For the same span and load, a steel W-beam is far shallower than a wood beam, and it keeps going long after built-up lumber runs out. In the 2024 Ontario Building Code tables, a beam carrying one floor with a 4.2 m supported joist length can span 3.46 m as a 3-ply 2x12 (38 x 286 mm) spruce-pine-fir beam, while a W150x22, about 152 mm deep, is listed for 4.8 m under the same conditions. For SPF lumber at that supported length, the largest built-up beam in the table (5-ply 2x12) stops at 4.46 m, while steel sections in the same code go on to 8 m and more.
Below we line up the two OBC tables so you can see the size difference for common basement and main-floor spans, then look at steel posts versus wood posts.
Where these numbers come from
The 2024 OBC (O. Reg. 163/24) has span tables for both materials in Part 9:
- Tables 9.23.4.2.-H to -K give maximum spans for built-up wood and glued-laminated floor beams. Tables -H, -I and -J cover built-up beams supporting not more than one, two and three floors.
- Table 9.23.4.3 gives maximum spans for steel W-beams supporting floors in dwelling units, for one or two storeys.
Both use the same key input: the supported joist length, which is half the sum of the joist spans on both sides of the beam. Joists spanning 4.0 m on one side and 4.4 m on the other give a supported length of 4.2 m. Both tables apply to residential floor loads, and both assume the load arrives through the joists as a uniform load. The steel table also assumes the top flange is laterally supported by the joists, and our steel beam span tables guide explains what that means.
The wood figures below are for spruce-pine-fir (SPF), No. 1 and No. 2 grade, the most common framing lumber in Ontario. Built-up beams are made from 38 mm thick plies, so a 3-ply beam is about 114 mm wide, a 4-ply about 152 mm and a 5-ply about 190 mm.
Side by side: one floor supported
Maximum spans in metres at a 4.2 m supported joist length, one floor (wood, Table 9.23.4.2.-H) or one storey (steel, Table 9.23.4.3):
| Beam | Depth | Max span |
|---|---|---|
| 3-ply 38 x 235 SPF (2x10) | 235 mm | 2.98 m |
| 4-ply 38 x 235 SPF | 235 mm | 3.44 m |
| 3-ply 38 x 286 SPF (2x12) | 286 mm | 3.46 m |
| 4-ply 38 x 286 SPF | 286 mm | 3.99 m |
| 5-ply 38 x 286 SPF | 286 mm | 4.46 m |
| W150x22 | about 152 mm | 4.8 m |
| W200x21 | about 200 mm | 5.7 m |
| W200x27 | about 207 mm | 6.3 m |
| W250x33 | about 258 mm | 8.0 m |
What that means for some common clear spans:
| Clear span | Smallest built-up SPF in the table | Smallest steel in the table |
|---|---|---|
| 3.0 m (about 10 ft) | 4-ply 2x10 or 3-ply 2x12 | W150x22 |
| 4.0 m (about 13 ft) | 5-ply 2x12 | W150x22 |
| 5.0 m (about 16 ft) | none listed | W200x21 |
| 6.0 m (about 20 ft) | none listed | W200x27 |
At 4.0 m, the wood beam is 286 mm deep and 190 mm wide. The steel beam is about 152 mm deep, which is roughly half the depth, with spare capacity.
Side by side: two floors supported
Two floors stacked on the beam (wood, Table 9.23.4.2.-I; steel, two storeys in Table 9.23.4.3), again at a 4.2 m supported joist length:
| Beam | Depth | Max span |
|---|---|---|
| 3-ply 38 x 235 SPF | 235 mm | 2.23 m |
| 3-ply 38 x 286 SPF | 286 mm | 2.59 m |
| 4-ply 38 x 286 SPF | 286 mm | 2.99 m |
| 5-ply 38 x 286 SPF | 286 mm | 3.34 m |
| W150x22 | about 152 mm | 3.8 m |
| W200x21 | about 200 mm | 4.3 m |
| W200x27 | about 207 mm | 5.3 m |
| W250x33 | about 258 mm | 6.5 m |
With two floors, the gap widens. A 5-ply 2x12 tops out at 3.34 m, while a W200x27 reaches 5.3 m at less depth. This is why steel is so common under the centre bearing line of two-storey houses, especially when the owner wants fewer basement posts.
