Building area and height

Joon-ho asks the question everyone has been waiting to ask. 'How big can a building be?' Kostyshyn smiles. 'That depends on what it is used for, what it is built of, whether it has sprinklers, how many sides a fire truck can reach, and how you measure it. Which is why the measuring comes first.' She draws a section through a building on a slope, with a basement half out of the ground at one end and fully buried at the other, a mezzanine inside, and a rooftop mechanical penthouse. 'How many stories is this?' Each student gives a different answer. 'You are all thinking like architects. The code thinks like a lawyer. It has its own definitions, and they are what count.'

Why size is limited

Codes limit the size of buildings because size is a measure of how many people a fire can reach and how hard the fire will be to fight. A tall building takes longer to evacuate and puts firefighters further from the street. A building with a large area lets a fire grow larger before it meets a wall, and leaves firefighters with longer distances to cover inside. The code's limits are a trade between size and protection. The more a building does to keep a fire small and slow, by sprinklers, by noncombustible construction, by fire-rated floors and walls and by access for firefighters, the larger it may be. A one-story wood building without sprinklers may be quite small; a sprinklered concrete building may be unlimited.

The limits in the International Building Code are set in Chapter 5 by three tables. Table 504.3 gives the allowable height in feet above grade plane, Table 504.4 the allowable number of stories above grade plane, and Table 506.2 the allowable area per story. Each table is organized by occupancy group and by type of construction, and each gives different values for buildings without sprinklers, with an NFPA 13 sprinkler system, and for residential buildings with an NFPA 13R system. A designer finds the allowable size by reading across for the building's occupancy and down for its construction type, then adding the increase for frontage where it applies.

Measuring a building

The International Building Code defines its terms carefully. Building area is the area included within the surrounding exterior walls, or the exterior walls and fire walls, of each story; allowable area is checked story by story. Building height is measured in feet from grade plane to the average height of the highest roof surface, and in stories above grade plane. Grade plane is a reference plane at the average of the finished ground level adjoining the building at its exterior walls. A story above grade plane is any story whose floor is entirely above grade plane, or whose floor is more than 6 feet above grade plane, or more than 12 feet above the finished ground at any point. A basement may therefore count as a story on a sloping site. A mezzanine of not more than one third of the floor area of the room or space in which it is located is not counted as a story, provided it meets the code's other conditions.

Rooftop structures add another trap. A penthouse is not counted as a story if it is limited in area and height and encloses only mechanical equipment, stairs, elevators or similar services, as set out in the code's section on rooftop structures; a penthouse with any other use counts as a story. Callum's library monitors on the hub, which bring north light down into the reading room, are part of the roof, not stories. A rooftop terrace with a small glass pavilion for parties, which Ingrid drew on Greenway Homes, is a different matter and needs checking.

Sprinklers and streets

Two things make the biggest difference to how large a building may be. The first is sprinklers. Sprinklers keep most fires small, and both codes allow much larger, and often taller, buildings when they are installed throughout. The second is access for firefighters. A building that can be reached from several sides lets firefighters attack a fire from more than one direction and protect the buildings next to it.

The International Building Code adds a frontage increase to the allowable area when more than a quarter of the building's perimeter fronts on a public way or open space at least 20 feet wide, with access for the fire department. The increase factor is the share of the perimeter that has frontage, less one quarter, multiplied by the width of the open space divided by 30, using a width of no more than 30 feet. The allowable area per story is then the tabular area plus the non-sprinklered tabular area multiplied by the increase factor.

Table 506.2 gives four values for many occupancies: NS for buildings without sprinklers, S1 for one-story buildings with an NFPA 13 system, SM for buildings of more than one story with an NFPA 13 system, and S13R for residential buildings with an NFPA 13R system, which gets no area increase over NS. For a building of more than one story, the allowable area per story is checked for each story, and the total area of all stories is also limited, generally to the allowable area per story times the number of stories above grade plane, but not more than three, for sprinklered buildings.

Checking Greenway Homes

The students check Greenway Homes first. Its four-story U-shaped block has a north bar and an east bar joined at the corner core, and a row of stacked townhouses on the west. Wren decides to separate the townhouses from the rest with a fire wall, so that each part is treated as a separate building for area. The script checks the main block.

