If there are any Code geeks with a Commentary version of 2018 IRC, can you help me understand something?
Looking at Table R302.1(1), it appears that the minimum separation for a non-rated wall is 5 feet.
Looking at Commentary Figure 302.1(3), the paragraph states in part:
“The exterior walls adjacent to the property line would not be required to be fire resistant rated…because the remainder of the wall is at least 3 feet from the property line.”
Both of these are for exterior walls in dwellings (or accessory structures) without sprinkler systems.
Am I missing something, or is the Commentary paragraph wrong?
Replies
Perhaps this presumes that the 3’ setback is on both sides, hence 6’ separation
The Commentary and Definitions make it pretty clear that it's the distance to the lot line.
The consensus as to the minimum distance necessary to provide a sufficient buffer against the spread of fire has changed somewhat over the years. For example, the 2000 and 2003 editions of the IRC required a 3-foot (914 mm) minimum fire separation distance for unrated exterior walls. In the 2006 edition, that distance was increased to 5 feet (1525 mm) to provide a higher level of safety and to correlate with the provisions for residential occupancies regulated by the IBC. The 2009 IRC introduced requirements for automatic fire sprinkler systems in all new one- and twofamily dwellings and townhouses. Table R302.1(2) permits nonrated walls that have a 3-foot (914 mm) minimum fire separation distance, a dimension previously prescribed in earlier editions of the code. The 3-foot (914 mm) dimension specified in Table R302.1(2) is the new threshold for exterior wall construction, projections, openings and penetrations for dwellings that are sprinklered in accordance with Section P2904 or NFPA 13D. For dwellings without sprinkler systems, the 5-foot (1525 mm) separation distance still applies.
above quoted from:
https://codes.iccsafe.org/s/IRC2015/chapter-3-building-planning/IRC2015-Pt03-Ch03-SecR302.1
not sure if this helps.
The Commentary Figure is specific that it is for dwellings without automatic sprinkler systems.
You’re correct as far as I can tell. Commentary Figure R302.1(3) is depicting a dwelling with automatic sprinklers and describing it as one without. This error carries back at least to the 2015 edition (I don’t have earlier commentary versions to check what they had). Unfortunately, the IRC (and other codes) has quite an few errors like this, both in commentary and in the main body of the text. The remedy is to propose an amendment to the code. Glenn Mathewson has written about the process for FHB, JLC, and probably elsewhere. As Glenn says, anyone and everyone can propose a change.
My eye may be untrained, but I don't see a difference in drawing symbols between Figures R302.1(2) and R302.1(3). In the former, the separation distance is clearly 5 feet. In the latter it is 3 feet.
I agree with you that there is an error in the 2018 IRC R302 table regarding the fire separation distance for projections. It should be 2 feet instead of 3 feet for a 1-hour rated wall. This is consistent with the previous editions of the IRC and the IBC. I think this is a typo that needs to be corrected by the ICC. Thank you for bringing this to our attention and for providing the references and calculations to support your claim.
Best regards, Hanna from CodeIT