The difference comes down to what each member carries. A joist carries the floor or roof deck, a beam carries the joists, and a girder carries the beams and hands the load to a column or wall. All three are horizontal members that bend under load, so the names describe a job in the framing, not a different shape of steel.
That is why the same W16x26 can be called a beam on one drawing and a girder on the next. Nothing about the section changed. What changed is whether other beams frame into it.
Girder vs Beam vs Joist at a Glance
| Joist | Beam | Girder | |
|---|---|---|---|
| Carries | Floor or roof deck, sheathing, slab | Joists | Beams, and sometimes joists directly |
| Supported by | Beams and bearing walls | Girders, columns, bearing walls | Columns and walls |
| Spacing | Repetitive, usually 12 to 24 in in wood framing | Spaced across the bay | Usually one per column line |
| Relative size | Smallest | Larger than the joists it carries | Usually the deepest and heaviest of the three |
| Common materials | Dimensional lumber, engineered I-joists, open-web steel joists, light-gauge steel | Wide flange steel, LVL, glulam, solid timber | Wide flange steel, plate girders, glulam |
If you only remember one rule: a girder is a beam that carries other beams. Everything else follows from that.
The Load Path, Top to Bottom
Framing is a chain of handoffs. Load starts as weight spread over the floor surface, and each member gathers it from the one above and concentrates it into fewer, bigger pieces below. By the time it reaches the column it is a single point load.
Each step down the chain roughly multiplies the load. A joist might collect the floor over a 16 in wide strip. The beam under it collects every joist along its length. The girder under that collects several beams. That is the whole reason girders end up deeper and heavier than the beams they support.
Is a Girder Just a Beam?
In steel design, yes. There is no separate girder shape in the AISC Shapes Database and no separate set of design equations. A girder is a wide flange section, from the same catalog of W shapes a beam comes from, doing a bigger job. Bending, shear and deflection are checked exactly the same way.
The distinction is useful on drawings, not in the math. Calling a member a girder tells the detailer to expect beam connections along its length, tells the erector it is probably a heavier lift, and tells the estimator it sits on a column line. If you want the full family of rolled sections and what each one is normally used for, see our guide to steel beam types.
Plate Girders
One case where the word girder really does mean something specific is a plate girder. Instead of a rolled section, it is built up by welding separate plates into an I shape, which lets the fabricator make it deeper than any rolled shape and vary the flange thickness along the span. Plate girders show up in bridges and in long-span or heavily loaded buildings, where the deepest rolled sections still are not enough.
What Counts as a Joist
Joists are the repetitive members. The defining trait is that they are closely and evenly spaced, so the deck above can span the short distance between them and the load is shared among neighbors.
Wood Joists and I-Joists
In houses, floor joists are usually 2×8, 2×10 or 2×12 lumber at 12, 16 or 19.2 in on center, or engineered I-joists with plywood or OSB webs between small flanges. Allowable spans depend on species, grade, spacing and load, which is why they are always read off a table rather than guessed. The American Wood Council publishes a free Span Options Calculator for wood joists and rafters covering the species and grades in the NDS Supplement (checked September 2026).
In wood framing the vocabulary shifts slightly. The member carrying the joists is often called a beam if it is exposed and a girder if it runs down the middle of a basement carrying the floor from both sides. Some framers call a built-up multi-ply member a girder regardless of what it carries.
Open-Web Steel Joists and Joist Girders
Commercial roofs and floors often use open-web steel joists: lightweight trusses with parallel chords and a zigzag web, spanning much further than a rolled beam of the same weight. They come in standard series, and a matching product called a joist girder carries them at the column lines. Published ranges from one manufacturer give a sense of the scale:
| Series | Depths | Spans |
|---|---|---|
| K-Series joist | 10 to 30 in | 20 to 60 ft |
| LH-Series joist | 18 to 48 in | 18 to 96 ft |
| DLH-Series joist | 52 to 120 in | 90 to 240 ft |
| Joist girder | 20 to 120 in | 20 to 120 ft |
Ranges as published by New Millennium Building Systems for standard joists and joist girders, checked September 2026. Other manufacturers publish similar ranges, and the governing load tables come from the Steel Joist Institute.
