Basements (Tables 504.3 and T504.4)

[rosegamble]

Hello All,

I am working on a small R-2 (condo) building. The code is the 2021 Building Code.

It is 2 stories with an attic that could easily be occupiable. However, it also has a partial basement. This basement I am considering to be a "story above grade plane" as defined in the IBC because the finished floor of the first floor (above) is more than 6' above grade plane, with grade plane being the average grade around the building periphery. This is despite the basement being lower than finished grade. The first floor is pretty elevated over grade (over a crawl space or in some parts this partial basement).

It is VB and sprinklered. Regardless of sprinklering type (standard vs S13R) we are only allowed 60' in height per 504.3 and three stories above grade plane per 504.4.

So... if the basement stays, then we cannot use the third floor attic, correct? That seems pretty clear. We are considering demolishing the basement as it is small, then we could use the attic. A crawl space unlike a basement does not count as a story above grade plane, right? The definition of "story" doesn't define "floor."

Also... is the 60' height limitation inclusive of the existing basement? Table 504.3 is for "allowable building height in feet above grade plane." I wish this was defined. Is this height limitation based on the height of the building from topmost point (ridge) to grade plane? Or is it ridge to basement? I see that "building height" is defined in the IBC as the vertical distance from grade plane to the average height of the highest roof surface" but is this what Table 504.3 is measuring?

Thanks!

 

[RLGA]

Building height is a defined term that is measured from the grade plane to the average height of the highest roof surface. You seem to understand the concept of the grade plane, so the average height of the highest roof surface is usually the midpoint between the lowest roof point and the highest roof point.

 

[Yankee Chronicler]

This also applies to height in stories, as well as feet. The building height limitations refer to the number of stories above grade.

The IBC Commentary has a lengthy commentary under "Height, Building." The first paragraph of that says:

This definition establishes the two points of measurement
that determine the height of a building. This measurement
is used to determine compliance with the
building height limitations of Sections 503.1 and 504
and Tables 504.3 and 504.4, which limit building
height both in terms of the number of stories and the
number of feet between the two points of measurement

 

[rosegamble]

Thank you. I am reading "building height" to mean "grade plane to average height of the highest roof surface is usually the midpoint between the lowest roof point and the highest roof point." Therefore, the basement while it counts towards story count (as the floor above it is over 6' above grade plane), building HEIGHT is measured at its lowest point as grade plane, not as the floor of the basement.

Another thought... and I did read the definition of "attic" which wasn't helpful... if the attic stays unoccupiable, it does not count as a story, right? I looked at the definition of "story" also and didn't see much to guide me.

 

[jar546]

If this reconfiguration ends up adding an occupied floor beyond what's allowed, it’s pushing the limits of the code’s intent. Expect building officials to take a hard look at it, especially if it affects egress, fire protection, or height and area restrictions.

 

[Yankee Chronicler]

Thank you. I am reading "building height" to mean "grade plane to average height of the highest roof surface is usually the midpoint between the lowest roof point and the highest roof point." Therefore, the basement while it counts towards story count (as the floor above it is over 6' above grade plane), building HEIGHT is measured at its lowest point as grade plane, not as the floor of the basement.

Another thought... and I did read the definition of "attic" which wasn't helpful... if the attic stays unoccupiable, it does not count as a story, right? I looked at the definition of "story" also and didn't see much to guide me.

If it is a trussed attic, then IMHO it would be an attic. If it's open, are there stairs leading up to it? If so, it's a story regardless of whether or not you declare an occupancy classification for it at this time.

[BG] ATTIC. The space between the ceiling framing of the top
story and the underside of the roof.

[BG] STORY. That portion of a building included between
the upper surface of a floor and the upper surface of the floor
or roof next above (see “Basement,” “Building height,”
“Grade plane” and “Mezzanine”). A story is measured as the
vertical distance from top to top of two successive tiers of
beams or finished floor surfaces and, for the topmost story,
from the top of the floor finish to the top of the ceiling joists
or, where there is not a ceiling, to the top of the roof rafters.

ICC Commentary on the definition of Story:

All levels in a building that conform to this description
are stories, including basements. A mezzanine is considered
part of the story in which it is located. See
Chapter 5 for code requirements regarding limitations
on the number of stories in a building as a function of
the type of construction.

