Interior Masonry Chimney Clearances

By definition, an interior masonry chimney is one that has any portion of hte chimney within the exterior wall of the building. An interior chimney (see above) requires a minimum 2 inch clearance to combustible materials. Plasterboard, all sheetrock, and plaster with wood lath backing are all considered combustible. Consequently, all of these materials require a 2 inch minimum clarance to the outer surfaces of interior masonry chimneys.

Firestopping with a noncombustible sheet material no greater than 1/2 inch thick, or galvanized steel not less than 26 gage, must be installed at every floor and ceiling through which the chimney passes. Except for firestopping and non-combustible trim to prevent entry of debris into the clearance space, the air space should not be filled.

Exterior

Exterior Masonry Chimney Clearances

An exterior masonry chimney is defined as one that has the entire chimney completely outside the exterior wall of the building. Because the chimney walls are exposed, it will operate at cooler outer surface temperatures than an interior chimney. A minimum 1 inch clearance is required from the outer surfaces of exterior masonry chimneys to all combustibles. (See above illustration.)

If in doubt about the classification of a chimney, it is best to consider it an interior chimney and use 2 inch clearances. Consult local code regarding mimimum clearance requirements for masonry chimneys.