Picture two identical event barns, one on each side of the Iowa–Minnesota line. The Iowa State Building Code adopted the 2024 International Building Code (IBC), effective September 10, 2025 (Iowa Administrative Code 481-301.3). Minnesota Rules 1305.0011 still adopts the 2018 IBC; the Minnesota Department of Labor and Industry asked for comments on a move to the 2024 edition in January 2025, but its rule docket lists no effective date yet. The wood is the same. The rulebook is not.
This guide walks through where IBC Section 2304.11 and its size table fit, what the section does not cover, and which other chapters a commercial timber frame must clear before it gets a permit. Section numbers below are from the 2024 IBC unless noted. Your city or county may amend them, so check with the building official (the authority having jurisdiction, or AHJ) early.
Words that change your permit path
Timber frame is the traditional post-and-beam craft: pegged mortises and visible braces. It describes how the structure looks and goes together. It is not a code category.
Heavy timber is a code label tied to minimum sizes and detailing. Under Table 2304.11, a column that supports floor loads must be at least 8 × 8 inches nominal in solid sawn lumber to count as heavy timber.
Mass timber covers both heavy timber members and engineered products such as glued-laminated timber (glulam), cross-laminated timber (CLT) and structural composite lumber (SCL). Table 2304.11 gives separate minimum sizes for sawn, glulam and SCL members.
Construction type classifies the whole building. Labels such as Type IV-HT, IV-B or III-A control fire-resistance ratings, height and area. Until the AHJ accepts that label, every height limit and fire rating in your plans is still an estimate.
If you want to see how exposed framing works in hospitality and other commercial projects, read Hamill Creek Timber Homes’ explainer on commercial timber frame construction, then come back for the code math.
When Section 2304.11 is triggered
Section 2304.11 applies “where a structure, portion thereof or individual structural elements are required by provisions of this code to be of heavy timber.” Common triggers in the 2024 IBC:
- Type IV-HT buildings (Section 602.4.4). Interior building elements are solid wood, laminated heavy timber or SCL, and their minimum sizes and details must comply with Section 2304.11.
- Heavy timber roofs in other construction types (Table 601, note c). In all occupancies, heavy timber complying with Section 2304.11 may be used for roof construction, including the primary structural frame, where a 1-hour or less rating is required.
- Projections and balconies near a lot line (Sections 705.2.3 and 705.2.3.1). Heavy timber complying with Section 2304.11 is one of the permitted options.
- Motor-fuel canopies (Section 406.7.2). Canopies and their supports over pumps may be noncombustible, fire-retardant-treated wood, “heavy timber complying with Section 2304.11” or 1-hour construction.
Once a trigger applies, the affected columns, beams and decks must meet the table sizes and the detailing rules in Sections 2304.11.1 through 2304.11.4.
Minimum member dimensions
Selected rows from Table 2304.11 (2024 IBC):
| Supporting | Member | Solid sawn, nominal (in.) | Glulam, net (in.) | SCL, net (in.) |
| Floor loads only, or floor and roof | Columns | 8 × 8 | 6 ¾ × 8 ¼ | 7 × 7 ½ |
| Floor loads only, or floor and roof | Wood beams and girders | 6 × 10 | 5 × 10 ½ | 5 ¼ × 9 ½ |
| Roof loads only | Columns (roof and ceiling loads) | 6 × 8 | 5 × 8 ¼ | 5 ¼ × 7 ½ |
| Roof loads only | Framed timber trusses and other roof framing | 4 × 6 | 3 × 6 ⅞ | 3 ½ × 5 ½ |
(1 in. = 25.4 mm. Source: 2024 IBC Table 2304.11.)
Two notes from the table matter in practice. Solid sawn sizes are nominal, while glulam and SCL sizes are net, so read the column that matches your product. Footnote a lets spaced roof members be built from two or more pieces at least 3 inches nominal thick, if they are blocked solidly or closed by a continuous wood cover plate at least 2 inches nominal thick. Footnote b lets the 4-inch nominal width of roof framing drop to 3 inches where approved automatic sprinklers protect the area under the roof deck.
Structural detailing the table assumes
Section 2304.11.1 adds detailing rules to the sizes:
- Columns (Section 2304.11.1.1). Columns must be connected in an approved manner and, in Type IV-HT construction, be “continuous or aligned vertically from floor to floor in all stories.”
- Beam-to-column joints (Section 2304.11.1.1). Girders and beams at column connections must be “closely fitted around columns,” and adjoining ends must be cross tied, or intertied by caps or ties, to transfer horizontal loads across joints. Wood bolsters may sit on top of columns only where the columns support roof loads only.
- Floor framing (Section 2304.11.1.2). Intermediate beams that support a floor must rest on top of girders or sit in an approved metal hanger. Where traditional heavy timber detailing is used, ledgers or blocks fastened to the sides of the girders are permitted.
