A steel beam is a primary member that carries load to columns or walls; steel joists are lighter members repeated at close spacing that hold up the floor or roof and bear on the beams. In commercial buildings, steel joists are usually open web steel joists, lightweight trusses designed by the joist manufacturer for the project's loads. In light framing they can be cold-formed C-joists, and in mezzanines and heavy floors a W-beam sometimes does the joist's job. Spans depend on the load, spacing and deflection limit, so there is no universal span chart for either.
This guide explains how the members differ, where each is used, and what the drawings need to say.
Beams vs joists: the basic difference
| Steel beam | Open web steel joist | |
|---|---|---|
| Role | Primary member: collects load, carries it to columns or walls | Secondary member: supports deck, carries load to beams or walls |
| Shape | Solid rolled section, usually a W-shape, sometimes HSS or channel | Truss: top and bottom chords connected by a zigzag web |
| Spacing | Wider, set by the column grid | Close and repeated across the bay |
| Who designs it | The project engineer sizes it | The engineer specifies loads; the joist manufacturer designs the joist |
| Weight for a long, lightly loaded span | Heavier | Lighter |
| Services | Must go around or below, or through designed openings | Ducts and pipes can pass through the open web |
| Typical use | Floors, roofs, transfer members, openings | Roofs and floors of warehouses, schools, retail and offices |
The two work as a team. A typical steel roof is a grid of columns, beams or joist girders between them, joists spanning between the beams, and steel deck on the joists. In a house, the steel beam usually carries wood joists instead.
Open web steel joists
An open web steel joist is a shop-built truss. The chords are usually pairs of angles, and the web members are bars or angles welded between them. The ends have bearing seats that sit on a beam flange, a wall bearing plate or a joist girder.
Why they are used:
- Weight. A truss puts steel only where it works hardest, so for long spans with roof or light floor loads it uses much less steel than a solid beam.
- Depth for services. Ducts, sprinkler mains and conduit can run through the web openings.
- Speed. Joists arrive ready to set and are welded or bolted at the seats.
What they need:
- Bridging. Rows of bracing between joists keep the slender bottom and top chords from buckling, especially during erection before the deck goes on.
- Load information. The structural drawings show the joist depth, spacing, specified loads, any concentrated loads and the deflection limit. The joist manufacturer then designs each joist and prepares its own design and shop drawings.
- Care with added loads. Hanging a rooftop unit, a hoist or heavy piping from random points on a joist can overload it. New loads go back to the engineer and, usually, the joist supplier.
Joist girders are the bigger cousin: deep trusses that act as primary members, carrying the ends of the joists at panel points in place of a W-beam.
Steel beams used as joists
"I-beam floor joist" and "steel beam joist" searches usually come from projects like these:
- Mezzanines and platforms, where W-beams or channels at regular spacing carry steel deck, grating or plywood. See our steel mezzanine guide.
- Heavy floors, such as mechanical rooms or storage areas, where a solid section handles high loads and concentrated loads better.
- Short spans and shallow floors, where a rolled section fits the depth available and connections are simple.
- Repairs and reinforcement, where a steel section is added alongside or under existing framing.
A W-beam used as a joist is sized by the engineer like any other beam. For long spans with light loads it is usually heavier than an open web joist, but it is stiffer for its depth, takes point loads anywhere along its length, and is easy to connect to, cut and modify. Our post on how far a steel beam can span explains what limits a beam's span.
Light-gauge steel joists
In steel-framed houses and some small buildings, floor joists are cold-formed C-shapes made from galvanized sheet. These are a different product from hot-rolled beams or open web joists. The 2024 Ontario Building Code requires cold-formed steel members to conform to CSA S136 (Article 4.3.4.2), and light-gauge joists are selected from the manufacturer's design data for the span, spacing and loads.
In most Ontario houses, though, the joists are wood and the steel is the beam underneath. The connection between them is where the detailing matters; see joist hangers for steel beams and attaching a wood top plate to a steel beam.
Why there is no single steel joist span chart
People search for a steel joist span chart, a maximum span or a span calculator. The honest answer is that the span depends on several things at once:
- Loads. Roof snow load varies by location in Ontario, and drifts form against higher roofs and parapets. Floors depend on occupancy. Equipment, partitions and ceilings add more.
- Spacing. Wider spacing puts more load on each joist and asks more of the deck.
- Depth. Deeper joists span further for the same weight of steel.
- Deflection limit. Floors that carry brittle finishes, or that need to feel solid underfoot, need stiffer joists than a simple roof.
- Vibration. Long, light floors can be lively. Floor vibration is checked separately from strength.
- Bearing and bracing. How the ends are supported and how the chords are braced.
Joist manufacturers publish load tables that help designers pick a depth and series early on. The final joist, though, is designed for the actual building. The same is true of beams: span tables exist for some simple residential cases, but most steel beams are sized by calculation. If you need longer spans than usual, see can a steel beam span 30, 40 or 50 feet.
How beams and joists are specified on the drawings
A clean set of structural drawings makes pricing and coordination much easier. Look for:
- Beams: section size (for example W310x39), length, camber if any, connection details or loads for connection design, and finish.
- Open web joists: depth, spacing, specified dead, live and snow loads, concentrated loads with their locations, deflection limit, bridging requirements and bearing details.
- Bearing points: the beams, bearing plates or embedded plates the joists sit on, which are usually part of the structural steel package rather than the joist package.
Because joists and beams often come from different suppliers, check that every bid covers the bearing seats, bearing plates, bridging and any reinforcement for openings. Gaps between packages show up on site.
Beams, columns and bearing steel for your project
Open web steel joists come from specialist joist manufacturers. We fabricate the structural steel they bear on and work with: beams, columns, posts, lintels, bearing and embed plates and connection steel, with in-house shop drawings, CWB certified welding and delivery across the GTA and Ontario. We also supply and install structural steel, from basement beams to commercial frames.
Send your structural drawings for a free take-off and quote, or browse our steel products.
Frequently asked questions
What is the difference between a steel beam and a steel joist?
A beam is a primary member that collects load and carries it to columns or walls. Joists are lighter secondary members repeated at close spacing to hold up the floor or roof deck, and they bear on the beams. In commercial buildings steel joists are usually open web steel joists, which are lightweight trusses rather than solid sections.
How far can a steel joist span?
There is no single answer. Open web steel joists are designed by the joist manufacturer for the span, spacing, loads and deflection limits on the structural drawings, and long-span joists can cover much greater distances than solid beams of the same weight. The engineer sets the requirements and the manufacturer designs each joist.
Can a steel I-beam be used as a floor joist?
Yes. W-beams are used as joists in mezzanines, heavy floors and short spans, often with wood or steel deck on top. For long spans with light loads they are usually heavier than an open web joist doing the same job, so they are chosen where loads, depth limits or connections favour a solid section.
Is there a steel joist span calculator?
Joist manufacturers publish load tables for preliminary selection, but the actual joist is designed for the project loads, including point loads from equipment, snow drifts and deflection limits. Treat any calculator as a starting point for a conversation with the engineer, not a design.