# Greenway Homes main block: height, stories and area per story (R-2)
grade_plane = 97.0       # average finished ground at exterior walls (ft, site datum)
floors = [97.7, 108.0, 118.3, 128.6]        # floor elevations (ft)
roof = 138.9
stories = sum(1 for f in floors if f >= grade_plane or f - grade_plane > 6)
height = roof - grade_plane
print(f"stories above grade plane {stories}, height {height:.1f} ft")

parts = {"north bar": (147, 36), "east bar": (92, 56)}
area = sum(w * d for w, d in parts.values())
P = 2 * (147 + 36) + 2 * (92 + 56) - 2 * 36      # joined at the corner
F, W = 0.62 * P, 30                                # frontage on the greenway, streets and court
If = max(0.0, (F / P - 0.25) * W / 30)
for name, NS, At, smax, hmax in [("Type V-A, NFPA 13R", 12000, 12000, 4, 60),
                                 ("Type V-A, NFPA 13", 12000, 36000, 4, 70)]:
    Aa = At + NS * If
    ok = area <= Aa and stories <= smax and height <= hmax
    print(f"{name}: allowable {Aa:,.0f} ft2 per story vs {area:,} ft2; stories {stories}/{smax}; "
          f"height {height:.0f}/{hmax} ft -> {'OK' if ok else 'not OK'}")

stories above grade plane 4, height 41.9 ft
Type V-A, NFPA 13R: allowable 16,440 ft2 per story vs 10,444 ft2; stories 4/4; height 42/60 ft -> OK
Type V-A, NFPA 13: allowable 40,440 ft2 per story vs 10,444 ft2; stories 4/4; height 42/70 ft -> OK

The main block passes both ways. With an NFPA 13R system, which costs less and is common in apartment buildings of up to four stories, the allowable area per story is about 16,400 square feet once the frontage increase is added, and the building uses about 64% of it. With a full NFPA 13 system the allowable area rises to about 40,400 square feet. Joon-ho notices how much hangs on the grade plane. If the site were lower along the south side, so that the ground floor sat more than 6 feet above grade plane, the bike room and parking level would count as a story and the building would be five stories, beyond what Type V-A allows.

Checking the community hub

The hub contains A-3 and E occupancies, two stories high and sprinklered with an NFPA 13 system, with a footprint of about 16,500 square feet. In Type V-A construction, Group A-3 is allowed 34,500 square feet per story and three stories, and Group E 55,500 square feet and two stories. Because Callum separates the daycare, each part is checked on its own and the ratios of actual to allowable area are added on each story; the sum on the ground floor is about 0.43, well under the limit of one. If the daycare had been on the second floor, the E occupancy would still be allowed two stories, but the daycare would have become Group I-4, with its own, much stricter, limits. Kostyshyn notes that the library monitors, which rise to about 36 feet, do not add a story, and the height is far below the 70 feet allowed.

Callum asks why it matters to leave spare capacity. Kostyshyn's answer is practical. Designs grow. A program that starts at 2,400 square meters often ends at 2,700 after the client adds rooms and the engineers add shafts. A building that is right at the limit of its article or table at schematic design will be over it by design development, and changing the construction type then is very expensive. Experienced architects check the limits early, choose a classification and construction with room to grow, and check again at every stage.

Firewalls and separate buildings

A fire wall is a fire-resistance-rated wall with structural stability that allows collapse of construction on either side without collapse of the wall, and that divides a building into parts treated as separate buildings for allowable height and area. It must have the rating required by the code, typically two, three or four hours depending on the occupancy, must extend continuously from the foundation to or through the roof, and must have rated protection at every opening, with openings limited to a share of the wall's length. Fire walls are expensive, but they allow a large development to be built as a series of smaller buildings, each within the limits for its group and type.

Exercise 4: building area and height

• For your Design Studio IV project, determine the building height and area as the code defines them, with a section showing the reference level for grade.
• Find the table values that apply to your building and record how much of the allowable area and height you use.
• Show how your result would change without sprinklers.
• Redraw your site so that the building faces one more street or open space, and calculate the effect on the allowable area.

Self-check

• Have I measured building area and height using the code's definitions, not everyday ones?
• Have I checked mezzanines, penthouses and sloping ground?
• Have I left room for the design to grow?