- Roof framing (Section 2304.11.1.3). Every roof girder, and not less than every alternate roof beam, must be anchored to its support to resist the forces required by Chapter 16.
Traditional joinery fits inside these rules when the engineer details it that way. Hamill Creek Timber Homes, which has designed and built commercial timber frames since 1989, joins large Douglas fir beams with mortise and tenon joints secured by oak pegs. Each frame is designed in 3D CAD, cut on CNC machinery at its Meadow Creek, BC facility and test-fit on the shop floor before it ships. Shop drawings at that level of detail make it easier for the plan reviewer to check each beam-to-column connection against Section 2304.11.1.1.
Walls, floors and roof decks
Sections 2304.11.2 through 2304.11.4 cover the flat planes that tie the frame together:
- Exterior walls (Section 2304.11.2.1). CLT at least 4 inches thick is permitted.
- Interior walls and partitions (Section 2304.11.2.2). Solid wood made of at least two layers of 1-inch matched boards, laminated construction 4 inches thick, or 1-hour fire-resistance-rated construction.
- Floors (Section 2304.11.3). Floors must have no concealed spaces, or concealed spaces that comply with Section 602.4.4.3. A CLT floor must be at least 4 inches thick. A plank floor must be splined or tongue-and-groove planks at least 3 inches nominal thick, covered with 1-inch nominal flooring, 15/32-inch wood structural panel or 1/2-inch particleboard (or planks at least 4 inches nominal wide set on edge, with a similar cover). Continuous lines of joints may occur only at points of support, and floors must stop 1/2 inch short of walls, with the gap covered by a molding.
- Roof decks (Section 2304.11.4). CLT at least 3 inches thick, splined or tongue-and-groove plank at least 2 inches nominal thick, or 1 1/8-inch wood structural panel with exterior glue, among the listed options. Roof decks supported by a wall must be anchored to it.
What 2304.11 does not answer
Table 2304.11 tells you how big the wood must be. It does not settle the building’s overall fire and life-safety plan. For that, open other chapters:
- Occupancy: Chapter 3.
- Height, stories and area: Chapter 5.
- Construction type and fire-resistance ratings: Chapter 6 and Table 601.
- Fire and smoke protection features: Chapter 7.
- Sprinklers and fire alarms: Chapter 9.
- Design loads: Chapter 16.
- Special inspections: Chapter 17.
Treat 2304.11 as a member-sizing and detailing section, not a permit shortcut.
Start with occupancy: Chapter 3
Chapter 3 classifies the building by use. A restaurant or bar is Group A-2, which covers “assembly uses intended for food and/or drink consumption” (Section 303.3). A building or tenant space used for assembly with an occupant load of less than 50 is classified as Group B (Section 303.1.1). Every later chapter looks back to this label, so confirm it with the AHJ before you size a beam or price a fire barrier.
Height, stories and area: Chapter 5
Once you set occupancy and construction type, Tables 504.3, 504.4 and 506.2 tell you how tall and how large the building may be. For a Group B office in the 2024 IBC:
| Construction type | Height, not sprinklered / sprinklered (Table 504.3) | Stories, not sprinklered / sprinklered (Table 504.4) | Area factor per story, not sprinklered (Table 506.2) |
| IV-HT | 65 ft / 85 ft | 5 / 6 | 36,000 ft² |
| IV-C | 65 ft / 85 ft | 5 / 9 | 45,000 ft² |
| IV-B | 65 ft / 180 ft | 5 / 12 | 72,000 ft² |
| IV-A | 65 ft / 270 ft | 5 / 18 | 108,000 ft² |
Sprinklers change the area factor too. For a one-story Group B building of Type IV-HT, the sprinklered factor (S1) in Table 506.2 is 144,000 ft², four times the 36,000 ft² non-sprinklered value.
A frontage increase can add more. Under Sections 506.3.1 and 506.3.2, at least 25 percent of the perimeter must face a public way or open space at least 20 feet wide. Table 506.3.3 then sets a factor from 0.17 up to 0.75, so the increase is up to 75 percent of the non-sprinklered area factor.
Set these numbers early. A switch from IV-HT to IV-B after schematic design brings fire-resistance ratings and noncombustible protection that can change your budget.
Construction type: Chapter 6
Chapter 6 assigns one construction type to the building, and Table 601 sets the fire-resistance ratings that follow from it. The four Type IV options in the 2024 IBC:
| Type | Primary structural frame (Table 601) | Floor construction (Table 601) | Exposed mass timber inside |
| IV-HT | HT (heavy timber sizes) | HT | Permitted; members meet Section 2304.11 |
| IV-C | 2 hours | 2 hours | Permitted to be unprotected (Section 602.4.3.2); shafts and concealed spaces need 40-minute noncombustible protection |
| IV-B | 2 hours | 2 hours | Limited: ceilings up to 100 percent and walls up to 40 percent of the floor area of a fire area or dwelling unit; columns and beams not part of a wall or ceiling without limit (Section 602.4.2.2.2) |
| IV-A | 3 hours | 2 hours | None: all interior faces of mass timber are protected (Section 602.4.1.2) |
Types IV-A, IV-B and IV-C first appeared in the 2021 IBC, built on the older heavy timber type, which was renamed Type IV-HT. The 2018 IBC, still the base of Minnesota’s code, does not include them. That is the real gap between the two barns at the state line.
