I-joist (engineered floor framing)
I-joist is engineered floor or roof framing with LVL or sawn flanges and an OSB or plywood web. High strength-to-weight; volume-builder default for upper floors.
Ask Chalkline about this →An I-joist is an engineered timber floor or roof framing member with an I-shaped cross-section: a top and bottom flange of LVL or sawn timber, and a web of OSB (oriented strand board) or structural plywood glued between them. The I-section is mechanically efficient: most of the bending stress is in the flanges, while the lightweight web only needs to resist shear. Result: I-joists span further than equivalent-depth solid timber, are far lighter to handle, and have very low dimensional variation.
Brand examples in Australian residential:
| Brand | Manufacturer | Common availability |
|---|---|---|
| Wesbeam I-Joist | Wesbeam | Wide WA / national |
| Tilling SmartJoist | Tilling Timber | NSW / VIC / national |
| CHH hyJOIST | Carter Holt Harvey | NSW / QLD national |
Each brand has slightly different flange dimensions, web thickness, and depths, and must be sized using the manufacturer’s own span tables: span tables are not interchangeable between brands.
Common residential applications:
- Upper-floor joists in two-storey dwellings (deeper, lighter and stiffer than solid pine for the same span).
- Floor joists over a long-span open ground floor, e.g. residential alterations where load-bearing walls are removed.
- Cathedral ceiling rafters where the depth/span ratio of solid timber would be uneconomic.
Typical I-joist depths and spans:
| Depth | Typical residential span (single span, 600 mm centres, residential floor load) |
|---|---|
| 200 mm | 4.5 to 5.5 m |
| 240 mm | 5.5 to 6.5 m |
| 300 mm | 6.5 to 7.5 m |
| 400 mm | 7.5 to 9.0 m (long-span applications) |
Install rules:
- Hangers, not nail-ledger. Most I-joists need engineered hangers (Strongtie LUS, MiTek LU) at the bearing, not toe-nail or end-grain bearing.
- No notching the flange. The flange is the bending element; cutting or drilling through it destroys the joist. Bottom-flange notching is always prohibited.
- Web-only holes for services. Manufacturers publish a “hole zone” chart showing where holes can be drilled through the web for plumbing and electrical without compromising shear capacity.
- Squash blocks at point loads (under a load-bearing wall above). Without squash blocks the joist crushes vertically at the bearing point.
Common defects on residential install:
- Flange notched for a service pipe: joist fails at the notch, slab above sags or cracks.
- Service hole drilled outside the manufacturer’s hole zone (too close to the bearing).
- Wrong hanger used: light-duty hanger under a heavy load joist.
- Mixed brand on the same floor: span tables conflict, joist depths don’t match exactly, hangers vary.
Also known as: TJI (a US trade name often used generically); engineered floor joist; OSB-web joist; deep-web joist.
Category: Structure.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.