IBC Wood Construction: NDS, Span Tables and Framing Requirements

TL;DR — Key Takeaways

• IBC Chapter 23 adopts AWC NDS (National Design Specification for Wood Construction) as the primary standard for engineered wood design.

• Two design approaches under IBC Chapter 23: engineered design (AWC NDS) and prescriptive conventional light-frame construction (IBC §2308).

• Prescriptive conventional framing (§2308) is limited to: 3 stories above grade plane, 10-foot floor-to-floor heights, and specific load limits — if the building exceeds these, full engineered design is required.

• Engineered wood products — LVL, PSL, LSL, I-joists, wood structural panels — all have their own ANSI/APA product standards and must be designed per the manufacturer's published design values.

• Wood in Types I and II construction is prohibited for structural elements; Types III, IV, and V construction allow wood framing subject to the fire resistance rating requirements of Table 601.

• Diaphragms and shear walls are the primary lateral-force-resisting elements in wood-frame buildings — designed per AWC SDPWS (Special Design Provisions for Wind and Seismic).

• Fire retardant treated wood (FRTW) may be required in specific locations per IBC §2303.2 — it must comply with AWPA C20/C27 and the fire retardant treatment must be approved for the moisture exposure condition.

• Seismic limitations in SDC D–F restrict the use of conventional light-frame construction and require engineered shear wall systems in many configurations.

Construction Types That Allow Wood Framing

IBC Chapter 6 determines whether wood framing is permitted based on construction type. Wood framing (combustible) is only permitted in:

• Type III: Exterior walls noncombustible; interior may be any material including wood

• Type IV (Mass Timber): As covered in the mass timber article — large-dimension solid or engineered wood members

• Type V: Exterior and interior walls, floors, and roof may all be combustible (wood) — only construction type with fully combustible framing

Types I and II (noncombustible construction) do not allow structural wood framing, though wood blocking, nailers, and trim are permitted with limitations.

The maximum height and area allowed for wood-frame buildings depends on construction type, occupancy, and sprinkler status per Tables 504.3 and 506.2.

IBC construction types and fire resistance

AWC NDS — Engineered Wood Design

IBC §2301.2 adopts AWC NDS (National Design Specification for Wood Construction) for engineered design. The current edition referenced by IBC 2024 is AWC NDS-2024.

NDS covers:

• Allowable stress design (ASD) for sawn lumber, glulam, LVL, and other products

• LRFD is also provided but less commonly used in wood

• Reference design values by species, grade, and product

• Adjustment factors (Cd, CM, Ct, CF, Ci, etc.) that modify reference values for service conditions

• Connection design — nails, screws, bolts, lag screws, split rings, and shear plates

Design values vary significantly by species and grade. Douglas fir-larch (Pacific Northwest) and southern yellow pine (Southeast) are the two most structurally efficient species groups and are widely available; hem-fir and spruce-pine-fir have lower published design values and are more common in commodity framing.

Span Tables and Prescriptive Framing (§2308)

IBC §2308 provides a prescriptive path for conventional light-frame wood construction — the standard wall stud + floor joist + rafter system used in residential and small commercial construction.

§2308 applicability limits:

• Maximum 3 stories above grade plane

• Maximum floor-to-floor height: 10 feet

• Roof live load: ≤ 25 psf

• Ground snow load: ≤ 50 psf

• Basic wind speed: ≤ 110 mph (Exposure B and C; lower limits for Exposure D)

• SDC: A through C (SDC D–F has additional limitations in §2308.12)

Span tables for prescriptive framing are provided by the American Wood Council (AWC) in the Span Calculator and in the Wood Frame Construction Manual (WFCM). Separate span tables cover:

• Floor joists by joist size, spacing, span, and design live/dead load

• Rafters by rafter size, spacing, slope, and snow load

• Headers over openings by tributary load and span

Within §2308 limits, these span tables allow framing without full structural engineering calculations.

