I-Joist Calculator — TJI, LVL & Posi-Joist Spans

You don't reuse sawn-lumber tables — I-joist geometry and design values are different animals. Three honest steps: (1) measure the clear span and pick spacing (16″ o.c. is the I-joist default); (2) pick the deflection limit — L/360 for ordinary floors, L/480 under tile; (3) read the depth class off the series span book: 9½" TJI-class members reach roughly 16 ft at 16″ o.c., 11⅞" reaches 19 ft, 14" reaches 22 ft, and 16" reaches 24 ft under 40/10 psf loads — the indicative range the calculator above uses. The delivered series' catalog is the binding source, and every manufacturer publishes a free span calculator that matches it exactly.

Per pound and per inch of depth, yes — and here is the mechanism rather than the slogan. An I-joist puts strong LVL or solid-sawn flanges at the extreme fibers, where bending stress peaks, and a thin OSB or plywood web where shear lives. There are no knots in the flanges and no grain slope past limits, which is where solid No. 2 lumber loses capacity. The result: an 11⅞" I-joist typically spans 15–25% farther than a 2×10 while weighing a third less. The trade is vulnerability — thin webs bruise, and every hole must follow the zone map.

The rule that matters: square holes and notches are prohibited in the web, and nothing may be cut into the flanges — ever. Round holes drilled in the web are allowed within the zone map, commonly up to about 2″ diameter in a 9½" member and 2½–3″ in deeper series, positioned in the middle portion of the span where shear is low. Keep holes at least a hole-diameter apart, outside the bearing ends. A length of duct that doesn't fit the zones is a reason to change the run — not to enlarge the hole.

The break-even lands in the 14–16 ft span range for most markets. Below it, SPF 2×10s are simply cheaper per stick and the span gap doesn't matter. Above it, the ledger flips: one 16 ft I-joist replaces the two sticks and the beam you'd otherwise buy, and the lighter board cuts labor. Add the service-run savings (electricians drill clean holes instead of fighting knots) and the basement-ceiling depth savings (11⅞" replaces a 2×12), and the engineered premium pays for itself from roughly 16 ft upward. Short spans on a tight budget: sawn lumber every time.

Bearing: a minimum of 1¾" of bearing length at each end — standard hangers and wall plates provide it — with web stiffeners required where the catalog says the bearing isn't laterally supported. Blocking: full-depth blocking or rim boards at every bearing and at cantilever back-spans is required; mid-span bridging generally is not for I-joists, which is a labor saving solid lumber doesn't offer. Cantilevers: shorter than sawn equivalents — typically 4 ft or less without catalog verification — because the thin web takes the back-span reaction differently.

I-joists (TJI, BCI, and similar) are the flange-and-web sections this page sizes — the default engineered floor member. LVL is solid engineered lumber: used as I-joist flanges, but also as a full-depth beam or flush header where a single ultra-strong member matters — LVL floor joists reach spans I-joists can't at the same depth, at a higher price per foot. Posi-joists (metal-web open joists, popular in the UK) put a steel web between solid chords: services pass straight through the open web, no drilling at all, and spans reach 20+ ft — the trade is cost and the fact that you never cut or modify a metal web.

Honest answer: worse per minute. Solid lumber chars from the outside at a predictable rate — roughly 1/32 of an inch per minute — and keeps carrying load through its core. An I-joist's thin OSB web loses section quickly once exposed, which is why code requires fire protection: a full ceiling of ½" drywall (the standard floor/ceiling assembly) protects I-joists adequately for residential assemblies, and encapsulated mass-timber-style details go further. Unprotected exposed I-joist ceilings in garages are the configuration to avoid — either drywall them or use sawn lumber there.