Chimney Fireblocking

During a reinspection on a new build, I observed what appeared to be non fire-rated oriented strand board installed as fire-blocking at the chimney penetration of the attic floor. There was no identification on the oriented strand board indicating its fire-rating. I called this out on the original inspection because there was a non fire-blocked 3’ by 3’ opening at this penetration. My question is whether any other inspectors come across this. Thanks.

Fire blocking looks good from here.


I’m not sure what code cycle you’re on in Texas or what local amendments there are, but the unmodified 2018 IRC allows non-treated OSB as fireblocking if it is the appropriate thickness. See #3

R302.11.1 Fireblocking Materials
Except as provided in section R302.11, Item 4, fireblocking shall consist of the following materials:

  1. Two-inch (51 mm) Nominal lumber
  2. Two thicknesses of 1-inch (25.4mm) Nominal lumber with broken lap joints
  3. One thickness of 23/32-inch (18.3mm) wood structural panels with joints backed by 23/32-inch (18.3mm) wood structural panels

R1003.19 Chimney Fireblocking. All spaces between chimneys and floors and ceilings through which chimneys pass shall be fireblocked with noncombustible material securely fastened in place. The fireblocking of spaces between chimneys and wood joists, beams or headers shall be self-supporting or be placed on strips of metal or metal lath laid across the spaces between combustible material and the chimney.

Apparently the IRC makes a distinction for fireblocking at chimneys. It says that the fireblocking must also be noncombustible at chimney penetrations of floors and ceilings. Therefore, I think my picture shows an improper installation of a combustible oriented strand board used as fireblocking in the space between the chimney and ceiling framing. It should be metal or metal lath.

Yes, but that’s under the main section R1003 Masonry Chimneys. I think this would fall under R1004/R1005 factory built fireplaces/chimneys if anything. The insulation shield is already installed. A masonry chimney and prefabricated fireplace are different beasts. What is this fireplace actually burning? gas or wood? Gas is a lot more likely nowadays. I’d like to see more fire caulking around the base of the metal firestop though. The OSB also looks thin and there is an open joint that does not appear to be blocked. Is the black stuff fire caulking though, typically they make them red to make it easy to inspect and verify. Any used caulking tubes lying around?

There is only one way to know 100% and that is to check the chimney’s installation instructions. If you called it out based on R1003, you’ve made a mistake, so I concur with Yu.

1 Like

Thank for all of your help.

1 Like