Floor Truss Calculator

Open-web parallel chords do floors and flat roofs the same way: two chords held apart, services routed through the web zone, spans joists can’t touch. Check depth, deflection budget, and count before you call the plant.

Floor Truss Spec Checker — Depth, Deflection Budget & Count

Screening numbers before you call the truss plant: depth against span, the deflection your finished floor allows, and the truss count.

Span (ft)Truss depth (in)Truss spacing (in o.c.)Deflection limit

Span-to-depth

12.0 : 1

Efficient band

Total-load deflection budget

0.67 in

L/360 at 20 ft

Live-load budget

0.67 in

stiffness under people & furniture

Truss count (24 ft room)

at 24 in o.c.

✓ Depth verdict: 20 in deep over 20 ft sits in the efficient screening band (span-to-depth ≤ 14). Manufacturer tables will likely show a chord choice at your spacing — this is a starting point, not a stamp.

What an Open-Web Floor Truss Actually Is

A floor truss is a parallel-chord truss: two horizontal chords held a fixed depth apart by a zig-zag of short webs. The top chord carries the subfloor screws; the bottom chord carries the ceiling (and, in floor service, the deck and everything on it); the webs do the quiet work of shear. Unlike a roof truss — a triangle over open space — the parallel profile means the member is a deck: constant depth, flush top and bottom, stackable into a floor plane that behaves like a single stiff plate.

The consequence of that geometry is the product’s whole pitch. Depth becomes a design variable chosen per span instead of dictated by the lumber yard’s longest board. Webs divide the chords into short unsupported lengths, so slender 2x3 and 2x4 chords do work that would demand 2x12 lumber. And the open center is a manufactured chase: a 6-inch duct, a 3-inch drain, a run of conduit — all pass through without touching a single structural fiber.

💡 Tip: order the mechanical chase locations with the trusses. A chase is free at the plant and impossible after delivery — every hole a plumber drills in a delivered truss is a warranty letter waiting to happen.

⚠️ Warning: the no-cut rule is absolute. Never notch a chord, never cut a web, never drill through a plate. If the layout changed since ordering, call the truss plant — they re-engineer single trusses quickly and cheaply.

❌ Don’t: don’t let the crane lift a truss by its top chord flat without spreaders past about 12 ft — bundles need proper picking. A truss bent sideways at delivery is a rejected truss, not a slight warp to straighten.

Floor Truss Depth vs Span: The Selection Table

The screening rule — depth in inches near span in feet — puts every row of this table within a step of what plants actually ship. Chord sizes and web counts are the typical package at residential 40 psf live / 10–15 psf dead loads at 24 in spacing; your supplier’s stamped table is the final word for your load zone.

Clear span	Typical depth	Chords	Web bays	Typical use
12 ft	12–14 in	2x4 / 2x3 chords	2 bays	Sheds, small additions
16 ft	14–16 in	2x4 chords	3–4 bays	Bedrooms over garages
20 ft	16–20 in	2x4 chords	5–6 bays	Great rooms, open plans
24 ft	20–24 in	2x4–2x6 chords	6–7 bays	Full-width family rooms
28 ft	24–26 in	2x6 chords	7–8 bays	Clearspan over ballrooms of the house
32 ft	26–30 in	2x6–2x8 chords	8–9 bays	Commercial-light, long clearspan

Two trades run against this table. Deeper is cheaper per span: a 24 in truss at 28 ft beats a 20 in truss at the same span on both deflection and price, because depth is the cheapest stiffness there is. But depth eats headroom — a 24 in floor sandwich plus finishes costs two inches of ceiling per foot of building height, which is why basement clearspan conversations are really headroom conversations.

Deflection Limits in Practice: L/360, L/480, and L/720

Deflection limits are the floor’s comfort contract, written as fractions of span. At 20 ft (240 in), the same physical floor gets three very different allowances: L/360 permits 0.67 in of total-load sag, L/480 permits 0.50 in, and L/720 — the brittle-finish zone — permits just 0.33 in. The spec checker above turns whatever limit you select into an inch budget for your exact span.

