Engineered Timber Joists: I-Joists and LVL Compared

Engineered timber joists span further, deflect less, and carry more than solid timber of similar depth. For modern floor structures — especially extensions and conversions — they're often the default choice.

The two main types

I-joists

OSB (oriented strand board) web bonded between LVL (laminated veneer lumber) flanges. Lightweight, dimensionally stable, very efficient.

• Depths: 200, 240, 300, 356, 406mm
• Flange widths: 38, 47, 63, 89mm

LVL (Laminated Veneer Lumber)

Solid engineered timber made from thin veneers bonded with grain aligned. Used for beams and long-span joists.

• Available in solid section sizes equivalent to traditional timber
• Strength 30–50% higher than C24 timber
• Used where I-joist depths are inadequate

Spans (typical residential floor 1.5kN/m²)

• 200×47 C24 solid: 3.4m
• 225×47 C24 solid: 3.9m
• 200mm I-joist (47mm flange): 4.8m
• 240mm I-joist: 5.6m
• 300mm I-joist: 6.7m
• 356mm I-joist: 7.5m

Advantages over solid timber

• Longer spans
• Less deflection — stiffer floors
• Straighter, more dimensionally stable
• Web allows easy services penetration
• Lighter to handle

Limitations

• More expensive per metre than solid timber
• Web requires careful end bearing — squash blocks at supports
• Holes for services must follow manufacturer's chart (not arbitrary)
• Susceptible to moisture damage if exposed

Cost comparison

• C24 solid 225×47: R160–R240 per linear metre
• I-joist 240mm: R360–R560 per linear metre

Per square metre of floor, engineered joists typically cost R300–R600 more — recovered through faster install and fewer intermediate supports.

Specification

Manufacturer engineering software produces job-specific joist schedules — sizes, lengths, hangers, blocking. Engineers verify and incorporate into building design.

MCFAR specifies engineered joists for residential and light commercial.

Frequently Asked Questions