Chimney Shoulders

A chimney shoulder forms the outside surface of each side of the smoke chamber. The purpose of the chimney shoulder is to decrease the width of a chimney that surrounds an open firebox to the size of a chimney that surrounds a chimney liner. The chimney shoulder typically runs parallel with the smoke chamber. Valuable information can be obtained by looking at a chimney shoulder.

First, you can use a pitch gage on your phone to verify the shoulder is rising at 45 degrees or more from vertical. If the shoulders are rising at a rate less than 45 degrees, then you should pay extra attention to the smoke chamber to confirm that it complies with R1001.8.1, which states that the smoke chamber shall not be inclined more than 45 degrees from vertical. The next piece of good information is the height of the shoulder. Take a measurement of the vertical change of elevation between the top and bottom of the shoulder. R1008.8.1 states that the smoke chamber height shall not be greater than the inside width of the fireplace opening.

The shoulder and smoke chamber typically run parallel, and thus, you can likely get the smoke chamber height by measuring the shoulder height. Another code that should be noted regarding chimney shoulders is R1003.5 Corbeling: “The projection of a single course shall not exceed one-half the unit height or one-third of the unit bed depth, whichever is less”. All these seem like pretty specific requirements for a chimney shoulder but the main thing you should worry about is leaking. Similar to a corbeled drip edge, if the chimney is corbeled too much then it exposes brick holes, and exterior rain water can leak into these holes and damage the smoke chamber.

Many times, masons understand this problem and will lay flat pavers to cover the shoulders, but if the mortar joints between these pavers separate, then the shoulders will leak. Repointing, sealing, and application of masonry water repellent is a good idea for chimney shoulders.

• Cracked or missing mortar on the sloped shoulder surfaces
• Loose, cracked, or shifted pavers on top of the shoulders
• Spalling brick on or just below the shoulders
• White staining (efflorescence) below the shoulders
• Moss, plants, or standing water on the shoulders
• Water stains on the inside wall around the fireplace, or water in the firebox
• Brick or mortar debris on the ground below the shoulders

Inspect. We check the slope, the brick and mortar, any pavers, and the smoke chamber inside, since shoulder leaks often show up there first.

Repoint. Worn or cracked mortar joints are ground out and refilled with new mortar. See our brick joints page.

Reset or replace pavers. Loose or cracked pavers are reset with tight, well-tooled joints, or replaced, so water runs off instead of soaking in.

Replace damaged brick. Spalled brick is cut out and replaced.

Waterproof. Once repairs have cured, a breathable masonry water repellent helps the shoulders shed water. Learn more on our chimney waterproofing and leak repair page.

Chimney shoulders are among the few sloped surfaces on a chimney, so they catch rain, snow, and debris that a vertical wall would shed. Water that soaks in here travels toward the smoke chamber and firebox, where it weakens mortar and rusts the damper. Keeping the shoulders sealed protects the structure inside. When shoulder damage is widespread, rebuilding that section may be the better long-term fix; see chimney rebuild. For more background, read our chimney shoulder repair article.