Eaves in building area calculation?

[CHardt&SmNmbrs]

Reading the definition of Area, Building in Chapter 2, states "The area included within the surrounding exterior walls, or exterior walls and fire walls, exclusive of vent shafts and courts. Areas of the building not provided with surrounding walls shall be included in the building area if such areas are included within the horizontal projection of the roof or floor above."

Does this mean that the building area is calculated by the drip-line of the roof and not by exterior walls? If not, where can one find a supporting statement?

 

[fatboy]

within the horizontal projection of the roof or floor above

That says it, "within the horizontal projection of the roof or floor above"

 

[RLGA]

The area under roof eaves/overhangs is not considered part of the building area. Eaves/overhangs are considered projections beyond the exterior walls. If there is an overhang over usable space (e.g., patios, decks, etc.), those areas should be included in the building area.

 

[CHardt&SmNmbrs]

I appreciate the response, but I'm still not entirely clear on this. "Areas of the building not provided with surrounding walls," Would the area under the eaves not be considered a part of the areas of a building? Is it because that is not a 'Functional Area' with no designated activities or processes?

 

[RLGA]

I appreciate the response, but I'm still not entirely clear on this. "Areas of the building not provided with surrounding walls," Would the area under the eaves not be considered a part of the areas of a building? Is it because that is not a 'Functional Area' with no designated activities or processes?

In essence, yes. The space under eaves are not occupied for a functional use. If the eave extends far enough to allow for seating or other uses beyond just shading of, or keeping rain water from, the exterior wall, then that area should be included.

 

[wwhitney]

Does this mean that the building area is calculated by the drip-line of the roof and not by exterior walls?

Yes, that is what the definition says, at least for the case that the area enclosed by the exterior walls is all under a roof.

Cheers, Wayne

 

[wwhitney]

In essence, yes. The space under eaves are not occupied for a functional use.

The definition of "Floor Area, Gross" does include the limiter "usable" in the phrase "useable area under the horizontal projection of the roof or floor above." So such a distinction is appropriate for determining "Floor Area, Gross".

The definition of "Area, Building" does not include any such "usable" limiter. Therefore it includes all area under the eaves, useable or not.

If that is not the intention of the ICC, the definition requires amendment.

Cheers, Wayne

 

[walker.t]

The definition of "Area, Building" does not include any such "usable" limiter. Therefore it includes all area under the eaves, useable or not.

That’s a good catch, you’d make a good quality control proofreader!

If that is not the intention of the ICC, the definition requires amendment.

I do think, however, that the intent is useable area even though it doesn’t use that word (and it should.) The 2018 IBC Illustrated Handbook has commentary on many of the terms defined in Chapter 2, regarding the definition of “Building Area” it says:

Where a column line establishes the outer perimeter of the usable space under the roof, it is also typically the extent of the building area. Beyond the column line, the overhead cover is simply viewed as a projection. If all the area beneath the roof above can be considered usable space, then the building area is measured to the leading edge of the roof above. [emphasis added]

 

[mtlogcabin]

The definition of a building includes structures with no exterior walls such as carports, open parking garages, storage facilities, stadium grand stands to name a few. That type of building would be measured to the drip line of the eaves/gable ends

[A] BUILDING. Any structure utilized or intended for supporting or sheltering any occupancy.

[BF] FIRE AREA. The aggregate floor area enclosed and bounded by fire walls, fire barriers, exterior walls or horizontal assemblies of a building. Areas of the building not provided with surrounding walls shall be included in the fire area if such areas are included within the horizontal projection of the roof or floor next above.

 

[Yikes]

Would you count any of these balconies as building area?

 

[wwhitney]

Would you count any of these balconies as building area?

Seems like the only way to avoid including the horizontal projection of those balconies in the building area would be to claim that they aren't part of the "floor". Looks like a floor to me.

I'm surprised that the UBC doesn't include definitions for basic terms like floor, roof, and walls (which has a definition applicable to Chapter 21 only). I also would expect that the definitions are carefully enough crafted that an omission of a term like "usable" from one definition but not another would be a signal of intention, rather than an oversight. But perhaps I'm expecting too much.

Cheers, Wayne

 

[Yankee Chronicler]

Seems like the only way to avoid including the horizontal projection of those balconies in the building area would be to claim that they aren't part of the "floor". Looks like a floor to me.

I'm surprised that the UBC doesn't include definitions for basic terms like floor, roof, and walls (which has a definition applicable to Chapter 21 only). I also would expect that the definitions are carefully enough crafted that an omission of a term like "usable" from one definition but not another would be a signal of intention, rather than an oversight. But perhaps I'm expecting too much.

