For a 20 foot (6.1 m) span carrying one floor of a house, the 2024 Ontario Building Code steel beam table calls for a W200x27 (W8x18) with joists of 3.6 m to 4.8 m; a 24 foot (7.3 m) span calls for a W250x33 (W10x22) at the same joist lengths. Carry two storeys and, at joist lengths of 4.2 m to 4.8 m, the 20 foot beam moves to a W250x33 while the 24 foot beam needs a W310x39 (W12x26) or an engineered solution.
Those answers come from Table 9.23.4.3, which covers steel beams supporting floors in dwelling units. Below are the lookups for 16 to 24 feet, what drives them, and what changes when a beam gets this long. Beams of this length almost always replace a wall or a post line, so they are designed by an engineer or qualified designer and installed under a building permit. Use this to understand the numbers, not to pick a beam.
Why there isn't one answer for 20 feet
A beam's size depends on how much load it carries, not just how far it spans. The Ontario table uses two inputs besides span:
- Supported joist length: half the sum of the joist spans on both sides of the beam. Joists of 12 ft (3.66 m) on one side and 14 ft (4.27 m) on the other give about 3.96 m.
- Storeys supported: one floor, or two floors stacked above.
A 20 foot beam under a narrow bungalow and a 20 foot beam under a wide two-storey house can differ by two section sizes. The table also only applies when the load arrives evenly through joists. Posts, bearing walls or roof load landing on the beam send it to an engineer.
Span lookup: 16 to 24 feet, one storey
Each cell is the shallowest section in Table 9.23.4.3 that reaches the span at that supported joist length. Spans are converted to metres and rounded up to the next 0.01 m.
| Span | Joists 3.0 m | 3.6 m | 4.2 m | 4.8 m | 5.4 m |
|---|---|---|---|---|---|
| 16 ft (4.88 m) | W150x22 | W150x22 | W200x21 | W200x21 | W200x21 |
| 18 ft (5.49 m) | W200x21 | W200x21 | W200x21 | W200x27 | W200x27 |
| 20 ft (6.10 m) | W200x21 | W200x27 | W200x27 | W200x27 | W200x31 |
| 22 ft (6.71 m) | W200x27 | W200x31 | W200x31 | W250x33 | W250x33 |
| 24 ft (7.32 m) | W200x31 | W250x33 | W250x33 | W250x33 | W250x33 |
Derived from Table 9.23.4.3, 2024 OBC, Division B.
At 22 and 24 feet, the W250x24 and W310x31 sections in the table can also work for one storey at some joist lengths. They are deeper but lighter per metre, and less common, so check availability before your designer settles on one.
Span lookup: 16 to 24 feet, two storeys
| Span | Joists 3.0 m | 3.6 m | 4.2 m | 4.8 m | 5.4 m |
|---|---|---|---|---|---|
| 16 ft (4.88 m) | W200x21 | W200x27 | W200x27 | W200x27 | W200x31 |
| 18 ft (5.49 m) | W200x27 | W200x27 | W200x31 | W250x33 | W250x33 |
| 20 ft (6.10 m) | W200x27 | W200x31 | W250x33 | W250x33 | W250x39 |
| 22 ft (6.71 m) | W250x33 | W250x33 | W250x39 | W250x39 | W310x39 |
| 24 ft (7.32 m) | W250x33 | W250x39 | W310x39 | W310x39 | none |
Derived from Table 9.23.4.3, 2024 OBC, Division B. "None" means no section in the table reaches that span.
The two-storey 24 foot row is where the table runs out. With a 5.4 m supported joist length, the heaviest section listed, a W310x39, reaches only 7.0 m. That beam would be engineered as a heavier or deeper section, or the span would be broken with a post.