Key §2308 Prescriptive Framing Requirements

Wall studs (§2308.5.3):

• 2×4 studs at 16 inches on center: maximum 10-foot height for exterior bearing walls

• 2×6 studs at 24 inches on center: maximum 10-foot height (energy-efficient framing in cold climates)

• Studs must be continuous from floor to floor; no notching or boring that exceeds AWC NDS limits

Foundation anchor bolts (§2308.3.1):

• Minimum 1/2-inch diameter anchor bolts embedded 7 inches minimum in concrete

• Maximum 6-foot spacing; maximum 12 inches from ends of sill plates

• In SDC D and higher: 5/8-inch minimum diameter, 7-inch embedment, increased placement requirements

Shear wall requirements (§2308.12):

Prescriptive braced wall panels must be provided along the full length of exterior walls and at interior braced wall lines. Panel types, lengths, and nailing requirements are specified in §2308.12.4 Tables.

Lateral Design: Diaphragms and Shear Walls (AWC SDPWS)

Wood-frame lateral design for wind and seismic is governed by AWC SDPWS (Special Design Provisions for Wind and Seismic), adopted by IBC §2301.2.

Wood structural panel (plywood or OSB) shear walls are the primary lateral-force-resisting elements. Design parameters:

• Panel grade and thickness

• Nail size and spacing (edge and field nailing)

• Hold-downs at wall ends to resist overturning

• Sill plate anchor bolt spacing

Allowable unit shear values for shear wall panels are tabulated in AWC SDPWS Table 4.3A (seismic) and 4.3B (wind). Values range from approximately 200 plf (3/8" OSB, 8d at 6") to over 1,500 plf (23/32" plywood, 10d at 2" — blocked diaphragm).

In SDC D–F, wood structural panel shear walls must meet additional requirements including:

• Maximum 2:1 height-to-width aspect ratio (reduced from 3.5:1 in lower SDC)

• Minimum panel thickness increased

• Nailing must be blocked (all edges of panels nailed to framing members)

• Hold-down hardware with tested allowable loads

Fire Retardant Treated Wood (FRTW)

IBC §2303.2 governs fire retardant treated wood (FRTW). Treatment modifies the wood to reduce flame spread (to Class A — flame spread index ≤25, smoke developed ≤450 per ASTM E84).

Where FRTW is required by IBC:

• Roof construction in Types I, II, III, and IV buildings where the roof deck is wood (§2303.2.2)

• Blocking and nailers within fire-resistance-rated assemblies

• Certain attic blocking requirements

Limitations of FRTW:

Fire retardant treatments reduce design values — typically 10–20% reduction in bending, shear, and compression parallel to grain. FRTW manufacturers publish adjusted design values per AWPA C20/C27 for their specific treatment type. The engineer must use these adjusted values, not the published base NDS design values, for FRTW members.

Research Wood Construction Requirements for Your Project

Wood framing requirements depend on construction type, SDC, jurisdiction, and whether the project qualifies for prescriptive §2308 design or requires full engineering per AWC NDS. Melt Code lets you search IBC Chapter 23, SDPWS requirements, and your jurisdiction's amendments together.

Search wood framing requirements for your project on Melt Code Try Melt Code →

Frequently Asked Questions

Q: Can wood-frame construction be used for a 4-story apartment building?

Yes, in some configurations. Type V-A construction (1-hour rating) allows up to 4 stories for Group R-2 residential in a sprinklered building per Table 504.3. Type III-A allows up to 5 stories. Some jurisdictions allow Type V-A "podium" construction — 1–2 stories of concrete (Type I) parking below 3–4 stories of wood frame — which is a very common affordable multi-family configuration.

Q: Is OSB structurally equivalent to plywood?

For shear wall applications, OSB (oriented strand board) and plywood of equivalent span ratings and thickness produce identical allowable shear values in AWC SDPWS Table 4.3A/B. OSB is more common (lower cost) for shear walls. Plywood retains a performance advantage in prolonged moisture exposure conditions.

Q: Does conventional light-frame (§2308) work in California (SDC D)?

With significant limitations. IBC §2308.12 allows conventional construction in SDC D under strict constraints on story height, braced wall panel lengths, and hold-down requirements. Many California projects that appear to be "conventional" are actually partially engineered because the building geometry, load, or configuration exceeds §2308 limits. The designer must carefully document which portions are conventional and which are engineered.

Q: When do I need an engineer for wood framing?

When any of the §2308 conditions are exceeded (height, spans, loads, SDC limits) — or when the building official requires it. Any building exceeding 3 stories, with spans exceeding the span table limits, or in SDC E/F requires full engineered design per AWC NDS.