IRC Roof-Deck Penthouse Access Height Questions

[MKolchins]

I am an architect working on a 3 story duplex development project in New Jersey and hoping to keep the structure in the IRC and avoid having to be the bad guy telling my client the structures have to be sprinkled. The IRC allows 3 stories and 40 ft. height in type 5A construction. I am proposing three habitable stories with a roof deck above accessed by a penthouse that is just large enough to accommodate stair and residential elevator access and a landing. The roof of the penthouse does extend over a small portion of the roof deck but this is not enclosed it's just covered deck.

If I measure to the peak of the penthouse roof I will exceed 40 ft. However the exception to the 2018 NJ edition of the IRC section 504.3 allows an additional 20 ft of height for "Towers, spires, steeples and other roof structures..." Section 503.1.4 "Occupied Roofs" states the occupied roof does not constitute additional floor area per section 506. It seems to me the penthouse could be construed as a "Tower" under section 504.3 and section 506 seems to imply roof decks are like mezzanines and not intended to contribute additional height or area.

While the IRC is not very flexible on the height and area limits it references the IBC which appears to me to potentially allow enough flexibility so that the penthouse access necessary for the roof-deck does not push the project out of R-5 and the IRC and into R-3 and the IBC/ sprinkler. I had the opposite issue years ago discussing a requirement in the IBC to extend fire protection to decks in R-5 5A construction class with New Jersey's Code Interpretation Unit arguing that because it was not specifically referenced by the IRC it did not apply. I was told just referencing the IBC for fire ratings in general was enough to get to this odd section and it did apply to R-5 structures. I'm hoping this interpretation might work in my favor this time.

 

[SH225]

For purposes of measuring building height under IRC there is a definition of HEIGHT, BUILDING in Chapter 2. It says the building height is measured from grade plane to the average height of the highest roof surface. Grade plane is also defined in Chapter 2.

 

[cda]

"""I am an architect working on a 3 story duplex development project in New Jersey and hoping to keep the structure in the IRC and avoid having to be the bad guy telling my client the structures have to be sprinkled""""

The entire building, or stuff on the roof????

 

[MKolchins]

For purposes of measuring building height under IRC there is a definition of HEIGHT, BUILDING in Chapter 2. It says the building height is measured from grade plane to the average height of the highest roof surface. Grade plane is also defined in Chapter 2.

The IBC has an identical definition; however, per my original post there are provisions in the IBC that allow for penthouses to exceed the maximum height. Section 1510 further states they don't constitute a story or add area. It seems as though the IBC is trying not to force the height and area limitation of chapter 5 onto a structure for just a penthouse. New Jersey's Department of Community Affairs (DCA) has a code interpretation unit that I contacted years ago about a provision in the IBC that required fire ratings for 5A construction class be extended to porches and decks. This provision is was and still is not specfically referenced in the IRC; however, some local officials were requiring decks and porches on IRC 5A projects be protected in accordance with this section. NJ DCA Code Interpretation stated the section did apply because the IRC refers to the IBC for fire ratings requirements and therefore this oddball section not even in chapter 7 was applicable to IRC 5A projects. I'm just trying to see if we can use the same logic to justify not pushing a three story duplex dwelling out of the IRC and into the IBC which is going to require suppression just because there is a penthouse to accommodate stair and residential elevator access to roof decks. It seems to me the IBC is trying not to count a penthouse in height as adding height or area and considers it more like a mezzanine.

 

[SH225]

The IBC has an identical definition; however, per my original post there are provisions in the IBC that allow for penthouses to exceed the maximum height. Section 1510 further states they don't constitute a story or add area. It seems as though the IBC is trying not to force the height and area limitation of chapter 5 onto a structure for just a penthouse. New Jersey's Department of Community Affairs (DCA) has a code interpretation unit that I contacted years ago about a provision in the IBC that required fire ratings for 5A construction class be extended to porches and decks. This provision is was and still is not specfically referenced in the IRC; however, some local officials were requiring decks and porches on IRC 5A projects be protected in accordance with this section. NJ DCA Code Interpretation stated the section did apply because the IRC refers to the IBC for fire ratings requirements and therefore this oddball section not even in chapter 7 was applicable to IRC 5A projects. I'm just trying to see if we can use the same logic to justify not pushing a three story duplex dwelling out of the IRC and into the IBC which is going to require suppression just because there is a penthouse to accommodate stair and residential elevator access to roof decks. It seems to me the IBC is trying not to count a penthouse in height as adding height or area and considers it more like a mezzanine.

My reason for pointing out the code definition is to say that under the code you don’t have to measure building height to the top of the penthouses. An AHJ interpreting differently is always a possibility. After I get an especially tough interpretation, I still go back to what I believe to be the correct interpretation on the next project, unless I know that AHJ is going to continue with that previous interpretation.

 

[MKolchins]

My reason for pointing out the code definition is to say that under the code you don’t have to measure building height to the top of the penthouses. An AHJ interpreting differently is always a possibility. After I get an especially tough interpretation, I still go back to what I believe to be the correct interpretation on the next project, unless I know that AHJ is going to continue with that previous interpretation.

It's been a while since I had to worry about the definition and I did misread it to be the peak instead of the average. Unfortunately since it's a relatively small roof this does not help enough to resolve the building height issue.

 

[MKolchins]

"""I am an architect working on a 3 story duplex development project in New Jersey and hoping to keep the structure in the IRC and avoid having to be the bad guy telling my client the structures have to be sprinkled""""

The entire building, or stuff on the roof????

If you exceed the allowed height and area of the IRC for one and two family dwellings you get kicked into the IRC and an R-3 use group for one and two family dwellings that do not qualify for the IRC. The IBC requires R-3 structures be suppressed and it's the entire structure. You can go with an NFPA 13D system if you don't exceed three stories which probably helps but I know from past experience clients don't like sprinkler systems in dwellings. It looks like I can get out of 5A construction class and into 5B if I do sprinkle so hopefully that will help ease the pain. Developers and contractors are creatures of habit and 5A construction is common where I practice so I'd like to stick to 5A and avoid having to tell them the need to suppress if at all possible.

 

[SH225]

It's been a while since I had to worry about the definition and I did misread it to be the peak instead of the average. Unfortunately since it's a relatively small roof this does not help enough to resolve the building height issue.

I don’t think we’re understanding each other. You referred to the peak of the penthouse. I’m saying the top surface of the penthouse is not what the code means by the roof of the building. As an example, look at a Type VA building under the IBC. The lid of the stair tower and the hoistway is not required to be fire rated, and yet the roof is required to be 1-hour rated.

 

[MKolchins]

I don’t think we’re understanding each other. You referred to the peak of the penthouse. I’m saying the top surface of the penthouse is not what the code means by the roof of the building. Look at a Type VA building under the IBC. The lid of the stair tower and the hoistway is not required to be fire rated, and yet the roof is required to be 1-hour rated.

In my response I recognized my mistake in reading peak instead of the average roof and stated the average still doesn't resolve the issue because it's a small roof and not enough of a difference to resolve my problem.

 

[TheCommish]

In my experience your are in one book or the other, what you are doing is ike using conduit from the electric code to design a plumbing system

 

[ADAguy]

A question for Webster's with an ever changing answer?

 

[SH225]

My understanding has been that the hoistways and stairway penthouses don’t count in the IRC (or IBC) building height. They might have roofing on them, but they’re not the roof of the building.

 

[steveray]

As penthouses are not addressed in the IRC, maybe ask if you can use those IBC provisions for relief? Under R104.11?