There is no fixed span-to-depth ratio for steel beams. In the Ontario Building Code's span table for house floor beams, the maximum span works out to about 21 to 38 times the beam's depth, depending mainly on how much floor the beam supports. The commonly quoted shortcut, depth in inches equals half the span in feet (about span/24), lands inside that range but can be well off for a specific beam. Use span-to-depth ratios for early planning of headroom and ceiling heights, then size the beam properly.
This post tests the rules of thumb against real code values, explains why the ratio moves, and shows where it stops being useful.
The common rules of thumb
You'll hear a few versions on site and online:
- "Depth in inches is half the span in feet." A 20 ft span gets a 10 in (W250) beam. In metric, that's a span-to-depth ratio of about 24.
- "Span over 20." A more cautious version for floor beams that carry heavier loads.
- For trusses, much deeper. UK guidance from SteelConstruction.info puts efficient roof trusses at a span-to-depth ratio of about 10 to 15, because a truss is light and needs depth to be stiff.
All of these are planning shortcuts, not code values. CSA S16 and the Ontario Building Code don't set a span-to-depth ratio for steel beams. They set strength requirements and deflection limits, and the depth falls out of those.
Testing the rules against the Ontario span table
The best public data for house beams is OBC Table 9.23.4.3., which gives maximum spans for steel beams supporting floors in dwelling units. Its stated assumptions are simple spans, a laterally supported top flange, 350 MPa steel, a span/360 deflection limit, a 1.9 kPa live load and a 1.5 kPa dead load.
Dividing the table spans by the actual beam depths (from the CISC section tables) gives these span-to-depth ratios:
| Section (actual depth) | 1 storey, 2.4 m joists | 1 storey, 6.0 m joists | 2 storeys, 2.4 m joists | 2 storeys, 6.0 m joists |
|---|---|---|---|---|
| W200x27 (207 mm) | 7.3 m, ratio 35 | 5.8 m, ratio 28 | 6.4 m, ratio 31 | 4.4 m, ratio 21 |
| W250x33 (258 mm) | 9.2 m, ratio 36 | 7.3 m, ratio 28 | 8.2 m, ratio 32 | 5.5 m, ratio 21 |
| W310x39 (310 mm) | 11.4 m, ratio 37 | 9.0 m, ratio 29 | 10.0 m, ratio 32 | 6.7 m, ratio 22 |
"Joists" here means the supported joist length: half the sum of the joist spans on both sides of the beam. Our guide to the Ontario steel beam span tables explains how to work it out.
Two things stand out:
- Within one load case, the ratio is fairly steady across sizes. For a lightly loaded beam (one storey, short joists), all three sections span around 35 to 37 times their depth. For the heaviest case in the table, all three drop to about 21 or 22.
- The load changes the ratio more than the size does. Going from one storey with short joists to two storeys with long joists cuts the allowable span-to-depth ratio by about 40 percent.
So "half the span in feet" (about 24) is conservative for a lightly loaded basement beam and slightly unconservative for a beam carrying two floors with long joists on both sides. And those are only the cases the table covers. A beam carrying a bearing wall, a post from above or a roof with snow can need a much lower ratio.
Why the ratio moves: stiffness and deflection
For a simply supported beam under uniform load, the maths behind deflection gives a neat result. If a beam with symmetric flanges is stressed to a given level in bending, its deflection as a fraction of the span is proportional to:
(stress / modulus of elasticity) x (span / depth)
Rearranged, the span-to-depth ratio a beam can reach is proportional to the deflection limit, times the modulus, divided by the working stress. That explains most of the behaviour you see in practice:
- More load means higher stress, so a lower ratio. That's the trend in the table above.
- A stricter deflection limit lowers the ratio in direct proportion. Change the limit from span/360 to span/600, as the OBC roof-and-floor tables do for beams carrying brick veneer, and at the same stress the beam can only reach 360/600, or 60 percent, of the span-to-depth ratio.
- Higher strength steel doesn't help stiffness. All structural steel has a modulus of about 200 000 MPa. A stronger grade lets the beam carry more stress, but if deflection governs, the beam still has to be as deep.
That's why depth is the most powerful lever on stiffness, and why rules of thumb are really rules about deflection. Our post on steel beam deflection limits covers the limits themselves.
Depth isn't everything: weight matters too
A span-to-depth ratio uses only depth, but beams in the same depth family vary a lot in weight. From the same OBC table, at a 4.2 m supported joist length and one storey:
| Section | Actual depth | Maximum span |
|---|---|---|
| W200x31 | 210 mm | 6.8 m |
| W250x33 | 258 mm | 8.0 m |
| W250x39 | 262 mm | 8.6 m |
The W200x31 and W250x33 weigh almost the same, but the deeper beam spans 1.2 m farther. The W250x39 is the same depth family as the W250x33 and only 4 mm deeper, but its heavier flanges add 0.6 m of span. A pure depth rule can't see that difference.
The trade-off matters in basements, where headroom is often the real constraint. A designer can usually choose between a deeper, lighter beam and a shallower, heavier one for the same span. Comparing steel with wood shows the same idea: steel gets the job done in less depth, as covered in steel beam vs LVL strength, span and depth.
Where rules of thumb break down
Span-to-depth ratios assume a simple span with a roughly uniform load. They mislead in these cases:
- Point loads. A post from the floor above landing near mid-span can govern the size outright.
- Cantilevers. A cantilever deflects far more than a simple span of the same length, so it needs a much lower span-to-depth ratio.
- Brick or stone above. The stricter deflection limit cuts the allowable ratio.
- Unbraced beams. A beam whose compression flange isn't held sideways may be governed by buckling, not deflection. See lateral bracing and unbraced length.
- Floor vibration. Long, shallow beams can meet deflection limits and still feel bouncy.
- Heavier occupancies. Offices, retail, storage and parking all carry more load than a house floor, so their beams run at lower ratios.
How to use span-to-depth ratios properly
Used well, a ratio is a planning tool:
- Early layout. Estimate whether a beam will be closer to 200 mm or 300 mm deep, to see if it fits below the joists or has to be flush.
- Comparing post layouts. Halving a span can cut the depth needed by half or more, which can be worth an extra post and footing.
- Checking drawings. If a beam on a drawing is far outside the usual range for its loading, ask why.
For the beam that actually gets built, the size comes from the code span tables within their limits or from a designer or engineer. Our homeowner sizing guide explains the inputs.
Getting your beam made
Once the section is on your drawings, we can supply and fabricate it, cut to length with end plates, bearing plates, holes and camber as specified, in our CWB certified shop. We deliver across the GTA and Ontario, and we supply and install basement beams and posts.
Send your drawings for a quote, or see the steel beams and other products we fabricate.
Frequently asked questions
What is a good span-to-depth ratio for a steel beam?
There is no single right ratio. In the Ontario span table for house floor beams, the allowed span works out to roughly 21 to 38 times the beam depth, depending on how much floor the beam carries. Heavier loads and stricter deflection limits push the ratio down.
What is the rule of thumb for steel beam depth?
A commonly repeated shortcut is that beam depth in inches should be about half the span in feet, which is a span-to-depth ratio of about 24. It is a rough starting point for planning headroom, not a sizing method.
Does a deeper steel beam span farther?
Generally yes, because depth adds stiffness much faster than it adds weight. But weight matters too: in the Ontario table a W250x39 spans farther than a deeper W310x31 when carrying two storeys at longer joist lengths.
What span-to-depth ratio is used for steel trusses?
Trusses are deeper relative to their span than beams. UK steel design guidance from SteelConstruction.info suggests a span-to-depth ratio of about 10 to 15 for efficient roof trusses.