Choose the type during schematic design and confirm local amendments with the AHJ. A late switch can rewrite every fire-rating detail.
Fire and smoke protection: Chapter 7
Heavy timber sizes do not by themselves give an hourly rating. Chapter 7 applies the same rating rules to wood that it applies to steel or concrete:
- How a rating is proved (Section 703.2). A rating comes from an ASTM E119 or UL 263 test (Section 703.2.1), an analytical method (Section 703.2.2), or the other methods the section lists.
- Supporting construction (Section 704.1.1). Members that support a rated assembly must have at least the same rating as the assembly they support.
- Shafts (Section 713.4). Shaft enclosures need at least 2 hours where they connect four stories or more, and at least 1 hour where they connect fewer than four stories.
- Penetrations (Section 714). A through penetration of a rated wall must be protected by the tested assembly or by an approved firestop system tested to ASTM E814 or UL 1479, with an F rating at least equal to the wall’s rating (Section 714.4.1.2). If you need a fire barrier near the frame, our earlier piece on fire doors covers the openings side.
- Joints and voids (Section 715). Joints and voids in or between rated assemblies need a tested joint system.
- Draftstops (Sections 718.3 and 718.4). Outside Group R, combustible floor/ceiling assemblies must be subdivided so no area exceeds 1,000 square feet, and combustible attics and concealed roof spaces so no area exceeds 3,000 square feet. Buildings sprinklered throughout under Section 903.3.1.1 are exempt.
Calculated fire resistance for exposed wood
When an exposed timber member needs a 1- or 2-hour rating, Section 722.1 allows the rating to be calculated for “exposed wood members and wood decking … in accordance with Chapter 16 of ANSI/AWC NDS.” WoodWorks notes that the NDS method uses a nominal char rate of 1.5 inches per hour and can calculate ratings of up to 2 hours.
For sawn lumber, glulam and SCL with that char rate, American Wood Council Technical Report 10 (Table 4.1.1.4A) lists an effective char depth of 1.8 inches for a 1-hour rating, 2.5 inches for 1½ hours and 3.2 inches for 2 hours. The effective depth is 1.2 times the char depth, which allows for a heated zone of weakened wood behind the char.
Worked example for a 6 ¾ × 10 ½ in. glulam beam that needs a 1-hour rating, exposed on three sides (top covered by the floor):
- Residual depth: 10.5 − 1.8 = 8.7 in.
- Residual width: 6.75 − 2 × 1.8 = 3.15 in.
The engineer then checks the residual section against the design loads. If it fails, the usual fixes are a larger member or added protection.
Connections need the same attention. AWC TR10 (Section 4.5) states that wood connections, “including connectors, fasteners and members, shall be protected from fire exposure for the required fire resistance time,” using wood, fire-rated gypsum board or other approved materials.
Sprinklers and alarms: Chapter 9
- Group A-2 sprinklers (Section 903.2.1.2). A sprinkler system is required where the A-2 fire area exceeds 5,000 square feet, has an occupant load of 100 or more, or sits on a floor other than a level of exit discharge.
- Group A fire alarms (Section 907.2.1). A manual fire alarm system is required where the assembly occupant load is 300 or more, or where more than 100 Group A occupants are above or below the lowest level of exit discharge. Section 907.2.1.1 adds requirements for Group A occupant loads of 1,000 or more.
Special inspections: Chapter 17
A statement of special inspections must be submitted as a condition of the permit (Section 1704.2.3). For mass timber in Types IV-A, IV-B and IV-C, Table 1705.5.3 lists the required inspections, including periodic inspection of anchorage and connections to timber deep foundations, of mass timber erection, and of threaded fasteners (installation equipment, pre-drilled holes and screw diameter, length, spacing, angle and depth), bolted connections and concealed connections. Adhesive anchors installed horizontally or upward to resist sustained tension need continuous inspection.
Conclusion
IBC Section 2304.11 sets the minimum sizes and detailing for heavy timber, but the permit path for a commercial timber frame building also runs through occupancy, height and area, construction type, fire ratings, sprinklers and special inspections. Check which IBC edition your state and city enforce, settle the construction type with the building official early, and prove any hourly rating by test or by the NDS calculation. Put the connection and penetration details in the permit set so the plan reviewer and inspector see them before the frame goes up.