Glulam (Glued Laminated Timber)

What Is Glulam?

Glued laminated timber — universally called glulam — is one of the oldest engineered wood products, first developed in Europe in the early 1900s and commercially produced in the United States since the 1930s. It is manufactured by stacking layers of kiln-dried dimensional lumber (called laminations), applying structural adhesive between each layer, and curing the assembly under pressure.

The genius of glulam is that by combining many small pieces of lumber, the statistical probability of a critical defect at the point of maximum stress is dramatically reduced compared to a single large solid timber. Glulam consistently achieves higher allowable design values than the solid lumber used in its laminations.

How Glulam Is Made

• Lumber selection and grading: Each lamination is graded for strength. Higher-grade laminations are placed in the outer layers (flanges) where bending stress is greatest. Lower-grade lumber is used in the core where stress is lower.
• Finger jointing: Individual lumber pieces are end-joined with structural finger joints to create laminations of any required length.
• Adhesive application: Structural adhesive (typically melamine or phenol-resorcinol for interior use, and phenol-resorcinol for exterior exposure) is applied between laminations.
• Pressing and curing: The assembly is clamped under pressure and cured. Straight members are clamped on flat beds; curved members use shaped forms.
• Finishing: Members are planed to final dimensions. Architectural grade receives additional surfacing and void filling.

When to Choose Glulam

• Exposed structural beams: Glulam in Architectural or Premium grade provides a warm, natural appearance that is architecturally desirable. It can be stained, finished, or left natural.
• Long spans: Glulam handles spans that LVL and PSL cannot economically reach. For spans over 20-25 feet, glulam is often the most practical wood beam option.
• Wide bearing requirements: Glulam comes in widths from 3-1/8" to 10-3/4" as standard products. LVL achieves wider widths only by multi-ply lamination, which is less elegant for exposed applications.
• Curved or tapered members: Only glulam can be manufactured in curved shapes (arches, tapered beams, pitched/tapered/curved combinations) for architectural applications.
• Exterior applications: With appropriate treatment, glulam performs well in covered exterior applications like porches, pavilions, and pergolas.

Design Considerations

Glulam design requires attention to a few details that differ from LVL:

• Volume factor: Glulam allowable stresses decrease slightly as the member gets larger (volume effect). The published Fb values assume a standard 5-1/8" x 12" section. Larger or smaller members require adjustment.
• Unbalanced layups (like 24F-V8): These have different top and bottom allowable stresses. The stamp indicates which side is the tension face. Installing an unbalanced glulam upside-down can reduce its capacity by 20-40%.
• Lateral bracing: Like all deep beams, glulam members require lateral bracing of the compression flange to prevent lateral-torsional buckling. In floors, the subfloor provides this bracing. Exposed beams may need intermediate bracing.
• Moisture considerations: Glulam should be stored under cover and protected from rain during construction. While the adhesive is waterproof, the lumber laminations will swell and check if repeatedly wetted.