Cheers, Wayne

It's probably unrealistic to expect the codes to be logical or consistent. After all, although the old BOCA Basic Building Code required construction drawings to be drawn to scale, the IBC does not. Neither does the IRC or the IEBC. But the IMC and the IPC do require the construction documents to be drawn to scale.

IMHO, that's borderline idiotic.

 

[walker.t]

Would you count any of these balconies as building area?

I would include them all without a second thought, they are useable space, some of them even have furniture on them.

For the balconies on the right, at first I was thinking people were crawling out the windows but I assume there’s a door on the left wall which we can’t see. If the balconies were not served by a door and were only for aesthetics I would not include them in the useable area.

 

[Yikes]

My problem is, if you count the balcony as floor area, then do you count the face of the balcony as building “walls” and the space above the guard wall when calculating % of allowable openings (based on distance to property line)?

Or is the % of allowable openings calculated at the inset door that goes out onto the balcony? Or maybe at the face of the main facade, where the smoke from an apartment fire can now dissipate towards the sides of the balconies?

 

[wwhitney]

My problem is, if you count the balcony as floor area,

You originally asked about building area, not floor area. But almost the same logic as for the building area applies: in order not to count the balconies, you'd have to declare that the balcony above is not part of "the floor above". As the balconies shown are clearly "useable area."

The one difference from the building area discussion is the topmost balcony in each stack. Any portion of it that is open to the sky above is excluded from the floor area. Which is a bit counterintuitive.

Cheers, Wayne

 

[walker.t]

if you count the balcony as floor area

I am assuming you meant “building area” when you said “floor area.”

if you count the balcony as floor area, then do you count the face of the balcony as building “walls” and the space above the guard wall when calculating % of allowable openings (based on distance to property line)?

I don’t think so, the guard forming the perimeter of the balcony does not fit the definition of “Exterior Wall” in IBC Chapter 2:

IBC Chapter 2 Definition of “Exterior Wall”
A wall, bearing or nonbearing, that is used as an enclosing wall for a building, other than a fire wall, and that has a slope of 60 degrees or greater with the horizontal plane. [Emphasis added.]

Or is the % of allowable openings calculated at the inset door that goes out onto the balcony? Or maybe at the face of the main facade

You’re talking about fire separation distance…

IBC Chapter 2 Definition of “Fire Separation Distance”
The distance measured from the building face to one of the following:

1. The closest interior lot line.
2. To the centerline of a street, an alley or public way.
3. To an imaginary line between two buildings on the lot.

The distance shall be measured at right angles to the face of the wall.

So for the building in the photograph, we have a fire separation distance measured to the frontmost wall (with the two sets of double windows) and a different fire separation distance measured to the recessed wall (with the three windows overlooking the balcony.)

I don’t know how roof overhangs and balconies that extend beyond the exterior wall are to be handled when it comes to fire resistance rating.

Any portion of it that is open to the sky above is excluded from the floor area.

Wouldn’t the uppermost balcony, if open to the sky, be included in the floor area for purposes of occupant load calculation using Table 1004.5?

 

[wwhitney]

Wouldn’t the uppermost balcony, if open to the sky, be included in the floor area for purposes of occupant load calculation using Table 1004.5?

I haven't studied Chapter 10, but if it is using the term "floor area" without any further modifiers, then I conclude it is referring to the definitions in Chapter 2, and the "floor area, gross" definition excludes the region of a balcony outside the exterior walls that is open to the sky directly above (straight up). So then I would say no, as written.

Again, if this is not the intention, then the definitions are sloppy, which is disappointing.

Edit: is there no language in Chapter 10 to deal with occupant loads attributable to spaces open to the sky above, like decks or rooftop parking in parking garages?

Cheers, Wayne

 

[walker.t]

is there no language in Chapter 10 to deal with occupant loads attributable to spaces open to the sky above, like decks or rooftop parking in parking garages?

1004.7 “Outdoor Areas” makes reference to outdoor areas but doesn’t address whether they are open to the sky or not. This section talks about the means of egress for them and says, “The occupant load of such outdoor areas shall be assigned by the building official in accordance with the anticipated use.”

Table 1004.5 is talking about function of the space, that doesn’t have to be tied to whether the space is open to the sky or not.

The closest I’ve run into this is a coffee shop which had an open patio in front of the building and a a small covered “porch” near the front door, both with tables and chairs, those areas needed to be included in the occupant load calculation for the restrooms. The total building area was so small we never discussed the building area in detail but the covered porch would have been included in the building area, the open patio would not have been.

if it is using the term "floor area" without any further modifiers, then I conclude it is referring to the definitions in Chapter 2, and the "floor area, gross" definition excludes the region of a balcony outside the exterior walls that is open to the sky directly above (straight up).

The Chapter 2 definition of “floor area, gross” would only apply to a reference to the use of the term “gross floor area” in a provision, those references are italicized to show they are defined terms.

