Our Blog

Glulam vs LVL vs Sawn Lumber: Choosing a Beam for Long Spans

A homeowner removes a wall to open the kitchen onto the living room. The new opening is 18 feet wide, the contractor proposes a built-up beam of 2×12 lumber, and the supplier mentions that a glulam or LVL beam would be shallower. All three options can work, and all three can fail if the choice is made on depth and availability alone. Long-span wood beams are governed by several checks at once: bending, shear, bearing, deflection, and lateral stability. The material that performs best in one check can perform poorly in another.

This article explains how glued laminated timber (glulam), laminated veneer lumber (LVL), and conventional sawn lumber differ, how engineers verify each one under the International Building Code (IBC) and the National Design Specification for Wood Construction (NDS), and what to confirm before a beam is ordered. JMVC Consulting Structural Engineers reviews wood beam designs on residential and light commercial projects regularly, and the same questions arise on most of them. Understanding those questions early helps owners, contractors, and designers avoid redesign during plan check.

What Each Beam Type Is

Sawn lumber is cut directly from a log and graded visually or by machine. Design values for common species are published in the NDS Supplement; for example, No. 2 grade Douglas Fir-Larch dimension lumber has a base bending value (Fb) of 900 psi and a modulus of elasticity (E) of 1.6 million psi. Sawn members are widely available and simple to connect, but natural defects and log size limit their capacity (NDS Chapter 4). Built-up beams made from several plies only perform as intended when the plies are fastened as detailed by the engineer and the load is delivered evenly to every ply.

Glulam is manufactured by bonding graded laminations with structural adhesive under controlled conditions (NDS Chapter 5). Defects are dispersed and the strongest laminations are placed where stresses are highest, so glulam achieves higher stress values and can be produced in large sections, long lengths, and with camber. A common 24F-1.8E combination has a tabulated bending value of 2,400 psi and a modulus of elasticity of 1.8 million psi, though the exact combination and values must be confirmed with the manufacturer.

LVL is a structural composite lumber made of thin wood veneers bonded with the grain running in the same direction (NDS Chapter 8). Its manufacturing process spreads defects, so it is more uniform than sawn lumber. Typical products are rated near 1.9 million psi for E, but LVL values are proprietary and come from the manufacturer’s evaluation report, not from NDS tables. One brand cannot be substituted for another without the engineer’s approval.

The Three Strength Checks: Bending, Shear, and Bearing

Loads come from IBC Chapter 16 and ASCE 7. Most residential living areas are designed for a 40 psf floor live load, with 30 psf for sleeping rooms (IBC Table 1607.1), combined with the dead load of framing, finishes, and any walls or roof above. The beam is then checked against three strength limits under the NDS.

Bending is the most familiar. The calculated bending stress must not exceed the adjusted allowable value, Fb’, which accounts for the size factor CF for sawn lumber (NDS 4.3.6), the volume factor CV for glulam (NDS 5.3.6), the load duration factor, and the beam stability factor CL (NDS 3.3.3). Shear is checked against Fv’ and often governs for short, heavily loaded beams.

Bearing is the check most often overlooked. Compression perpendicular to grain (Fc perpendicular) controls how much length a beam needs where it rests on a post, wall, or hanger. A powerful beam that bears on a post with only 1.5 inches of contact can crush the wood fibers even though bending and shear are satisfactory. High bending capacity does not come with proportionally higher bearing capacity, so engineered beams frequently need longer bearing or a wider post. The load must then continue through that post to the foundation, which is why the supporting load path is reviewed along with the beam.

Deflection and Creep: Why Stiffness Often Governs

On long spans, stiffness usually decides the size before strength does. IBC Table 1604.3 limits floor member live load deflection to span divided by 360. For an 18 foot (216 inch) span, that is 0.6 inch. Many engineers also limit total load deflection to reduce cracking in drywall, tile, and finishes, which is common practice rather than a Table 1604.3 requirement.

Wood also creeps under sustained load. NDS 3.5.2 applies a creep factor (Kcr) of 1.5 for seasoned lumber, glulam, and structural composite lumber in dry service, and 2.0 for unseasoned lumber. The long-term deflection is then checked as Kcr multiplied by the dead load deflection, plus the live load deflection.

Depth is far more effective than material grade for controlling deflection. Stiffness depends on E multiplied by the moment of inertia, and the moment of inertia grows with the cube of depth. Increasing a beam’s depth by 20 percent increases its stiffness by about 73 percent, while changing from an E of 1.6 million psi to 1.9 million psi increases it by only about 19 percent. This is why a shallow LVL or glulam beam chosen for a low ceiling can fail the deflection check even when its strength is ample.

Lateral Stability, Moisture, and Fire

A deep, narrow beam can buckle sideways before it reaches its bending capacity. NDS 4.4.1 sets lateral support requirements based on the ratio of depth to breadth; for sawn rectangular beams, a ratio not exceeding 2 needs no lateral support, and higher ratios require bracing of the compression edge and restraint at the ends. A 3.5 inch wide LVL that is 14 inches deep has a ratio of 4, so it must be braced by joists, sheathing, or blocking in the field. Missing blocking at the supports is a common construction-stage defect.

Moisture is a second consideration. Where a beam will see sustained high humidity or weather, such as an exterior deck or porch, NDS design values are reduced by the wet service factor CM, and untreated LVL should not be used outdoors unless the manufacturer’s evaluation report permits it. Preservative treatment, protected end grain, and flashing details all matter.

Fire is the third. Beams in fire-resistance-rated assemblies must satisfy IBC Chapter 7, and NDS Chapter 16 provides a calculation method for the fire resistance of exposed wood members. Exposed glulam is often used for this reason, since large sections char at a predictable rate and retain structural capacity.

Choosing Between Them in Practice

Sawn lumber suits shorter spans and moderate loads where depth is not restricted. Glulam suits long spans, heavy loads, exposed architectural beams, and cases where camber is useful to offset deflection. LVL suits concealed floor and roof framing where a compact depth is needed and where multiple plies can be fastened together to carry larger loads.

Finally, the stamped calculation applies to the specified product. Substituting a different brand, grade, or section on site, even a deeper one, can change connections and bearing without anyone noticing. Any change should be sent to the engineer of record for review before installation.

Conclusion

The right beam is the one that passes every check, not only the one that looks suitable by depth or price. Sawn lumber, glulam, and LVL each have a proper place, and the governing check on a long span is often deflection or bearing rather than bending. Confirming the loads, the supports, the lateral bracing, and the connection details at the design stage prevents corrections at plan check or during construction.

If you are planning an opening, an addition, or a long-span framing system and want the beam sizing verified, JMVC Consulting Structural Engineers is available to review your project and provide calculations that your building department can accept.