Fireblocking in "cornices"

[Glenn]

History and technical question for those around during the ICC merger (not trivia, I'm asking for help)

IRC 302.11 Fireblocking. Item 6:
"Fireblocking of cornices of a two-family dwelling is required at the line of dwelling unit separation."

I am searching the origin and modern necessity of this provision. The term "cornice" is not very common in the IRC. In searching this term online, it appears it can be used to describe a sort of horizontally projecting parapet around flat roofs, but can also be just the normal "eave" "overhang" "projection" "soffit and fascia" we see in design.

- This first appeared in the 2000 IRC. No discussion in commentary.
- This was NOT in the "September 98 final draft" of the IRC.
- So I thought maybe it was a last minute thing slipped in by the demand of one of the legacy orgs.
- I can't find anything like this in the 99 BOCA, 97 UBC, or 94 SBCCI codes though. I was surprised to not find it. That's where maybe someone can help me.

So... do any of you mentors remember this subject during those wild years?
Is this still relevant considering all the changes over the last two decades?
Take a look at the stock photo example of a two-family home and lets assume the separation wall is between the garages. Under exception 2 of R302.3 the rated wall can terminate at the ceiling and only "draft stopping" is required above the wall to divide the attic. However, under item 6 of R302.11, the peak of the rake edge eave ("cornice?") would need to be "fire blocked" at the dividing line.

I can't explain why one requires draft stop material and one fireblock material.

I can't explain why this eave fireblocking would not be necessary for townhouse separation but would be for two-family dwellings. Yes the rating is different, but if fire is going to breach units from the exterior, within a soffit, I don't see the relationship to the rating of the separation inside.

I can't explain why this fireblocking is not mentioned in the section about two-family dwelling unit separation when that is the actual trigger to require the fireblocking. This makes me question it's modern relevance.

I'd really love some commentary on this subject.
Thank you in advance.

 

[Paul Sweet]

There's nothing like this in the 1992 CABO code. I wonder if it was meant to apply where one dwelling unit had a higher roof than the other, so the eave of the upper roof projected over the lower roof.

Another possibility is that interior decorators often refer to a tray ceiling or other projecting decorative moulding at the ceiling as a cornice.

 

[Glenn]

There's nothing like this in the 1992 CABO code. I wonder if it was meant to apply where one dwelling unit had a higher roof than the other, so the eave of the upper roof projected over the lower roof.

Another possibility is that interior decorators often refer to a tray ceiling or other projecting decorative moulding at the ceiling as a cornice.

Since posting, I have discovered the following:

"Cornice" is defined in the IBC since 2012 as "A projecting horizontal molded element located at or near the top of an architectural feature". That is not a very helpful defintion, but I have yet to research the proposal that got it in.

Both the IRC and IBC use the term cornice when referring to attic ventilation. The phrases are "...eave or cornice vents..."

I can't find anything in any of the CABO codes or IRC drafts. It just shows up in the 2000 IRC. The idea about it being a cornice running parallel to the separation wall is an interesting thought. Thanks.

 

[Glenn]

Bump: So will you oppose me if I propose to delete item 6 in R302.11 in the 2027?

If so, please tell me your thoughts now so we can work together for meaningful code that can be understood, validated and trusted.

Thanks.

 

[Paul Sweet]

Clarify it if one of the other items doesn't cover the condition, otherwise delete it. I wonder whether the fire blocking would interfere with required attic ventilation.

 

[steveray]

I think it should be added to townhouses, not deleted.....funny, because I was just doing a review of some townhouses and noticed that they are less restrictive than 2 families in some ways....I believe the intent is lateral fire propagation in a hollow cavity...I think of it as large exterior crown moulding...

 

[KTall454]

Any further thoughts on this? I just went on a 30 min dive into this because a google search regarding the requirement brought me here...

 

[SCBO1]

Glenn,

Good to hear from you!

In the 80's we used the Uniform Code, "87". We enforced solid blocking in the soffits between units at the front and back of the structure. It was explained to me to prevent fire jumping. I would suspect smoke could also be drawn into the other side as well.

Maybe the wording "Cornice" came from that code? When I think of a cornice I think of the ornated material added to a roof/wall connection like in federal style building design.

 

[SCBO1]

Wouldn't this duplex require the soffit (cornice) to be blocked between units at the party wall?

 

[SCBO1]

Sorry, I didn't notice that Glenn left the building in June of "23"!