# Area Calculation Clarification



## chad

I have a somewhat basic (dumb?) question about area calculations -

Reading through the 2009 IBC it defines building area as:



> 502.1AREA, BUILDING. 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.
> 
> 1002.1
> 
> FLOOR AREA, GROSS. The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, 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.
> 
> FLOOR AREA ,NET. The actual occupied area not including unoccupied accessory areas such as corridors, stairways, toilet rooms, mechanical rooms and closets.


If I'm reading this correctly - area calculations for code purposes should be calculated from the interior face (finish face or core face?) of exterior walls correct?  If this is true - how to party walls get measured?

I ask because we frequently run our calculations using BOMA (exterior face / centerline of party walls) and the additional thickness of exterior walls will create a variation in the occupancy allowed in the building.  I know the developers / property owners want BOMA calcs for rental reasons but it seems that for code purposes that is not the correct way to measure.

Am I completely off base here or have we been doing it incorrectly?

Thanks


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## fatboy

I've always considered the building area definition is for use in calculating allowable area, based on the last part of the section.

"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."

Where the net and gross floor areas definitions are used for occupant load calculations.

BTW, welcome to the forum!


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## FM William Burns

I've always looked at keeping it simple for me as "Gross" = exterior to exterior measurement and "Net" = Interior to interior measurement scooping out all the built in features/rooms with the "net"


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## Coug Dad

Welcome to the board.

I somewhat agree with fatboy, but I would not include a roof eave overhang where an enclosing wall is provided.  Do not try to correlate between IBC measurements and BOMA.  That exercise has been know to cause serious brain damage.


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## jj1289

Building area calculations are base on the interior face.  For simplicity sake we use the gross footprint of the building including the walls.  If the floor area is at one of the threshold of placing the building into a different construciton type or any other triggers then we calculate excluding the exterior walls.


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## RLGA

"Gross" and "Net" areas apply when the code specifically uses those terms, which are primarily in the sections used to determine occupant load.

"Building area" is used when determining the the area of a building, regardless of occupant load, and is primarily applicable when determining allowable area for a building.

Party walls are "fire walls" by IBC description (See Section 706.1.1, 2009 IBC). Therefore, buildings area is measured to the face of the wall on the side that is being measured; thus, the party/fire wall is not included in the building area (see the "building area" definition).

If you're referring to what some call "demising walls" that define the limits of a tenant space, then those are only applicable when the code addresses tenant spaces and usually the "gross" area applies in that case.

Like Coug Dad states, don't mix code definitions with real estate definitions--they don't correlate.


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## permitguy

And to throw another wrench in the works, fire area is different from all of the above.  I agree with the others: get familiar with the IBC definitions and ignore the BOMA definitions for purposes of applying code provisions.

Welcome to the forum!


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## chad

Thanks.

I get enough headaches trying to read through one code - mixing won't happen.


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## Architect1281

Allowable building area is the GROSS includes roof overhang for using 503 to determine building class use compliance.

occupied area is use for occupant load floor area Gross when gross net when net in ch 10


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## RLGA

Architect1281 said:
			
		

> Allowable building area is the GROSS includes roof overhang for using 503 to determine building class use compliance.occupied area is use for occupant load floor area Gross when gross net when net in ch 10


The definition of "Building Area" does not use the term "Gross," although it would imply that based on a comparison of the definitions of "gross floor area" to "building area."

Also, you have be careful when mentioning roof overhang as being part of the building area.  Eaves and other architectural projections are not included in building area.  The building area definition only states that "areas of the building not provided with surrounding walls" shall be included in the building area if they "are included within the horizontal projection of the roof or floor above."  A roof overhang that constitutes an eave would not be included in building area.  Neither would awnings or canopies that are attached to buildings, since they are not a projection of the roof above.


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## JENLO

RLGA said:
			
		

> The definition of "Building Area" does not use the term "Gross," although it would imply that based on a comparison of the definitions of "gross floor area" to "building area."  Also, you have be careful when mentioning roof overhang as being part of the building area.  Eaves and other architectural projections are not included in building area.  The building area definition only states that "areas of the building not provided with surrounding walls" shall be included in the building area if they "are included within the horizontal projection of the roof or floor above."  A roof overhang that constitutes an eave would not be included in building area.  Neither would awnings or canopies that are attached to buildings, since they are not a projection of the roof above.


So then, How do I define a roof eave.  I have a contemporary style project with large architectural overhangs, I'd consider those architectural projections. I also have lugs that are architectural elements, and cantilevered balconies.  I am trying to determine if the cantilevered balconies and lugs would need to be included in the building area for allowable area calcs.


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## north star

*~ & ~ & ~ &*





JENLO,

Which codes & editions are you using ?

*~ & ~ & ~ & ~*


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## zigmark

Do these features you are inquiring about make enough of a difference in your code analysis to trigger other requirements?  I often find that I start down the very path you are on only to find it didn't matter.  Run the numbers both ways and see what the impacts are.  If the building is teetering on some significant determination based on these features, such as fire sprinklers, often times they get modified.  I would personally include balconies in the allowable/fire area calculations.

ZIG


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## JBI

A balcony and the area beneath it would count, as the balcony is a projection of the floor.

When I was in direct enforcement,I also would start with the outside dimensions and only subtract out the wall thicknesses if a threshold was crossed. Didn't happen often.

Most properly, building area is within surrounding walls, so inside face to inside face would be correct.


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