5
Building Heights and Areas
As noted in Chapter 3, building designers almost invariably start a project with a given occupancy. After a building's occupancy classification is determined, the code analysis task becomes one of determining what heights and areas are allowable for the occupancy classification, given various types of construction. Criteria for allowable building heights and areas are set forth in Chapter 5 of the IBC. Economics and utility generally dictate that buildings be built using the least costly and complicated type of construction that will meet the criteria set forth in the code.
Two types of design choices typically impact the use of Chapter 5 for determining allowable heights and areas. The first is when the design of a given occupancy must provide enough area to contain the known uses. The second comes into play in speculative buildings built for economic gain. The goal in these facilities, such as office buildings or retail uses, is to maximize the allowable height and area using the most economical construction type.
When addressing the question of maximizing the economically viable building size, the designer must use an iterative process to maximize the space for a given building. One must make assumptions regarding construction type and analyze the relative cost and return for various construction types before the owner can make a decision. This iterative process may also be used to maximize the economic efficiency of a building where the program size requirements are the primary consideration. In either case, the goal is to achieve the maximum area with the minimum investment in construction materials while still meeting or exceeding the code-mandated requirements to protect public health, safety, and welfare.
The descriptions for the allowable heights and areas for buildings that are contained in Chapter 5 were completely reorganized in the previous code cycle from the way they were presented in previous editions of the code. For those readers still familiar with the old table and methods presented in prior code editions, it is important to remember that this reorganization was NOT intended to make any substantive revisions to the heights or areas that would have been determined under prior codes. The reorganization was undertaken only to clarify and make more consistent how allowable heights and area are interpreted by code users and authorities having jurisdiction. There are no changes in the new code edition that alter the fact that using older height and area analysis methods will yield the same results as the newer methods for the same constuction types.
The organization of Chapter 5 in the code is based on a set of basic criteria that are then modified by mitigating factors to allow increases or trade-offs between heights, areas, construction types, fire protection, and life safety systems. Upon first reading, the tables in Chapter 5 may seem very restrictive, but they contain modifications that give the designer greater flexibility.
In summary:
- The designer usually knows the occupancy classification of the building.
- The program area is usually set either by program needs or by budget.
- A construction type will usually be determined during schematic design.
- Note that, as described in Chapter 2 regarding fire extinguishing systems, code requirements for fire sprinkler systems from Chapter 9 must be determined at this time to fully understand how to use the provisions of Chapter 5.
- Since almost any building has a mix of incidental and accessory occupancies, and perhaps a mixture of occupancy classifications as described in Chapter 3, the provisions of § 508 must be carefully examined.
Definitions
As for other chapters, all cross references to definitions have been removed. All definitions now occur only in Chapter 2. Definitions applicable to this chapter are discussed below. The definitions have very specific code-related criteria that may differ from the colloquial meanings of the terms. They should be studied carefully for applicability when determining allowable heights and areas.
Height limitations for buildings contained in planning and zoning regulations do not necessarily use the same definitions or criteria for determining heights. The definitions in the code are developed to facilitate uniform application of regulations. Read the documents that pertain to the regulations in question. Never apply building-code criteria to planning issues, or vice versa.
Building Area is usually considered to exclude the thickness of the exterior walls of a building. This is based on the language describing gross floor area as “the area included within surrounding exterior walls.” The wording seems to imply the wall is not a part of the area to be considered. Therefore the building area is the area enclosed within the building and the area begins at the inside face of the exterior wall. Another real-estate term often used for building area is “gross building area,” which is typically calculated to the exterior side of the exterior walls. The most conservative way to calculate building area is to measure from outside face of wall to outside face of wall. This generates the most building area when nearing the upper limit of allowable area, which should never be challenged by the Authorities Having Jurisdiction.
The definitions of Basement, Grade Plane, and Building Height are correlated and based on the relationship of building parts below or above the grade plane as it is defined. Note that building height, as measured in feet, takes roof profiles into account. Building heights are measured to the average height of the highest roof, thus allowing for pitched roofs, varying parapet heights, and rooftop equipment enclosures. The height is calculated using different criteria than those used to determine the height of a building in stories. “Story” is intended to mean occupiable or usable space located inside the building contained by a floor below and a plane (next story or roof) above. Thus, this would exclude such unoccupied spaces as depressed loading docks. This definition is found with other definitions in Chapter 2.
Exceptions to Building Height and Area Limitations
§ 503.1.1 exempts low-hazard, high-bay spaces, such as mills or foundries, from the height and building area limitations in § 504 and § 506. This recognizes the special-process requirements for such uses as well as their limited occupant loads with little or no public access.
