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.