June 27, 2019
We're adding an addition to the side of our house with a wood burning masonry fire place & chimney (now becomes an interior chimney). The contractor framed the first floor opening with the floor header and trimmer right against the masonry chimney. Reading IRC R1001.11, it requires 2" clearance on the sides and 4" clearance on the rear of the masonry for combustibles.
Exception #2 states that when masonry fireplaces are part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete walls less than 12" from the inside surface of the firebox.
I measured the firebox and put it on the floor plan. The rear and one trimmer are less than 12" in-plane.
If I account for the fact that the firebox is 10" above the finished floor, then the total length of masonry between the closest firebox lining and the outside masonry is over 12".
Does this meet the code? My thought is yes since I assume the 12" distance in the code is based on some heat transfer calculations to determine maximum masonry temperatures 12" away from the fire.
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