IRC 2024 Beam and Girder Sizing: Span Tables for Built-Up and LVL Beams

Quick Answer

IRC 2024 Section R502.5 governs girder and beam sizing for floor framing, referencing span tables in the code and the NDS for Wood Construction. For typical residential floor beams, Table R602.7 (also applicable to floor beam sizing) and the prescriptive built-up lumber tables provide maximum spans for doubled and tripled 2x lumber beams. A common doubled 2x10 Southern Yellow Pine beam can span roughly 7–9 feet under typical tributary loads — but the exact span depends on the tributary width and live load it carries.

Under IRC 2024, LVL beams are governed by manufacturer ICC-listed tables, not the IRC tables, and consistently outperform built-up lumber for longer spans. IRC 2024 reinforces the requirement that LVL documentation be on site at rough inspection — not just referenced on the plan, but physically present at the job site. This documentation requirement reflects the fact that LVL products differ significantly between manufacturers: a 3.5-inch by 11.25-inch LVL from one manufacturer may have a different allowable span than a visually identical product from another, because the species composition, grade of veneers, and adhesive system vary. The inspector cannot verify compliance without the specific product’s ICC Evaluation Report and the manufacturer’s span table for the installed size and loading condition.

What IRC 2024 Actually Requires

Section R502.5 requires that girders and beams supporting floor loads be sized in accordance with Tables R502.5(1) and R502.5(2). These tables organize beam spans by:

• Number and size of plies (e.g., two 2x8, three 2x10, three 2x12)
• Lumber species and grade
• Tributary width (the width of floor the beam supports — typically half the span on each side)
• Building width and story condition (one story, first floor of two stories, etc.)

Beyond span limits, the code sets minimum bearing lengths:

• Wood post bearing: Minimum 3.5 inches (the full width of a 4x post) of bearing length for the beam on a wood post or plate
• Steel post or column: Minimum 3 inches of bearing on steel
• Masonry or concrete: Minimum 3 inches of bearing on masonry

Section R502.5 also requires that built-up beams be constructed with nails of sufficient size and spacing to make the plies act compositely. IRC Table R602.3(1) specifies the nailing for built-up headers and beams: typically 20d nails at 32 inches on center along each ply edge, staggered top and bottom.

Why This Rule Exists

A floor beam carries the tributary loads from the floor joists framing into it and delivers those loads to the posts and foundation below. Unlike a joist, which carries load along its length and delivers it at each end, a beam collects concentrated loads from multiple joists and must have sufficient bending strength and stiffness to do so without excessive deflection. An undersized beam deflects at mid-span, causing the floor joists framing into it to slope toward the center, which creates a visually apparent sag or dip in the finished floor — one of the most common and most expensive defects in residential construction.

The bearing length requirements exist because wood and masonry crush under concentrated bearing stress if the contact area is too small. A beam delivering 15,000 pounds to a post over a contact area of 1 square inch will crush the wood fibers at the bearing point, causing gradual settlement. Minimum bearing lengths distribute the reaction force over enough area to keep the bearing stress within allowable limits.

What the Inspector Checks at Rough and Final

Beam and girder inspection is a critical part of the rough framing review. Inspectors verify:

• Beam size matches permitted plans — ply count, depth, and species
• Bearing length at posts meets the 3.5-inch minimum for wood posts
• Built-up beams nailed with appropriate fasteners at correct spacing — plies must not be separated by gaps
• Post-to-beam connection hardware present (post cap or post base per the nailing schedule)
• No notches in the beam tension zone (bottom third of depth)
• Beam splice locations, if any, located directly over a support (never at mid-span)
• LVL beams accompanied by manufacturer documentation showing the installed size meets the span and load condition

At final inspection, the inspector may observe finished floors for visible sag or bounce that indicates beam deflection beyond the L/360 limit.

Inspectors who specialize in structural review develop a practiced eye for beam deflection: they will sight down the beam from one end, looking for a visible curve at mid-span. A beam that has deflected beyond L/360 under construction loads (the weight of the floor framing, subfloor, and any stored materials) will be visibly bowed even before finish floor loads are applied. This sighting technique is fast and reliable for obvious deflection, though marginal cases require measurement. If the inspector requests a measurement, it is typically done with a straight string line stretched from bearing to bearing, and the gap between the string and the bottom of the beam is measured at mid-span. A 15-foot beam showing more than 0.5 inches of mid-span deflection under construction loads is a concern that warrants further investigation, even if the framing plan shows a theoretically adequate size.

What Contractors Need to Know

The choice between a flush beam (same depth as the joists, joists frame into it with hangers) and a dropped beam (joists sit on top of the beam, beam drops below the floor plane) has significant implications for floor system depth, ceiling height below, and connection hardware costs.