Code minimum

L/360 — the code floor

The minimum most codes accept for floor members under total load. It protects plaster ceilings and keeps doors latching, and truss plants quote it as the base table. It says nothing about how the floor feels: vibration under footsteps is a stiffness-and-mass question the deflection number alone answers poorly, which is why long-span floors at bare L/360 are the ones owners complain about.

Most ordered

L/480 — the comfort upgrade

One-third stiffer than code. In practice this is the highest-volume spec in custom residential: the premium over L/360 is modest because the fix is usually a 2-inch depth increase or one stronger chord grade, and the payoff is a floor that doesn’t announce every crossing. When a plan says 'solid-feel floor', L/480 is what the builder actually ordered.

Special finishes

L/720 — brittle finish territory

Large-format tile, natural stone, and double-height glass all punish deflection far beyond the feel threshold; L/720 halves the allowance again. At this level the conversation usually includes closer spacing and engineered chord upgrades — and sometimes a steel beam below — because every inch of depth and every foot of spacing is now working against the budget.

💡 Live-load versus total-load: the two budgets in the spec checker are different contracts. Total-load deflection includes the weight of the floor itself and protects things below (ceilings, partitions). Live-load deflection — people and furniture — is what you feel, and the vibration conversation always begins there. When a supplier table quotes both, match each to its check instead of comparing one number against both limits.

The Open-Web Advantage: Routing Services Through the Floor

On any floor with HVAC below ducted rooms, a bathroom group offset from the stack, or low-voltage runs to a media wall, the web zone stops being a structural detail and becomes floor plan freedom. The comparison table puts the three framing options side by side at a representative 20 ft span — the span where the decision usually gets made.

Aspect	Open-Web Floor Truss	I-Joist	Dimensional Lumber	Edge
Depth for 20 ft span	16–20 in open-web	16 in I-joist series	2x12 (11¼ in) — usually impossible	Truss & I-joist tie
Routing ducts & plumbing	Through the webs — anywhere, no drilling	Punched knockouts, size-limited	Drill rules: holes ≤ ⅓ depth, mid-span only	Floor truss, decisively
Stiffness control	Depth and web pattern tuned per span	Fixed series depths	Crowned and packed by hand	Floor truss
Length availability	Any span to ~40+ ft, custom order	Stock lengths, spans to ~26 ft typical	2x12 tops out near 15–16 ft at 40 psf	Floor truss
Site flexibility	No cutting, notching, or drilling allowed	Limited knockouts and flange nailing rules	Forgiving — cut and sister on site	Dimensional lumber
Cost structure	Engineered package, crane-ready, labor-light	Mid material cost, mid labor	Cheap material, expensive framing labor at long spans	Depends on span — trusses win above ~16 ft

Open-web floor trusses installed over a basement with ductwork routed through the web openings

Ducts through the webs, not through bulkheadsPhoto: Unsplash

What the chase is worth

Count what a dropped soffit really costs: framing, drywall, tape and finish, lost ceiling height, and the visual bulk in a finished room — routinely hundreds of dollars per bulkhead before the space tax. A chase routed through trusses costs a line on the order sheet.

The second dividend is speed: mechanical trades self-serve through open webs without waiting on drill-pattern approvals, and inspection passes because there is nothing to inspect — no bored holes, no notches, no judgement calls near bearing. Framers report floor systems going from crane-set to walkable in a day at 24 in spacing, and the mechanical rough-in starting the same afternoon.

The Same Member Upside-Down: Flat Roof Trusses

Run the parallel-chord profile across a room instead of under one and it becomes the flat truss of low-slope roofs: long clearspans between supports with the web zone carrying ductwork above the ceiling line instead of below the deck. The geometry, the depth trade, and the chase advantage all transfer — only the load direction and the drainage conversation change.

Slope is still mandatory

A 'flat' roof never drains flat. Detailers build in 1/8 to 1/4 inch per foot via tapered insulation over the deck or a shallow-sloped top chord, so membrane water reaches scuppers or drains. The truss itself can stay parallel — the insulation does the tapering — which is why parallel-chord flat trusses pair so well with tapered rigid insulation systems.

Loads flip direction

Roof service lands on the top chord (membrane, insulation, snow), wind tries to lift the whole assembly, and ponding water is the design case that quietly doubles loads on a roof that drains too slowly. Floor trusses never think about uplift; flat roof trusses check it first — parapet corners especially.

