact on the durability and performance of a structure. Mortar selection is particularly critical with projects such as chimneys, which experience significant temperature changes and exposure to outdoor elements. Before you build or repair your chimney, take the time to choose a mortar that allows you to maximise strength, performance and aesthetic appeal throughout the life of the chimney.

"Masonry Magazine" describes the four basic types of mortar used in brick and stone construction. They include Types N, S and M, which are used for new work, and Type O, which is used for repairs and indoor projects. The magazine suggests that installers should always use Type N mortar as the default for all projects unless there is a compelling reason not to do so. This may include severe exposure or other unusual circumstances. For basic chimney applications, stick with a Type N mortar to balance workability, strength and moisture-resistance.

The U.S. National Parks Service also recommends Type N mortar for the majority of chimney applications. The NPS describes Type N mortar as a mixture of 1 part cement, 1 part lime and 5 to 6 parts sand. For very severe weather applications, consider a Type S mortar mix made from 1 part cement, 1/2 part lime and 4 to 4-1/2 parts sand.

Chimney repair projects may require a slightly different mortar mix. Before attempting this type of project, chisel out any loose or damaged mortar to create a minimum 1-inch deep space in the remaining mortar. Look for a specially-prepared chimney mortar mix from a hardware or home improvement store and use it to fill these gaps.

Mortar plays an important role in the construction of a chimney or other brickwork. According to "Masonry Magazine," mortar keeps air and moisture out of the brick, as well as the chimney itself. Without mortar, moisture would penetrate the brick and lead to cracks, spalling and failure over time. This moisture could also seep into the chimney space, where it could impact the floor, walls or other materials within the building structure. Mortar also creates a secure bond between the individual bricks, and helps to make up for unevenness in the stones.