Masonry chimneys constructed of hollow masonry units must be grouted solid with a nominal thickness of at least how many inches?

Masonry chimneys constructed of hollow masonry units are required to be grouted solid to ensure structural stability and to aid in fire resistance. Grouting the hollow units provides a monolithic structure that enhances the chimney's overall strength while preventing the penetration of water and the accumulation of combustibles within the voids.

The requirement for a nominal thickness of at least 4 inches is specified in order to achieve adequate bonding and to minimize the potential for cracks or failures under thermal expansion and contraction. A thicker grout joint contributes to the durability and load-bearing capacity of the chimney, aligning with safety and building codes designed to protect both the structure and the occupants.

In terms of building practices, the choice of 4 inches reflects a balance between durability and practicality in construction methods, ensuring that the chimney meets the necessary fire safety standards while remaining feasible to work with on the job site. Grouting to a lesser thickness may not provide the same level of structural integrity or fire resistance, which is why the other options do not meet the requirements outlined in typical building codes.