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Engineered lumber products have become the preferred framing materials for modern residential and light commercial construction. Their increased strength, dimensional stability, and consistency over conventional sawn lumber make them ideal for today’s open-concept designs. Engineered products—such as laminated veneer lumber (LVL), parallel strand lumber (PSL), laminated strand lumber (LSL), and engineered I-joists (TJI)—eliminate common issues like warping, crowning, or shrinkage over time.
However, even with these advantages, engineered lumber members have limitations when it comes to penetrations for mechanical, electrical, and plumbing (MEP) systems. Improperly placed holes can significantly reduce structural capacity or lead to serviceability issues such as excessive deflection or vibration.
Manufacturer Guidelines
Most engineered lumber manufacturers provide detailed guidelines or tables outlining acceptable penetrations. For example, Weyerhaeuser’s published criteria for LVL, LSL, and PSL products are notably restrictive. Typically:
- Only round holes are permitted,
- The maximum hole diameter is approximately one-third of the member’s depth,
- Holes must be located within the middle one-third of the member’s depth and middle one-third of the span, and
- For LVL and PSL, these provisions apply only under uniform loading conditions.
These restrictions are intended to protect against excessive shear and bending stress concentrations near supports and within high-stress zones.
TJI Joists
TJI (wood I-joists) offer greater flexibility for penetrations compared to solid-section engineered lumber. Their structural behavior mirrors that of a steel wide-flange beam:
- The flanges resist bending moments, and
- The plywood or OSB web carries shear forces.
Because of this configuration, larger and more frequent openings can be accommodated. Penetrations—either round or rectangular—can typically be located anywhere within the web depth, provided that a minimum of 1/8 inch of web material remains adjacent to the flanges. In some cases, holes can also be positioned closer to supports than the one-third-span limit applied to solid engineered members.
Design and Coordination Considerations
In practice, these general guidelines often conflict with MEP layout requirements. For example, a sloped drain line serving a bathroom must maintain continuous fall toward its outlet. If the hole for this line is limited to the middle third of the member depth, the required slope may not be achievable without violating the allowable hole location zone across multiple joists.
When standard manufacturer tables do not accommodate project-specific needs, the project’s Professional Engineer (PE) should review and analyze the proposed penetrations. Most manufacturers, including Weyerhaeuser, offer design software and technical support to assist engineers in evaluating such conditions.
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