§ 503.1.3 states that Type I buildings, permitted to be of unlimited height and area per § 504 and § 506, do not require the mitigations for unlimited-area building imposed on other construction types by other sections of this chapter. Note, however, that certain Group H occupancies have height and area restrictions even for Type I buildings.
§ 503.1.4 regarding “occupied roofs” has been added to the code to recognize the increased use of building roofs for open space in urban areas. The occupancy classification of the roof is to be per the uses allowed for the story immediately below the roof unless the building is fully sprinklered and an occupant notification alarm system is provided at the roof. The area of an occupied roof is not added to the area of the building as long as there is no roof above the uses or the enclosed areas are less than 48'' (1219) high.
- Occupied roof
- Penthouse per § 1510.2
- Enclosure less than 48'' (1219) high
§ 503.1.4 regarding “occupied roofs” has been added to the code to recognize the increased use of building roofs for open space in urban areas. The occupancy classification of the roof is to be per the uses allowed for the story immediately below the roof unless the building is fully sprinklered and an occupant notification alarm system is provided at the roof. The area of an occupied roof is not added to the area of the building as long as there is no roof above the uses or the enclosed areas are less than 48'' (1219) high.
Building Height and Area Limitations
The provisions for determining allowable heights and areas are based on several factors. The first question to be determined is the type of occupancy (or multiple types in mixed-occupancy buildings) present in the proposed building. The second is whether the building is to be sprinklered or not. Next come determinations or assumptions about what construction type is desired or required for the proposed occupancies. The new organization of Chapter 5 uses the following abbreviations for conditions:
![Figure depicts S13R = building equipped throughout with an automatic sprinkler system installed in accordance with § 903.3.1.2 [NFPA 13R sprinkler systems in Group R occupancies up to and including four stories in height in buildings not exceeding 60 feet (18288) in height above grade plane].](images/c05/61-5.gif)
In prior editions of the IBC, Table 503 combined the basic information for heights of buildings in feet above grade plane, number of stories, and basic allowable areas per floor. These basic allowances were based on the intersection of occupancy groups and building construction types. Now there are tables in the code that address heights in § 504 and areas in § 506. They are organized around occupancies and construction types, and the tables are separated to address only one aspect of height and area allowances in each table. As in prior codes, the construction types are based on those described in Chapter 6, ranging from Type I fire-protected construction to Type V unrated construction. As in Table 601, “A” denotes construction that is “protected” with increased fire resistance and “B” denotes construction not provided with fire-resistance protection.
We will first discuss the standards in the tables as general limitations. We will then examine in detail the allowable modifications contained in the tables. For readers familiar with the presentation of these requirements in Table 503 in prior code editions, it is important to remember that while the presentation of the information has changed, the technical requirements have not. A building designed under the 2018 IBC will be the same size and height as that same building would have been under the 2012 IBC
- Allowable building height and
building area, defined in Chapter
2, are determined by the intersection of
occupancy group and construction type.
- The illustrations starting on the facing page are taken from Table 504.3 to show the relationship of occupancy and construction type to allowable building heights in feet. The examples are chosen from building types typically encountered by designers with the construction types chosen to highlight the differences as one proceeds from Type I fire-protected construction to Type V unrated construction. Entries in the table represent allowable heights in feet, as adjusted for inclusion of fire sprinklers or not.
Allowable Building Heights
§ 504 pertains to the determination of allowable building heights in feet and stories above the grade plane. As with building areas, building heights in feet and in stories are dependent on the occupancies to be included in the building and on the building's construction type. The inclusion of automatic sprinklers in the building also results in increases in allowable heights in feet and stories. Heights are described in two tables.
The first, Table 504.3, shows allowable building heights in feet only, based on occupancy type, construction type, and sprinkler conditions. The second, Table 504.4, shows allowable building heights in terms of the number of stories, also based on occupancy type, construction type, and sprinkler conditions. The tables include all of the criteria necessary to determine allowable values for these variables. One looks in the tables under the chosen construction type, then reads down the chosen column to the occupancy in question and then makes a selection based on whether the building is sprinklered or not. The intersection of those variables in the tables gives the code user the allowable values for height in feet (Table 504.3) or height in stories (Table 504.4). There are not any adjustments necessary to the values in the tables for provision of sprinklers; those determinations are built into the two new tables.