What the comparison leaves out
The tables are a fair comparison because they use the same inputs, but they aren't identical in every assumption, and they're narrower than people expect:
- Uniform loads only. If a post, a bearing wall or another beam lands on the beam, neither table applies and the beam is engineered.
- Bearing is different. The wood table notes require at least 76 mm of bearing, or 114 mm for 3-ply beams with supported lengths over 4.2 m. Steel beams need at least 89 mm of even, level bearing.
- Concrete toppings. Where floors have a concrete topping of not more than 51 mm, the wood table spans must be multiplied by 0.8. Heavier floors take steel beams outside its table too.
- Flush beams. Both tables picture joists bearing on top of the beam. Flush beams with joist hangers need their own design and connection details.
- Other products. Glulam has its own table (9.23.4.2.-K), and LVL has no OBC table at all. For engineered wood vs steel, see our steel beam vs LVL comparison.
In real renovations, one of these usually applies, which is why most replacement beams are sized by an engineer. The tables are still the best way to understand how much depth steel can save.
Metal post vs wood post
The posts under the beam follow Section 9.17 of the 2024 OBC, and the minimum sizes are quite different:
| Steel column | Wood column | |
|---|---|---|
| Minimum size | Pipe at least 73 mm outside diameter with a 4.76 mm wall (Article 9.17.3.1) | At least 140 x 140 mm, or 184 mm round, and no narrower than the beam it supports (Article 9.17.4.1) |
| Other sizes | Permitted where capacity is shown to be adequate | Smaller permitted only with calculations |
| Ends | Bearing plates at least 100 x 100 x 6.35 mm, top plate across the full width of a wood beam (Article 9.17.3.2) | Separated from concrete in contact with the ground by polyethylene or Type S roll roofing (Article 9.17.4.3) |
| Garages and carports | Section 9.17 | 89 x 89 mm wood columns permitted (Article 9.35.4.2) |
A steel post takes up a fraction of the floor space of a 140 mm wood column and doesn't care about a damp slab the way wood does, which is why fixed steel posts are the usual choice under steel beams. Wood columns are still common under built-up wood beams. Either way, a column needs a proper footing under it. For more on choosing, see lally column vs wood post, and for adjustable posts, can teleposts be permanent.
Fewer posts, bigger beam
The real trade-off is between beam size and post spacing. A 3-ply 2x10 may work at 3.0 m spans, which means a post every 3 m down the basement. Change to a W200x27 and the same beam line can run 6 m between posts in a one-storey case. Fewer posts means fewer footings, but each remaining post and footing carries more load, so they have to be checked too. That's a design decision, and it's worth getting an engineer to compare layouts before you commit.
From size to fabricated beam
Once your designer has chosen the beam and posts, send us the drawings or beam schedule and we'll quote the steel. We supply W-beams and steel posts cut to length with base and cap plates, holes for nailers and connections, delivered across the GTA and Ontario. See our steel products for details.
Frequently asked questions
How much smaller is a steel beam than a wood beam for the same span?
Usually much shallower. In the 2024 OBC tables, a 4.0 m span carrying one floor with a 4.2 m supported joist length needs a 5-ply 38 x 286 mm SPF built-up beam, while a W150x22, about 152 mm deep, is listed for 4.8 m in the same conditions.
Is there a steel beam equivalent to a wood beam span table?
There is no one-to-one conversion chart, but the 2024 OBC has span tables for both. Table 9.23.4.2.-H to -K cover built-up wood and glulam floor beams and Table 9.23.4.3 covers steel W-beams. Using the same supported joist length and number of floors lets you compare them.
What is the minimum size of a steel post or a wood post in a basement?
Under the 2024 OBC, steel pipe columns need at least a 73 mm outside diameter and 4.76 mm wall unless their capacity is shown to be adequate. Wood columns must be at least 140 x 140 mm (or 184 mm round) and no narrower than the beam they support, unless calculations show a smaller size works.
Can I use the span tables to replace my wood beam with steel myself?
No. The tables help you understand the sizes involved, but replacing a beam is a structural alteration that needs a designer or engineer and usually a building permit. Most renovation beams also fall outside the tables because of point loads or flush framing.
