Floor Truss Calculator — Open-Web Span Tables & Sizing

A floor truss span chart is organized the same way as any joist table, with one extra column: truss depth. Find your load series (usually 40-10-360: 40 psf live, 10 psf dead, L/360), then read across the row for your span to the depth that carries it — a 20 ft span typically lands on a 14–16″ deep truss in a 2×4-chord series. The chart is printed by the truss manufacturer because the chords, plates, and web pattern are theirs; two brands can differ 10–15% at the same nominal depth. That is why this page teaches the span/depth ratio instead of printing a universal chart: depth ≈ span ÷ 18 is the sizing instinct, the catalog is the confirmation.

Use the span ÷ depth ratio: at 40 psf live load a 20 ft span wants about 13–14″ of depth, 24 ft wants 16″, and 28 ft wants 19″ — the calculator above rounds each to the next even depth and checks your selection. Compare that to sawn lumber, which tops out near 15 ft, and the truss advantage is obvious: 24 ft of clear floor from a 16″ member, no beam, no posts. Real spans still come from the manufacturer's chart with your exact load series and chord spec — the ratio gets you to the right column of that chart the first time.

No — and this is the one rule with no exceptions. Chords and webs are the structure; cutting a web or notching a chord removes a member that was designed to be there, unlike sawn joists where IRC hole rules allow drilling through the neutral zone. What open-web trusses offer instead is engineered chases: openings between the webs, ideally in the middle third of the span, with length kept under the truss depth. Anything beyond the chase shown on the shop drawing goes back to the truss manufacturer — they will usually re-engineer a panel point for a duct route at little cost if it is requested before fabrication.

Both are engineered-wood floors and both massively out-span sawn lumber. Choose the truss when mechanical runs are the hard problem — HVAC branches, plumbing wet walls, long duct runs — because the open web routes them without drilling anything. Choose the i-joist when spans are moderate, budget is tight, and the mechanical layout is simple: i-joists are typically cheaper per square foot at common spans and install fast by hand. Mixed floors are common and legitimate: trusses over the great room where ducts converge, i-joists in the bedroom wing.

The truss price is mostly lumber and plates by volume, and that price is nearly flat as span grows — a 24″ truss costs meaningfully more than a 14″ one, but nowhere near proportionally. Competing systems do not scale that way: beyond about 16 ft, sawn lumber needs a beam and posts (structure, footings, head height), and i-joists jump a series or double up flanges. By 20–24 ft the truss is usually the cheapest floor per square foot even before counting the duct-routing savings; under 14 ft it rarely wins. That crossover is the whole economic story of open-web floors.

Two parallel chords — most residential series use 2×4 lumber — joined by diagonal steel-plated webs in a W or similar pattern. The chords carry bending (tension on the bottom, compression on the top), the diagonals carry shear, and because the section is open, a 14–16″ truss behaves like a mini steel joist: deep, light, and full of routing space. Trusses bear on a 4″+ heel at each end, run at 16–24″ o.c. like any joist, and arrive shop-built to the exact length — no cutting on site, which is a feature the schedule learns to love.

Same off-by-one rule as any joist layout: count the bays, add the end truss. Trusses = ⌈(floor length × 12) ÷ spacing⌉ + 1, and a 36 ft floor at 19.2″ o.c. needs ⌈432 ÷ 19.2⌉ + 1 = 24 trusses, plus two rim or band members closing the edges. Spacing follows the load series on the chart — 19.2″ o.c. is the volume-builder default because it divides the 4×8 subfloor sheet cleanly into five bays; 16″ o.c. buys margin under heavy loads; 24″ o.c. only on light-load floors.