Fire rating of roof penthouse walls

[nealderidder]

I've got a Type II-B office building (B) over a couple of levels of parking (S-2) in CA. I'm looking at a stair connecting all levels and also accessing the roof. The stair shaft requires a 2HR rating. The roof requires no rating. Is there anything that requires me to provide a fire rating for the stair enclosure walls above the roof (highlighted on the attached)? I don't think so but am I missing something? These walls are all at least 30' from the edge of the roof/parapet.

 

[RLGA]

Look at Section 1023.7 and see if that answers your question.

 

[nealderidder]

I'm guessing we all picture a plan view in our minds when reading that section. This section allows me to put unprotected windows in a stairwell when part of the stairwell is an exterior wall of the building (with limitations).

I read "...exposed by other parts of the building at an angle less than 180 degrees" and look at my attached section. I see that the roof is exposed to the walls in question at less than 180 degrees but vertically, not horizontally.

I read on about what to do when I've got this situation (parts of the building at an angle less than 180 degrees) and I see "...the building exterior walls within 10' horizontally of a nonrated wall..." This seems to imply that this only applies in plan? Otherwise it would say "...the building elements within 10' horizontally of a nonrated wall..."

You gave me a code section Ron, I assume because you thought that would give me the obvious answer. But maybe you underestimate how dense I can be, because I'm missing the obvious answer! Are you saying those highlighted walls should be rated or not?

 

[RLGA]

Sorry, I had several deadlines and no time to write a long explanation. I thought maybe you just missed that code section.

The IBC Code and Commentary offers Section 705.8.6 for vertical exposure situations. However, the reference to Section 705.8.6 is not directly made in Section 1023.7 other than "shall comply with the requirements of Section 705 for exterior walls." Further, the Commentary does not explain how Section 705.8.6 is applied in this particular situation since the stair is within the same building and Section 705.8.6 applies to two or more buildings on the same lot. If the building has a 1-hour roof, the section would not apply (Exception 1). Also, if the buildings are considered as portions of one building, the section would not apply (Exception 2). If the stair is part of the same building, by default Exception 2 should apply every time. Therefore, this reference in the Commentary is nonsensical (in my opinion).

Therefore, my interpretation is that if the roof has a 1-hour rating, then the situation is moot, and you should treat the exterior walls of the stairway enclosure according to their fire separation distance. Otherwise, as Section 1023.7 states, provide 1-hour protection of the roof (with 3/4-hour openings) within 10 feet of the unrated walls or provide 1- or 2-hour construction (as appropriate) for the stairway enclosure (with protected openings) up to ten feet above the landing.

Some may say Section 1023.7 only applies to walls, but, as you pointed out, it states "exposed by other parts of the building at an angle of less than 180 degrees..." The roof is considered a part of the building and would be less than 180 degrees from the stairway enclosure walls.

(FYI: I used to interpret this section as meaning once you are above the roof, the walls are treated as exterior walls only according to Section 705. A plan review comment changed my mind when I could not defend my position.)

 

[RLGA]

To follow up on my previous post, the 1-hour wall protection and 3/4-hour opening protection does only apply to exterior walls since that is what is mentioned in Section 1023.7. However, you could conceivably justify the use of that statement to support enclosing the stairway above the roof with 1-hour walls and 3/4-hour openings even for a 2-hour stairway enclosure.

My interpretation would only apply to Construction Types IIB, IIIB, and VB since all other Construction Types (even the new Type IV types) require at least 1-hour roof construction.

I think the intent here is to protect the interior of the stairway enclosure from fire exposure. Walls that are less than 180 degrees will expose the stairway to heat and flame. I think the exposure at the roof is the same if not worse; however, the IBC falls short of addressing this specific situation explicitly.

 

[nealderidder]

I see what you mean about the commentary reference being nonsensical. I'd go so far as saying that's not opinion but verifiable fact

This is a type II-B and the roof is not rated. I do think that exposure of these walls to a roof is probably a bigger threat for fire spread than you would get from an adjacent wall.

I'll probably wind up just keeping the 2HR stairwell assembly all the way up to the penthouse roof rather than try to argue with the BO whether or not a 1HR is allowed.

Thanks for the thoughtful response.