 

[wwhitney]

The Chapter 2 definition of “floor area, gross” would only apply to a reference to the use of the term “gross floor area” in a provision, those references are italicized to show they are defined terms.

Your position is that while "gross floor area" and "net floor area" are defined terms, "floor area" is not? That is defensible from a literal point of view, but it seems reasonable to me that say that a definition of "floor area" is implied.

1004.7 “Outdoor Areas” makes reference to outdoor areas but doesn’t address whether they are open to the sky or not.

OK, so I guess that's how I see the (implied) definition of "floor area" interacting with Chapter 10. A balcony that is covered above (by another balcony) is a "floor area" covered by 1004.5, while a balcony that is open to the sky above is an "outdoor area" covered by 1004.7. In practice I would expect the same occupant load to be assigned to that "outdoor area" as all the other balconies that are "floor area".

Cheers, Wayne

 

[walker.t]

Your position is that while "gross floor area" and "net floor area" are defined terms, "floor area" is not?

Yes, that is correct, only “gross floor area” and “net floor area” are defined terms in Chapter 2. if there’s a reference to just “floor area” without “gross” or “net” then we’d have to infer the meaning from the context.

That is defensible from a literal point of view, but it seems reasonable to me that say that a definition of "floor area" is implied.

If a “floor area” is not to be defined as gross floor area or net floor area, how would it be defined?

And I just thought of “building area” as another defined term addressing the general topic of floor areas.

IBC Chapter 2 Definitions for Area
Building Area
The area included within surrounding exterior walls, or exterior walls and fire walls, exclusive of vent shafts and courts. Areas of the building not provided with surrounding walls shall be included in the building area if such areas are included within the horizontal projection of the roof or floor above.

Floor Area, Gross
The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vents shafts and courts, without deduction for corridors, stairways, ramps, closets, the thickness of interior walls, columns or other features. The floor area of a building, or portion thereof, not provided with surrounding exterior walls shall be the usable area under the horizontal projection of the roof or floor above. The gross floor area shall not include shafts with no openings or interior courts. [Note reference to “floor area of a building” = “building area.”]

Floor Area, Net
The actual occupied area not including unoccupied accessory areas such as corridors, stairways, ramps, toilet rooms, mechanical rooms and closets.

In practice I would expect the same occupant load to be assigned to that "outdoor area" as all the other balconies that are "floor area".

That seems like a very reasonable expectation to me.

 

[wwhitney]

Note reference to “floor area of a building” = “building area.”

I don't think I agree. For the simple case of a cuboid building, isn't Gross Floor Area = Building Area * Number of Stories?

Yes, that is correct, only “gross floor area” and “net floor area” are defined terms in Chapter 2. if there’s a reference to just “floor area” without “gross” or “net” then we’d have to infer the meaning from the context.

Seems to me the definition may be inferred from the definition of "gross floor area", rather than the context of the section using the term "floor area". Namely "Floor Area: any portion of the Gross Floor Area." In the sense that gross means total, so if it's not counted in the Gross Floor Area it can't be a floor area.

Cheers, Wayne

 

[walker.t]

[Note reference to “floor area of a building” = “building area.”]

I don't think I agree.

Now that I think more about it, it was a mistake for me to read a reference to “building area” into the definition of “gross floor area,” if that’s what they meant they could have easily included an italicized instance of “building area.”

isn't Gross Floor Area = Building Area * Number of Stories?

Actually, the terminology used in Equation 5-2 is Allowable Area = Allowable Area Factor x Number of Stories. I don’t see anything in 506 that clarifies what is in the “allowable area” - namely exterior walls. 506.1 starts by only saying, “The floor area of a building…”, then 506.2 says, “The allowable area of a building…”. So maybe that supports my editorial comment “floor area of a building = building area”? But yes, allowable area factor times the number of stories gives us the total area, whatever it is we’re going to call that.

A commentary in the 2018 IBC Illustrated Handbook says, “The term building area describes that portion of the building’s floor area to be utilized in the determination of whether or not a structure complies with the provisions of Chapter 5 for allowable building size.” So this would indicate that “Allowable Area” is “Building Area.”

I think in concept the definitions of “building area” and “gross floor area” are basically the same, except the perimeter of the building area is also defined by fire walls, gross floor area is not. I think the Chapter 2 definition for “gross floor area” does a better job of listing what is included, it would be nice if the definition for “building area” also said there were no deductions for corridors, walls, et cetera. I also think the definition of “building area” should include a reference to “inside perimeter” similar to the definition of “gross floor area.”

I do think a significant difference, however, is when the terms “building area” and “gross floor area” are used. Building area is used for allowable building area in 506. Gross floor area is used (along with net floor area) to determine occupant load calculations in 1004.