Mezzanines and Equipment Platforms
§ 505 considers mezzanines that meet the definition in Chapter 2 to be part of the story below them. If they meet the criteria limiting their area to one-third of the floor below, then they are not considered part of the overall building area, or as an additional story. However, the area must be counted toward the overall “Fire Area” as defined in Chapter 2.
Mezzanines are conceived of by the code as open areas set above other spaces in a room. The code makes an absolute-sounding statement that all mezzanines shall be open and unobstructed to the room in which they are located, except for a railing-height wall at the edge. The statement is then followed by numerous exceptions. The basic idea is that if the mezzanine is small in area or occupant load, is furnished with a clearly defined separate exit path, or has two means of egress, it may be enclosed.
Allowable Building Heights and Areas
Allowable Building Areas per Table 506.2
As discussed, in code editions prior to 2015, Table 503 combined allowable building heights in feet and stories with basic allowable areas. Each of the sets of tabular values for heights and areas were then adjusted for sprinkler conditions and location on property. The code now breaks apart height in feet, height in number of stories, and allowable areas per level into three tables. It also includes modifications inside of each table based on whether the buildings are sprinklered or not.
The allowable areas are determined per Table 506.2. This table shows allowable building areas based on occupancy type and construction type. It also shows areas based on whether the building is nonsprinklered or is a sprinklered single-story or multiple-story building. It is very important to realize that under the increase formulas, the “NS” (nonsprinklered) values in this table will be used in area-increase calculations, even for sprinklered buildings. When doing a code analysis for a proposed building, it is important to note the allowable area values for nonsprinklered buildings based on anticipated occupancies and proposed construction types, so that you are able to insert them into the formulas from § 506.2 as described below.
Combined Table 504.4 and Table 506.2
The tables below and on the following pages show graphically the allowable areas taken from Table 506.2, based, as for heights, on the building's construction type, occupancy, and whether or not it is sprinklered. This table also graphically combines the number of allowable stories from Table 504.4 to represent the comparative heights and areas for the various combinations of the variables in the tables. This is a graphic tool to convey the use and implications of the choices for construction types for the various occupancies that may occur in a building. The intent of the code is that the determinations of the values from each table are made independently and then combined into a set of code criteria for design of the building.
Allowable Building Area Determination
The allowable area of a building is to be determined in accordance with the applicable provisions of § 506.2.1 through 506.2.4 and § 506.3 using the values from Table 506.2. The calculations for area increase have been changed dramatically from the way area increases were done under editions of the IBC prior to 2015. As noted, the increases for heights in feet and stories based on sprinklers and construction type are now built into the tables in § 504. For those code users familiar with the old calculation methods, it is very important to remember that the new methods are intended to yield the same results in terms of allowable building areas, and heights as well, as were allowed under previous editions of the IBC. Experienced code users may want to verify their calculations for allowable heights and areas using the new formulas against determinations for the allowances using formulas from the prior version of the IBC. This will help familiarize experienced users with the new methods while minimizing calculation errors. We have presented here only the new formulas to avoid confusing new code users for whom such comparisons would not be helpful. This will also familiarize experienced code users with the procedures found in the new edition of the code.
Frontage Increases
§ 506 specifies that the areas set by Table 506.2 may be modified based on how much open space adjoins the building, including streets and open space. This is known as “frontage.” As in prior editions of the code, the equation numbering in Chapter 5 is somewhat misleading. The values for frontage increase (If) are determined per Equations 5-4 and 5-5 and then used in Equations 5-1, 5-2, and 5-3.
We will address the use of Equations 5-5 and 5-4 in reverse order and then go on to describe how to use them in calculating area modifications.
- Every building must adjoin or have access to a public way in order to receive area increases. When a building has more than 25% of its perimeter opening onto a public way or open space at least 20' (6096) wide, the building's area may be increased based on the added width of the public way(s) and/or open space(s) and the extent of the building perimeter surrounded by them.
Area Modifications
§ 506.2 contains three formulas for calculating area modifications. The formulas make use of the frontage increase allowable per Equations 5-4 or 5-5. It is very important to keep in mind that while part of the area determination is based on whether the building is to have sprinklers, the “NS” value is to be used as well, regardless of whether the building is sprinklered. Be sure that for sprinklered buildings you do not duplicate the tabular values for sprinklered buildings; instead, use the NS values in the appropriate location in the equations.
The first formula is Equation 5-1 from § 506.2.1, which is for single-occupancy, one-story buildings. The building is to have no more than a single story above the grade plane. The basic tabular area is taken from Table 506.2 and is based on the construction type and sprinkler status of the building in question.
- Equation 5-1:
Aa = At + (NS × If)
where:
Aa = Allowable area (square feet).