Span bands widen

With snow loads typically lighter than floor live loads and depths free to grow, parallel-chord roof trusses reach further than their floor siblings — 30–40 ft clearspans are routine over retail and light-commercial bays, with the web zone doing the HVAC distribution the floor version does under it.

Open the Preloaded 20 ft Floor Truss

A flat 20 ft parallel-chord truss, 24 in deep, nine web bays, floor load of 1.5 kip on every interior bottom-chord joint — the bottom-loaded pattern that distinguishes floor service from roof service.

▶ Load the 20 ft floor truss model

Floor Truss Calculator FAQ

What depth floor truss do I need for my span?▼

Use the screening rule: depth in inches roughly equals span in feet — a 20 ft span starts at a 20 in deep truss, a 24 ft span at 24 in. Manufacturer tables routinely beat the rule by 2–4 inches with upgraded chords or closer spacing, but a span-to-depth ratio beyond about 14 is where deflection and vibration complaints start. The spec checker above converts your span and depth into the ratio instantly and flags the stretch zone.

What is the difference between L/360 and L/480 deflection limits?▼

The limit sets how much total-load sag the floor is allowed: L/360 permits span÷360 inches (0.67 in at 20 ft), L/480 permits span÷480 (0.50 in). Code accepts L/360, but floors that feel bouncy usually pass L/360 while failing the feel test — floor vibration depends on stiffness and mass beyond the deflection number. Choosing L/480 in the spec checker tightens the live-load budget by a third, and brittle finishes like large-format tile push toward L/720. Truss plants price the upgrade honestly: same truss, stronger chords, modest premium.

Why do floor trusses beat joists for plumbing and HVAC?▼

The open web is a manufactured chase: ducts, drains, and supply lines pass through the truss profile without drilling anything, and routes can turn within the truss depth. Compare the alternatives: dimensional lumber allows holes no bigger than a third of the joist depth and only in the middle portion of the span; I-joists offer fixed-size knockouts; neither accepts a 6 in duct cleanly. On a 24 ft great-room floor, the truss chase is frequently the difference between a flat ceiling below and a dropped bulkhead.

Are floor trusses suitable for flat roofs too?▼

Yes — the parallel-chord profile is the same member doing flat roof duty: long spans between supports with services running in the web zone, exactly the 'flat truss' of commercial-light construction. One design note transfers from roofs: truly flat means no drainage, so flat-roof trusses usually carry a slight built-in slope (often 1/8 to 1/4 per foot via tapered insulation or a shallow-sloped top chord) even though the truss is called flat. Load character flips too — roof trusses see snow and uplift, floor trusses see live load, and the solver handles either once the loads are entered.

How are loads applied to a floor truss model?▼

Floor load arrives on the bottom chord — the deck nails to it — which is the opposite of a roof truss. The preloaded 20 ft model on this page applies 1.5 kip at each interior bottom-chord joint, representing a 40 psf live plus 10 psf dead floor on 2 ft truss spacing across 22.5 ft of tributary length. Reading the force table of a bottom-loaded truss is instructive: the bottom chord tension is the dominant force, and the webs redistribute it — the mirror image of the roof-truss patterns most diagrams show.

Can floor trusses be drilled or modified on site?▼

No — and this is the one rule with no exceptions: open-web trusses are engineered assemblies, and cutting any chord or web voids the design. The compensation is that site modification should never be needed: give the truss plant the mechanical layout during ordering and the chases arrive where the plumber and HVAC contractor need them. Dimensional lumber remains the right choice for projects where field improvisation is likely; the comparison table above makes that trade-off explicit.

How far can floor trusses span without interior support?▼

Standard wood open-web floor trusses commonly reach 28–32 ft clearspan with 24–30 in depths, and engineered profiles beyond that; compare dimensional lumber, which effectively stops near 15–16 ft at residential loads. The honest method is to read the span table for the exact chord and web configuration at your spacing — the depth-vs-span table on this page is the screening version. At every span, cost per square foot of clearspan favors trusses because the alternative is a beam and a bearing wall below.