Seems to me the definition may be inferred from the definition of "gross floor area", rather than the context of the section using the term "floor area". Namely "Floor Area: any portion of the Gross Floor Area." In the sense that gross means total, so if it's not counted in the Gross Floor Area it can't be a floor area.

I would consider modifying that to say that “floor area” is “any portion of the building area,” I’d reserve “gross floor area” for calculation of occupant load.

 

[wwhitney]

Actually, the terminology used in Equation 5-2 is Allowable Area = Allowable Area Factor x Number of Stories. I don’t see anything in 506 that clarifies what is in the “allowable area” - namely exterior walls. 506.1 starts by only saying, “The floor area of a building…”, then 506.2 says, “The allowable area of a building…”. So maybe that supports my editorial comment “floor area of a building = building area”? But yes, allowable area factor times the number of stories gives us the total area, whatever it is we’re going to call that.

Looking only at the definitions, "Building Area" makes no mention of floors. So the obvious reading from my point of view is that adding a story to a building does not change the Building Area, it just changes the Floor Area. Building Area would be the footprint on the earth occupied by the building, independent of number of stories.

However, looking at Section 506, it seems the IBC occasionally uses the term Building Area when it means what I would call Floor Area. Section 506 is titled "Building Area" but as far as I can see doesn't regulate Building Area (in the above sense) at all, just floor area.

A commentary in the 2018 IBC Illustrated Handbook says, “The term building area describes that portion of the building’s floor area to be utilized in the determination of whether or not a structure complies with the provisions of Chapter 5 for allowable building size.”

Since the word "floor" does not appear in the definition of "Building Area", I have to say that is inconsistent with the definitions.

However, all of this is sounding like Building Area and Gross Floor Area are supposed to be almost the same, with the difference relating to whether the thickness of the exterior walls is included. In which case if that is the usage in the IBC, then "Building Area" should be updated to include a reference to floor area to make that clear.

Cheers, Wayne

 

[walker.t]

Looking only at the definitions, "Building Area" makes no mention of floors. So the obvious reading from my point of view is that adding a story to a building does not change the Building Area, it just changes the Floor Area. Building Area would be the footprint on the earth occupied by the building, independent of number of stories.

Interesting that you mention the term “footprint,” I was going to say something about that in my previous post. I have wondered what the proper term in the IBC is for the footprint, meaning the perimeter of the part of a multistory building that touches the ground. Even if the Chapter 2 definition of “building area” doesn’t mention the number of floors, the use in 506 supports the interpretation that it means “total area of all stories in a building” which is what you would call “floor area.”

The term “footprint” has given me some heartburn in the past. I found the term “footprint” used in two commentary documents:

1. 2018 IBC Study Companion, in a section discussing building area and referencing 503.1 and 202: “The building area is considered, in very general terms, the ‘footprint’ of the building…”
2. IBC commentary sent to me by a building official when I was asking about the concept of the building “footprint”: “A building area is the ‘footprint’ of the building; that is, the area measured within the perimeter formed by the inside surface of the exterior walls.”

My heartburn is that a footprint caused by a shoe is to the outside perimeter of the shoe, not the inside. And I think the general use of the term is to the outside of an object, such as the footprint of a piece of equipment. But in the IBC, “building area” is to the inside face of the walls, so I’d prefer that they use a term other than “footprint” to help describe the concept.

With all that said, the IBC doesn’t seem to care specifically about the building footprint (area of the ground covered by the building,) only the building area (total inside perimeter area of all floors of the building) and gross floor area as it applies to occupant load. Ground contact footprint of a building would be of interest to matters related to lot coverage for things such as zoning and stormwater control.

Since the word "floor" does not appear in the definition of "Building Area", I have to say that is inconsistent with the definitions.

I would agree with that, I’d like to see the definition of “building area” refined.

However, all of this is sounding like Building Area and Gross Floor Area are supposed to be almost the same, with the difference relating to whether the thickness of the exterior walls is included. In which case if that is the usage in the IBC, then "Building Area" should be updated to include a reference to floor area to make that clear.

No, they are the same in that regard, neither “building area” nor “gross floor area” include the exterior walls based on the commentaries. One of the refinements I’d make to the definition of “building area” is to say that it is taken to the inside perimeter of the exterior walls. In fact, I think I’d go so far as to say the definition of “building area” could be the same as for “gross floor area.” So they describe the same thing, but they use two different terms depending on what we’re doing:

1. “Building area” for 506. This makes sure that the entire building is within allowable constraints dictated by occupancy, construction type, and use of sprinklers.
2. “Gross floor area” for 1004. They use this term when determining the occupant loads for some of the functions of smaller areas within the “building area.”