At = Tabular allowable area factor (NS, S1, or S13R value, as applicable) in accordance with Table 506.2.
NS = Tabular allowable area factor in accordance with Table 506.2 for nonsprinklered buildings (regardless of whether the building is sprinklered).
If = Area factor increase due to frontage (percent) as calculated in accordance with § 506.3. [Equations 5-4 and 5-5]
- Frontage Increase per § 506.3 (with Equation 5-5)
If = [F/P – 0.25] W/30
= [(50' + 50')/(50' @ all 4 sides) – 0.25] 50/30*
= [100/200 – 0.25] 1*
= [0.5 – 0.25] 1*
= [0.25] = 25%
*W/30 cannot exceed 1.0 per § 506.3.2 with one exception: W/30 may be up to 2 when the building meets all criteria of § 507 except for compliance with the 60' (18 288) public way or yard requirement.
- Per Equation 5-1:
Aa = At + (NS × If)
Aa = 114,000 sf [10,591 m2] + ( 28,500sf [2648 m2] × 0.25)
= 121,125 sf [11,253 m2]
The second formula is Equation 5-2, which is used for multistory buildings with a single occupancy group. This takes the allowable areas for single-story buildings and adds a factor for stories greater than one.
For this example, we assume a 3-story, B occupancy, Type IIIA building with sprinklers and the same frontage increase of 25% from the previous example.
- Equation 5-2:
Aa = At + (NS × If) × Sa
where:
Aa = Allowable area (square feet).
At = Tabular allowable area factor (NS, S1, or S13R value, as applicable) in accordance with Table 506.2.
NS = Tabular allowable area factor in accordance with Table 506.2 for nonsprinklered buildings (regardless of whether the building is sprinklered).
If = Area factor increase due to frontage (percent) as calculated in accordance with § 506.3. [Equations 5-4 and 5-5]
Sa = Actual number of stories above grade plane, not to exceed 3 for nonsprinklered buildings and not to exceed 4 for sprinklered Group R buildings with a 13R sprinkler system.
- Frontage Increase per § 506.3 (with Equation 5-5)
If = [F/P – 0.25] W/30
= [(50' + 50')/(50' @ all 4 sides) – 0.25] 50/30*
= [100/200 – 0.25] 1*
= [0.5 – 0.25] 1*
= [0.25] = 25%
- Per Equation 5-2:
Aa = At + (NS × If) × Sa
Aa = 85,500 sf [7943 m2] + (28,500sf [2648 m2] × 0.25)
= 92,625 sf [8605 m2] × 3
= 277,875 sf [25 814 m2]
Building Height and Area Calculation Procedure
The height and area calculation procedure as described in the supporting statement for the 2015 IBC, where the procedure was changed, is as follows:
Unlimited-Area Buildings
§ 507 allows buildings of other than Type I construction to be of unlimited area when certain conditions are met. These exceptions overlay the height, story, and area tables in Chapter 5 and are summarized in the table below.
Unlimited-Area Buildings
Mixed Use and Occupancy
§ 508 Mixed Use and Occupancy
This section addresses three concepts that involve having more than one occupancy in a single building. They are:
- Accessory Occupancies (§ 508.2)
- Nonseparated Occupancies (§ 508.3)
- Separated Occupancies (§ 508.4)
Per exceptions to § 508.1, the following are not considered mixed occupancies for the purposes of applying § 508: occupancies separated per § 510; hazardous materials in separate buildings when required by Table 415.6.2; and the areas within live-work spaces complying with § 419.
Accessory Occupancies
When the occupancy contains a distinctly different ancillary use to the main occupancy of a building, and aggregate accessory occupancies do not occupy more than 10% of the floor area of the story in which they are located based on the tabular area allowed for various nonsprinklered occupancies by Table 506.2, the accessory use need not be separated from the primary occupancy. Exceptions exist for hazardous uses or when required as an “incidental use.”
Mixed Occupancies
When a building has a mix of occupancies that are each distinct or extensive enough to be considered as separate uses, each use is considered a separate and distinct occupancy. The mix of occupancies is addressed in several ways in addition to being treated as accessory occupancies:
- Nonseparated Occupancies
For nonseparated occupancies, the entire building is regulated according to the most restrictive of the height, area, and fire-protection requirements for each of the multiple occupancies under consideration.
- Separated Occupancies
Separated occupancies may require that an occupancy separation with a fire-resistance rating as defined by Table 508.4 be provided between the occupancies.