It seems clear to me, but others may not agree.

 

[rosegamble]

Not trying to exceed 3 stories.

No stairs up to the framed attic (not finished). Just attic access doors required by code. I think this is a code gray area whether the attic is considered a "story." I don't think it counts towards story count limitations or area count limitations.

 

[Yankee Chronicler]

Not trying to exceed 3 stories.

No stairs up to the framed attic (not finished). Just attic access doors required by code. I think this is a code gray area whether the attic is considered a "story." I don't think it counts towards story count limitations or area count limitations.

The lack of stair access is an argument in your favor. Ultimately, what any of us here think or what you think doesn't matter. The determination lies with the Building Official. Have you discussed this with him?

How much would you have to raise the grade plane (or lower the building) so the basement would not be a story above grade?

 

[rosegamble]

Thanks. I have a meeting with the building official but want to come prepared. In order to the basement to not be considered a story over grade plane, first floor above would have to lower 1' approximately, or we demo the basement. It is really tiny (not the footprint of the whole building), so that is a factor.

Thank you!

 

[Yankee Chronicler]

Can you raise the grade plane by a foot?

How do you demo an existing basement, even a partial basement, under an existing multi-story building?

 

[rosegamble]

Interesting idea, but the building is quite close to the street (the feet is within 6' of the street) so probably not possible. The demo would not require any structural work. This is a small building.

 

[Yankee Chronicler]

Interesting idea, but the building is quite close to the street (the feet is within 6' of the street) so probably not possible. The demo would not require any structural work. This is a small building.

A basement is ordinarily located below the first(ground) floor and is the foundation supporting the above-grade exterior walls. How can you remove a building's foundations without involving any structural work?

 

[Raymond Redington]

A basement is ordinarily located below the first(ground) floor and is the foundation supporting the above-grade exterior walls. How can you remove a building's foundations without involving any structural work?

Ah, the labyrinthine intricacies of building codes—a dance as delicate as a fine Bordeaux. Yankee Chronicler, your insights into the measurement of building height are as precise as a Swiss timepiece. Indeed, the International Building Code (IBC) meticulously defines "building height" as the vertical distance from grade plane to the average height of the highest roof surface. This measurement governs compliance with the limitations set forth in Sections 503.1 and 504, as well as Tables 504.3 and 504.4, which delineate permissible building heights both in stories and feet.

However, the crux of the matter lies in the classification of the basement. If the basement qualifies as a "story above grade plane," it influences the total story count, thereby affecting the utilization of the attic space. The IBC defines a "story above grade plane" as any story with its finished floor level:

1. Located more than 6 feet above grade plane;
2. More than 6 feet above the finished ground level for more than 50% of the total building perimeter; or
3. More than 12 feet above the finished ground level at any point.

If the basement meets any of these criteria, it is indeed considered a story above grade plane. Consequently, in a Type VB, sprinklered building, the inclusion of such a basement would limit the structure to two additional stories above grade plane, as per Table 504.4. This limitation would preclude the use of the attic as an occupiable space.

Therefore, the proposal to demolish the basement warrants careful consideration. Replacing it with a crawl space—which does not count as a story above grade plane—would allow the attic to be utilized, provided all other code requirements are satisfied. As always, a thorough analysis of the specific site conditions and consultation with local code authorities is advisable to ensure compliance and optimize the building's design potential.

In the grand tapestry of construction, each thread—be it a basement or an attic—must be woven with precision to create a masterpiece that stands the test of time.

 

[Yankee Chronicler]

If the basement meets any of these criteria, it is indeed considered a story above grade plane. Consequently, in a Type VB, sprinklered building, the inclusion of such a basement would limit the structure to two additional stories above grade plane, as per Table 504.4. This limitation would preclude the use of the attic as an occupiable space.

Therefore, the proposal to demolish the basement warrants careful consideration. Replacing it with a crawl space—which does not count as a story above grade plane—would allow the attic to be utilized, provided all other code requirements are satisfied. As always, a thorough analysis of the specific site conditions and consultation with local code authorities is advisable to ensure compliance and optimize the building's design potential.

We get that. A basement is a story, a crawlspace isn't. That's not the point.