Imperial names for these sections
US and older drawings name the same beams in inches and pounds per foot. Our guide to reading steel beam sizes explains the conversion.
| Metric | Imperial |
|---|---|
| W150x22 | W6x15 |
| W200x27 | W8x18 |
| W200x31 | W8x21 |
| W250x33 | W10x22 |
| W250x39 | W10x26 |
| W310x39 | W12x26 |
Worked example: a 20 foot opening on the main floor
A homeowner wants to remove the wall between the kitchen and living room, leaving a 20 foot (6.1 m) opening. The second floor joists run 3.8 m to one side of the wall and 4.4 m to the other, so the supported joist length is 4.1 m. Use the 4.2 m column to stay conservative.
- If the wall above carries only the second floor joists: one storey supported, so the lookup gives a W200x27 (W8x18).
- If the roof also bears on a wall directly above: the beam carries a roof and a floor, and Tables 9.23.4.3.-A to -J apply instead, with sizes that depend on snow load and wall type. Our post on sizing a beam to support a second floor walks through that case.
- If a post from above lands mid-span: the beam is engineered.
The full table, including how much margin each section has, is in our OBC steel beam span tables guide.
What a 20 to 24 foot beam means on site
Long beams bring practical questions that short ones don't.
Weight. The second number in the metric name is kg per metre. A 6.1 m W200x27 weighs about 162 kg; a 7.3 m W250x33 about 240 kg; a 7.3 m W310x39 about 283 kg, all before plates. That's a crew and lifting gear, not two people and a dolly. See how to lift a steel beam into a house without a crane.
Getting it in. A 7 m beam has to come through a door, window or opening in one piece, or be fabricated in two pieces with a designed splice. Splices are an engineering detail, not something added on site, and the beam length also has to suit the truck and the route to the door.
Depth. Going from a W200 to a W250 adds about 50 mm, and to a W310 about 100 mm. A dropped beam hangs that far below the ceiling plus any wood plate on its bottom. A flush beam needs the joists hung from its sides, which changes the lateral support assumptions in the table.
Ends and posts. At these spans each end of the beam carries a large share of the floor. The posts, the bearing under them and the footings all get checked. Sentence 9.23.8.1.(1) requires even, level bearing at least 89 mm long at end supports, and the code table assumes a beam laterally supported along its top flange (Sentence 9.23.4.3.(3)).
Breaking the span: one post vs one big beam
At 24 feet, adding a single post near mid-span halves the span and can drop the beam by two or more section sizes, from a W310 to a W150 or W200 for many houses. The trade-off is a post in the room and a new footing under it in the basement. Owners opening up a main floor usually accept the deeper beam; owners finishing a basement often accept a post. Your designer can price both options with the engineering.
For spans beyond 24 feet, see steel beams for 7 m to 10 m spans.
Get a long beam made and delivered
We fabricate beams cut to length with cap plates, drilled flanges for wood nailers, splices where the engineer details them, and the posts and plates that go with them. We deliver across the GTA and Ontario and supply and install basement beams and posts. Send us your drawings or beam size for a quote, or see the steel beams we fabricate.
Frequently asked questions
What size steel beam do I need to span 20 feet?
Twenty feet is about 6.1 m. In Table 9.23.4.3 of the 2024 Ontario Building Code, a beam carrying one storey with a 3.6 m to 4.8 m supported joist length needs a W200x27 (W8x18), and two storeys with a 4.2 m to 4.8 m joist length need a W250x33 (W10x22). Your designer confirms the final size.
Can a steel beam span 24 feet in a house?
Yes. Twenty-four feet is about 7.3 m, and the code table reaches it with W250 and W310 sections. Carrying two storeys with joists longer than about 5 m on average, no section in the table reaches 7.3 m and the beam is engineered or a post is added.
What size I-beam spans 16 feet?
Sixteen feet is about 4.9 m. For one storey, a W150x22 (W6x15) works up to a 3.6 m supported joist length. For two storeys, a W200x27 (W8x18) works from 3.6 m to 4.8 m. Point loads or roof loads put the beam outside the table.
Is a W8 beam enough for 20 feet?
A W8x18 (W200x27) reaches 20 feet in the Ontario table only for one storey with a supported joist length up to about 4.8 m. With two storeys, or with posts or roof loads landing on the beam, a deeper or heavier section is needed.
