Steel Beam Size for 3 m to 6 m Spans (Metric Guide)

For a house floor beam in Ontario, Table 9.23.4.3 of the 2024 Ontario Building Code puts the answer for 3 m to 6 m spans in the W150 to W250 range: a W150x22 covers spans up to 4.5 m for one storey at supported joist lengths up to 5.4 m, and a 6 m span carrying two storeys needs a W250x33 or heavier once the supported joist length reaches 4.2 m. The exact section depends on how much floor the beam carries and how many storeys sit on it.

This guide flips the code table around so you can start from your span. It covers the lookup, how to read it, and the conditions that push a real beam outside the table. Treat it as a way to understand and check a proposal, not as a design: a structural beam in an existing house still needs a designer or engineer and, in GTA municipalities, a building permit.

The two numbers you need first

Table 9.23.4.3 uses two inputs besides the span.

Supported joist length. This is half the sum of the joist spans on both sides of the beam. If the joists run 3.6 m on one side and 4.8 m on the other, the supported joist length is (3.6 + 4.8) / 2 = 4.2 m. The longer the joists, the more floor area loads the beam, and the shorter the span a given section can reach.

Storeys supported. One storey means the beam carries one floor. Two storeys means two floors stacked above it, for example a basement beam under a main floor with a bearing wall above it that carries the second floor. Our post on sizing a beam to support a second floor explains how to count storeys when walls and roofs are involved.

The span itself is the distance between supports, such as a foundation wall and a post, or two posts.

Span-first lookup: one storey supported

The table below lists the shallowest W-section in Table 9.23.4.3 whose maximum span meets or exceeds the span on the left. Shallowest matters in a house because every millimetre of depth comes out of headroom. Where two sections share a depth, the lighter one is shown.

Derived from Table 9.23.4.3, 2024 OBC, Division B. Joist columns are supported joist length.

Two things stand out. Up to about 4.5 m, a single storey of floor is light work for steel, and the smallest section in the table handles it. Past 5 m the table moves into the W200 group, which adds roughly 50 mm of depth over a W150.

Span-first lookup: two storeys supported

Derived from Table 9.23.4.3, 2024 OBC, Division B.

With two storeys on the beam, the jump to deeper sections comes sooner. A 6 m span under a typical two-storey house with joists of 4 m to 5 m on each side lands on a W250, which is about 250 mm deep.

How to use the lookup

1. Measure the span between supports.
2. Work out the supported joist length. The code allows straight-line interpolation between the table columns, so a 4.0 m joist length sits between the 3.6 m and 4.2 m columns. If you don't want to interpolate, use the next column to the right, which is conservative.
3. Count storeys supported.
4. Read the section, then check the full code table in our OBC steel beam span tables guide to see how much margin it has.

Worked example: a 5 m basement beam

A bungalow basement has a built-up wood beam on three posts. The owner wants one post removed, leaving a 5.0 m span. Joists run 3.8 m on one side and 4.2 m on the other, and the beam carries only the main floor.

• Supported joist length: (3.8 + 4.2) / 2 = 4.0 m. Use the 4.2 m column to stay conservative.
• Storeys supported: one.
• Lookup: W200x21, which the full table allows to 5.7 m at 4.2 m.

The same span under a two-storey house reads W200x27 (5.3 m allowed at 4.2 m). The roughly 50 mm of extra depth over a W150 is often what decides whether the beam is installed below the joists or set up into the floor framing.

The beam is only part of the job. Removing a post means the two remaining supports each take more load, so their posts and footings are checked too. Longer spans between posts always mean more load on each post and footing.

What the table assumes, and when it stops applying

The appendix note to Table 9.23.4.3 lists the design basis: simply supported spans, a laterally supported top flange, 350 MPa yield strength, a deflection limit of span/360, and a floor live load of 1.9 kPa with a dead load of 1.5 kPa. Sentence 9.23.4.3.(2) also requires the steel to at least meet CSA G40.21 Grade 350W.

The lookup above is only valid when all of that is true. These common situations push a beam outside the table:

• A post or bearing wall lands on the beam. That is a concentrated load, and the table assumes uniform load through joists.
• The beam carries roof load through a wall above. Tables 9.23.4.3.-A to -J cover some roof-and-floor cases; others are engineered.
• Flush beams with joists hung off the web. The top-flange lateral support detail in Sentence 9.23.4.3.(3) (joists bearing on the top flange at 600 mm or less, with 19 mm x 38 mm strips nailed on both sides) no longer applies as written.
• Heavy floors, such as tile on a concrete topping, or uses above residential live loads.
• Supported joist lengths over 6.0 m or more than two storeys.

In any of those cases the beam is designed under Part 4, which in practice means a structural engineer. The engineered size can come out smaller or larger than the lookup.

Choosing between sections that both work

When two sections both pass, the choice usually comes down to:

• Depth vs headroom. A W150 is about 150 mm deep, a W200 about 200 mm and a W250 about 250 mm. A dropped beam hangs that far below the joists, plus any wood plate on its bottom. Our W-shape size chart lists exact depths and flange widths.
• Weight per metre. The second number in the name is mass in kg/m, so a 5 m W200x27 weighs about 133 kg before plates. Lighter beams are easier to carry into a basement by hand.
• Flange width. Wide flanges give more room for bolting a wood nailer and for joist bearing. Narrow-flange sections are lighter but leave less to work with.
• Availability. Not every section in the code table is equally easy to source. Ask us what's practical for your job before the drawings are finalized.

If your opening is longer than 6 m, see steel beam size for a 20 to 24 foot span and steel beams for 7 m to 10 m spans.

Get the beam made to your design

Once your designer or engineer has picked the section, we fabricate it to length with cap plates, bolt holes for a wood nailer and any connections the drawing shows, and deliver it with the posts and plates. We also supply and install basement beams and posts. Send us your drawings or beam size for a quote, or see the steel beams and posts we fabricate.