This is (if I understand the situation correctly) an existing building. How do you demolish the foundation under a two- (or three-_ story building? Sure, anything is possible if you throw enough money at it, but is it practical? I'm not seeing it.

 

[Raymond Redington]

We get that. A basement is a story, a crawlspace isn't. That's not the point.

This is (if I understand the situation correctly) an existing building. How do you demolish the foundation under a two- (or three-_ story building? Sure, anything is possible if you throw enough money at it, but is it practical? I'm not seeing it.

Ah, practicality. A word often whispered in the hallowed halls of pragmatism, yet so frequently ignored when the fever dream of compliance collides headfirst with the cold, unyielding reality of engineering. Yankee Chronicler, your skepticism is both warranted and, dare I say, refreshingly astute. Indeed, the demolition of a foundation under an existing two—or perhaps three—story structure is no small feat. It is not unlike attempting to disassemble an aircraft mid-flight, only with considerably more rubble and, one hopes, fewer casualties.

But let us not underestimate the sheer audacity of human ingenuity. There are ways, my dear Chronicler—methods so bold, so daring, they border on the theatrical. A surgical removal, if you will. One could, for instance, embark upon the Herculean endeavor of underpinning, carefully installing new foundation elements as the existing one is dismantled—a delicate ballet of excavation and reinforcement, each step a high-stakes gamble against gravity itself. Or, for those with an appetite for the truly extravagant, one might even consider lifting the entire structure, replacing the foundation in its entirety, before gently lowering it back into place like a Fabergé egg into its velvet-lined case.

But let’s speak candidly. In the absence of a near-limitless budget or an inexplicably enthusiastic structural engineer with a penchant for peril, such endeavors are, for most, the stuff of legend. Which brings us to the more sobering reality—perhaps, just perhaps, this is not a problem that should be solved by swinging sledgehammers at the past, but rather by embracing the existing conditions and navigating the labyrinth of the code with a different key. A variance, an alternative means of compliance, an appeal to the omnipotent wisdom of the Authority Having Jurisdiction—each a path fraught with its own unique perils, yet far more achievable than playing Jenga with a building’s foundation.

So I leave you with this, Chronicler—yes, it can be done. But the more pertinent question is whether it should be done. And that, I suspect, is where the true battle lies.

 

[Yankee Chronicler]

So I leave you with this, Chronicler—yes, it can be done. But the more pertinent question is whether it should be done. And that, I suspect, is where the true battle lies.

You're not leaving me with that question -- it's not my building. I have no dog in this fight. The question belongs to rosegamble. I never said it can't be done. I know it can be done -- but at what cost?

It occurs to me that "demolition" of the basement may not be necessary to convert it from a basement to a crawlspace. Assuming the AHJ would agree, it would be far simpler to bring in fill, compact it in lifts with the smallest plate compactor that will fit, and once it has been raised high enough that the remaining headroom no longer qualifies as a story -- pour a rat slab and call it a day.

 

[steveray]

I thought they made a stupid change that took attics out of being a story or calculated as height a cycle or 2 ago....Maybe I was remembering this from the IRC...

R316.3​

A habitable attic shall be considered a story above grade plane.
Exceptions:A habitable attic shall not be considered to be a story above grade plane provided that the habitable attic meets all the following:

1. 1.The aggregate area of the habitable attic is either of the following:
   
   1. 1.1.Not greater than one-third of the floor area of the story below.
   2. 1.2.Not greater than one-half of the floor area of the story below where the habitable attic is located within a dwelling unit equipped with an automatic sprinkler system in accordance with Section P2904.
2. 2.The occupiable space is enclosed by the roof assembly above, knee walls, if applicable, on the sides and the floor-ceiling assembly below.
3. 3.The floor of the habitable attic does not extend beyond the exterior walls of the story below.
4. 4.Where a habitable attic is located above a third story, an automatic sprinkler system in accordance with Section P2904 shall be installed in the habitable attic and remaining portion of the townhouse unit or dwelling unit or units located beneath the habitable attic.

 

[bill1952]

If there were no slab, would it be a basement or a crawl space? Or more importantly, would it be a story? I couldn't find where a crawl space was defined not that the height of a crawlspace was limited. It seemed it all relies on it being a story which seems to require a floor. Do you allow first floors otherwise?