When framing a floor with TJI I-joists instead of sawn lumber, you must adjust your approach to accommodate their engineered properties. Key differences include:
- Blocking and Bridging: I-joists require solid blocking or lateral bracing at bearing points (ends and over supports) to prevent rotation and transfer loads. Use 2x4 or 2x6 blocking between joists, fastened with nails or screws per manufacturer specs. Unlike sawn lumber, I-joists don’t rely on depth for stiffness — blocking is critical for diaphragm action and preventing rollover.
- Holes and Notches: Never notch, cut, or drill flanges (top/bottom chords). Holes are allowed only in the web, following the manufacturer’s hole chart (e.g., Weyerhaeuser, LP, or Boise Cascade). Typically, holes must be centered in the web, at least 2x joist depth from ends, and sized per span/load tables. Use only manufacturer-approved hole locations — guessing risks failure.
- Bearing Requirements: I-joists need minimum bearing length (usually 1.5” on wood or metal, 3” on masonry). Use squash blocks or rim joists to transfer loads properly. Avoid point loads on flanges — distribute via blocking or hangers.
- Hangers and Connectors: Use joist hangers specifically designed for I-joists (e.g., IU, LUS, or MIU series). Standard sawn lumber hangers may not fit or provide adequate nailing. Follow nailing schedules exactly — often more nails are required due to lower flange density.
- Deflection and Span: I-joists can span farther than sawn lumber of same depth, but verify spans using the manufacturer’s load tables (based on live/dead load, spacing, and deflection limits — usually L/360 for live load). Don’t assume equivalence.
- Moisture and Storage: Keep I-joists dry and flat before installation. Avoid bowing or twisting — store off ground, covered, and stick-stacked.
Always consult the manufacturer’s installation guide (e.g., TJI® Joist Installation Guide from Weyerhaeuser). If unsure about hole placement, bearing, or connections — especially for cantilevers, point loads, or irregular layouts — contact the supplier’s tech support or a licensed engineer. Improper modification can compromise structural integrity.
Note: While framing I-joists is DIY-friendly for experienced carpenters, structural design (span, load, hole approval) should be verified by a pro if deviating from standard plans.