Allowable Building Height and Building Area for Nonseparated Occupancies with M and A-2 Occupancies
Example for a separated-occupancy height and area determination. Assume the same construction type and occupancies as on preceding page: Type V-A construction, unsprinklered, M-Occupancy (Retail/ 10,000 sf/929 m2), and A-2 Occupancy (Restaurant/ 3000 sf/279 m2), located on second floor.
| Criteria | A-2 Occupancy | M Occupancy | ||
| Allowable (Chapter 5) | Proposed | Allowable (Chapter 5) | Proposed | |
| Area Height (feet) | 50' (15 240) | <50' (15 240) | 50' (15 240) | <50' (15 240) |
| Area Height (stories) | 2 stories | On first floor | 3 stories | 3 stories |
| Floor Area/Story (sf/m2) | 11,500/1068 m2 | 3000/279 m2 | 14,000/1300 m2 | 10,000/929 m2 |
Calculation for the second floor per the example and § 508.4.2 :
Incidental Uses
Uses or occupancies that are incidental to the main occupancy are not considered to have enough impact to warrant their classification as an occupancy. The structure or portion thereof must be one of the uses listed in Table 509. These uses are typically considered to have a higher level of fire or safety risk than the occupancies within which they occur and Table 509 requires addtional protection or isolation from the areas within which they occur.
When the occupancy of a building contains one or more incidental uses as defined in Table 509, the areas are considered part of the main occupancy but require fire-resistance- rated separations from the rest of the occupancy according to the requirements listed in the table. Note that separation requirements for incidental accessory occupancies are not applicable to dwelling units.
Designation of incidental use areas is limited to those uses listed in Table 509. Note that incidental uses do not change the occupancy group of the area where they occur. That is why they are called “incidental.”
The layout of the code places Table 509 after the text for that section. Do not confuse Table 508.4 regarding occupancy separations with the incidental-use Table 509.
As examples we have highlighted the incidental uses most likely to be encountered in building design:
Special Provisions
§ 510 is devoted to exceptions to the provisions of Tables 504.3, 504.4, and 506.2, and the other sections in Chapter 5. These exceptions apply only when all of the conditions in the subsections are met. These conditions are based on specific combinations of occupancy groups and construction types. These special provisions were code responses developed over time to meet construction conditions found in the jurisdictions of the model codes that preceded the IBC. These special provisions will usually give the designer greater flexibility in meeting the requirements of the building program than will § 503.1 alone. These conditions should be annotated in that table for reference by the designer to be certain that they are not overlooked when commencing design and code analysis.
Per § 510.2, parts of buildings shall be considered as separate and distinct buildings for the purpose of determining area limitations, continuity of fire walls, limitations on the number of stories, and type of construction when all of the following conditions are met:
Group S-2 Enclosed Parking Garage with S-2 Open Parking Garage Above
All parking garages for the storage of private motor vehicles are classified as S-2 occupancies; thus, they need not be separated. § 510.3 uses the same principle as that for mixed-use buildings in § 510.2. The criteria are different but § 510.3 basically allows an open garage to be built over an enclosed garage while treating the transition between them as a new ground plane when fire-resistance requirements are met.
Special Provisions for Groups R-1 and R-2
§ 510.5 and § 510.6 were written to address hotel and apartment construction issues. Where fire-resistance ratings are met, fire separations are provided, and/or buildings are separated by required distances, these sections allow greater design flexibility than noted in Tables 504.3 and 504.4.
Open Parking Garage Beneath Groups A, I, B, M, and R
§ 510.7 allows constructing the use groups as indicated above an open parking garage and treating them as separate buildings for allowable height and area purposes. The height and area of the open garage are regulated by § 406.5, and the heights and areas of the groups above the garage are regulated by § 503.1. There is a restriction that heights in feet and stories for the part of the building above the garage be measured from the grade plane for the entire group of stacked uses. This section also requires fire resistance to be provided for the most restrictive of the uses. It also requires egress from the uses above the garage be separated by fire-resistance-rated assemblies of at least two hours.
Group S-2 Open Parking Garage Above Group B or M
Per § 510.8, where Group B or M occupancies are located on the first story above grade plane, they may be considered as separate and distinct buildings for the purpose of determining the allowable construction types when there is an open parking garage above these occupancies and all of the noted conditions are met.
Note that while Building A and Building B may be considered as separate buildings, Item 4 of § 510.8 says the building below the horizontal assembly is to be of Type I or Type II construction, “but not less than the type of construction required for the Group S-2 parking garage above.” How this is to be applied should be confirmed with the AHJ.
