in a fireplace can have 45 shoulders (offset 45 from the vertical between the damper and the bottom of the flue liner). However, in the chimney itself,
offsets of 30
from vertical are usually considered maximum. More than this may disturb the draft of the chimney.
Another problem with offsets is the mortar debris which falls from above and accumulates on the shoulder. This can reduce the flue size, restricting flow and creating draft problems.
In modern construction, every masonry chimney must have a flue liner. The flue liner can be clay, concrete, firebrick, asbestos cement or metal. Up until roughly 1950 (depending on where you are) liners were not required. Many older homes have unlined brick chimneys, used with furnaces, boilers, water heaters and/or fireplaces. In some jurisdictions, these are considered unsafe and must be lined. Other jurisdictions are more lenient and retrofitting liners is only recommended. Eight inch thick masonry chimney walls may not require a liner.