Flush beams allow a flat floor system and are structurally efficient, but require joist hangers at every joist connection — typically a significant hardware cost on a long beam. Dropped beams allow joists to simply rest on top and be toenailed, which is faster and requires no hangers, but the beam drops into the space below, reducing basement ceiling height and requiring a bulkhead if the ceiling is finished.

LVL beams are the dominant choice for spans exceeding about 12 feet in modern residential construction. LVL provides more consistent strength than built-up lumber (no knots or splits within the ply), and a single LVL member is far easier to handle than a tripled or quadrupled built-up beam. The tradeoff is cost — LVL runs roughly 3–5 times the cost of built-up dimensional lumber per board foot — and the requirement for manufacturer documentation on site. The inspector will ask for the ICC Evaluation Report and the manufacturer’s span table at rough inspection.

Beam splices must always be located directly over a support. A built-up beam that has a butt splice at mid-span carries no tensile force across the splice — the two halves act as independent shorter beams, each spanning only half the designed distance. This is a catastrophic design error that will fail inspection and, if undiscovered, will eventually cause excessive mid-span deflection.

What Homeowners Get Wrong

Homeowners who remove a bearing wall as part of a kitchen or living room open-plan remodel almost always underestimate the size of beam required to replace the wall’s function. A bearing wall distributes its load continuously over its entire length; a beam must carry the same total load between two point supports. As span doubles, the required beam depth increases dramatically — not linearly. A wall that was replaced by a beam that “looked right” to an unlicensed contractor frequently creates a beam that is 30–50% undersized, causing progressive sag that cracks finishes and eventually deflects visibly.

The second most common homeowner misconception is that a beam sitting on a post is “strong enough” without hardware. Post-cap connectors and post-base connectors are required because gravity alone does not prevent the beam from sliding off the post laterally under seismic or wind loading. The connector provides the lateral and uplift resistance that simple bearing cannot.

State and Local Amendments

California’s high-seismic zone provisions require post-to-beam connections with specific shear and uplift ratings — standard post caps may not meet the required force levels in SDC D. The CRC requires holdown hardware at posts supporting beams in certain seismic conditions, which must be specified by an engineer. Florida’s wind provisions require uplift-rated post-cap connectors throughout, with minimum rated uplift capacities specified in the wind design documents. Pacific Northwest jurisdictions often require engineers of record to stamp beam designs for any beam spanning more than 16 feet, regardless of what the IRC tables would permit.

North Carolina and South Carolina have adopted high-wind zone requirements that affect beam-to-post connections throughout coastal counties, requiring connectors with specific minimum uplift and shear ratings that often exceed what a standard post cap provides. In these zones, the structural drawings must show the connector model number and its rated capacity, not just a generic reference to “post cap hardware.” Inspectors in coastal North Carolina specifically look for uplift-rated connectors at all floor beam-to-post junctions as part of the wind-resistant construction requirements, and will cite installations that use only face-nailed post caps without uplift ratings. Contractors working in coastal markets should maintain familiarity with the Simpson Strong-Tie and USP Structural Connectors catalogs for uplift-rated hardware — the difference in cost between a standard post cap and an uplift-rated cap is negligible, but the inspection consequence of the wrong cap can be significant.

When to Hire a Professional

Hire a licensed structural engineer for beam design when: (1) the span exceeds what the IRC prescriptive tables cover (typically beyond 20 feet); (2) the beam supports a concentrated point load rather than uniformly distributed floor load; (3) the tributary width is wider than the table ranges; (4) an existing beam is being evaluated for adequacy after a change of use or load increase; (5) the beam is part of a moment-frame or other lateral-load-resisting system; or (6) the project is a bearing wall removal — the engineer must verify the entire load path from the beam to the foundation, not just the beam size. Always pull a permit for bearing wall removals; unpermitted structural work is a disclosure obligation and a liability in any future real estate transaction.

Common Violations Found at Inspection

• Beam size smaller than required by the span table for the actual tributary width and span
• Built-up beam plies not nailed per code schedule — plies held together with construction adhesive only
• Bearing length less than 3.5 inches on wood post — beam teetering on the corner of a post
• No post cap or post base hardware — beam sitting on post without lateral connection
• LVL beam installed without documentation — inspector cannot verify compliance
• Beam splice located at mid-span rather than directly over a support
• Notch in the bottom of the beam at mid-span (tension zone) — severely reduces bending capacity
• Species or grade lighter than assumed in the design — SPF installed where SYP was specified
• Post not centered under beam — bearing eccentricity creating torsion in the post