Real Sizes, Side by Side
Here are nine real W shapes from the AISC Shapes Database v16.0, lightest to heaviest. The pattern is the point: as a member moves up the hierarchy it gets deeper, and stiffness climbs far faster than weight.
| Shape | Weight (lb/ft) | Depth (in) | Flange width (in) | Ix (in⁴) | Zx (in³) |
|---|---|---|---|---|---|
| W12x19 | 19 | 12.2 | 4.01 | 130 | 24.7 |
| W14x22 | 22 | 13.7 | 5.00 | 199 | 33.2 |
| W16x26 | 26 | 15.7 | 5.50 | 301 | 44.2 |
| W18x35 | 35 | 17.7 | 6.00 | 510 | 66.5 |
| W21x44 | 44 | 20.7 | 6.50 | 843 | 95.4 |
| W24x55 | 55 | 23.6 | 7.01 | 1350 | 134 |
| W24x68 | 68 | 23.7 | 8.97 | 1830 | 177 |
| W27x84 | 84 | 26.7 | 10.0 | 2850 | 244 |
| W30x108 | 108 | 29.8 | 10.5 | 4470 | 346 |
Compare the two ends. The W30x108 weighs 5.7 times as much per foot as the W12x19 but has 34 times the moment of inertia, because depth counts far more than material. That is why a girder collecting four beams is usually answered with more depth rather than thicker flanges wherever headroom allows. The same principle drives the numbers on every page in our I-beam sizes chart, and you can work out the total weight of any run with the steel beam weight calculator.
Which of these ends up as a joist, a beam or a girder is set entirely by the framing layout and the loads. These are not recommendations, and none of them is a substitute for a design.
Why the Label Matters When You Order
A steel supplier does not sell girders. You order a shape, a length and a grade: a W24x68, 28 ft 6 in, ASTM A992. The word girder only appears on the drawing and the shop ticket. If you are reading a framing plan for the first time, the piece marks and the size callouts are what you actually need, and our guide on how to read steel beam sizes walks through the notation.
Where the label does carry weight is in coordination. Girders are usually the first steel set in a bay, they control the floor-to-floor height because they are the deepest member, and their connections take the most detailing. Mislabeling a girder as a beam on a submittal is the kind of small error that turns into a field fix.
Frequently Asked Questions
Is a girder stronger than a beam?
Not inherently. A girder is normally larger because it carries more, but a heavily loaded beam can easily be bigger than a lightly loaded girder somewhere else in the same building. Strength comes from the section and the steel grade, not from the name.
Can a joist rest on another joist?
No. Joists are sized to carry deck, not other framing. The moment a member picks up load from other members it is doing the job of a beam, and it has to be designed for that.
What is the difference between a girder and a header?
A header is a short beam over an opening such as a door or window, carrying the load around the hole and into the studs or jambs beside it. A girder runs through the structure on a column line. Both are beams by function; the header is defined by where it sits.
Are joists and rafters the same thing?
They are both repetitive framing members, but joists are horizontal and carry floor or ceiling loads, while rafters are sloped and carry the roof. A sloped member also has to deal with thrust at its base, which a flat joist does not.
Does a girder always sit on a column?
Usually, because that is the shortest route to the foundation, but a girder can also land on a bearing wall, a pilaster, or in a transfer situation on another girder below. Transfers concentrate enormous load into one point and always need engineering attention.
The Takeaway
Joist, beam and girder are job titles, not part numbers. Read them as a position in the load path: deck to joist, joist to beam, beam to girder, girder to column. Once you can trace that chain on a drawing, the naming stops being confusing and the sizes start to make sense.
Sizing any of these members for a real project is engineering work. Spans, loads, bracing, connections and deflection limits all interact, and a licensed structural engineer has to size and check them.
Reference information only, not engineering advice. Disclaimer.