5/22/14 Jim, Sorry to keep bothering you. Can you help with a quick code interpretation? It turns out I did not read the nj irc 2006 very carefully. In exception 3 for the air gap, it only requires 6 inches from the flue or firebox for trim to abut the fireplace/chimney. It does have the clause about overlapping the corners by 1". What does the thing about overlapping the corners 1" mean? Does my timber frame count as trim or sheathing that can abut the chimney? Do I only need 6" of masonry to allow the chimney to be built right against the timbers and tongue and groove ceiling? Would you still recommend 8"? Would you recommend 8" in the firebox and 6 in the chimney maybe? Also, can the natural stone veneer count as part of the masonry thickness? Thank you so much for your help. I will keep you up to date on exactly what we end up doing and I will be sure to send you pics. You can find the appropriate section of the nj irc at http://ecodes.biz/ecodes_support/free_resources/NewJersey/NJ_Residential/PDFs/NJ_Res_Chapter10.pdf Thanks, Evan Manaresi ************* Evan, That's interesting. The NJ code is almost identical to the 2006 IRC but in exception 3 they have apparently reduced the clearances from 12" to 6".... 3. Exposed combustible trim and the edges of sheathing materials, such as wood siding and flooring, shall be permitted to abut the masonry chimney side walls, in accordance with Figure R1003.18, provided such combustible trim or sheathing is a minimum of 6 inches (152 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm). Here is the IRC version: 3. Exposed combustible trim and the edges of sheathing materials, such as wood siding and flooring, shall be permitted to abut the masonry chimney side walls, in accordance with Figure R1003.18, provided such combustible trim or sheathing is a minimum of 12 inches (305 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm). Our code change proposal, which was approved unanimously last month at the IRC Committee hearings in Dallas, changed the IRC to 8" and eliminated the the last sentence which didn't seem to make sense to anyone. 3. Exposed combustible trim and the edges of sheathing materials, such as wood siding and flooring, shall be permitted to abut the masonry chimney side walls, in accordance with Figure R1003.18, provided such combustible trim or sheathing is a minimum of 12 inches (305 mm) 8 inches (203 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm). So, to answer your question, I'd comply with NJ code and make the chimney walls at least 6" in thickness and yes, the stone counts as part of the "solid masonry" chimney walls. As for the fireplace, the NJ code has also reduced the IRC's 12" to only 6" thick walls if they are to be in contact with combustibles. I find that surprising since firebox walls are required to be at least 8" thick. I will follow up to see what justification they used for those reductions in wall thickness but code is code and all that you are required to do is to follow the NJ code that allows 6". Thanks, Jim Buckley

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