A steel beam that supports a second floor is sized from three things: the span, the length of floor joists bearing on it, and whether a roof also comes down onto it. Under the 2024 Ontario Building Code, a 5 m main floor beam carrying only second floor joists can be as light as a W200x21, but if the same wall line also carries the roof, the roof-and-floor tables call for something closer to a W200x42 or W250x33. Getting the load path right matters more than any single table value.
This post explains how to tell which situation you have, which code table applies to each, and when the beam has to be engineered. It's for understanding a design and checking a quote, not for skipping the designer: a beam carrying a second floor is a structural alteration that needs a qualified designer or engineer and a building permit.
What "supporting a second floor" can mean
People search for this phrase in three quite different situations.
| Situation | Where the beam is | What it carries | Code table that may apply |
|---|---|---|---|
| Opening a main floor wall | Main floor ceiling | Second floor joists | Table 9.23.4.3, one storey |
| Same, with roof load | Main floor ceiling | Second floor joists plus a wall carrying the roof | Tables 9.23.4.3.-A to -J |
| Basement beam in a 2 storey house | Basement ceiling | Main floor, plus a bearing wall carrying the second floor | Table 9.23.4.3, two storeys |
The difference between rows can be two or three section sizes. Here's how to work out which row is yours.
Look at what's directly above. If the wall being removed has another wall standing on top of it on the second floor, follow that wall up. Does it hold up ceiling joists, rafters or a roof beam in the attic? If the roof is framed with trusses spanning from exterior wall to exterior wall, interior walls often carry no roof load. With rafters and ceiling joists, interior walls often do.
Look at what's below. A main floor bearing wall usually sits on a beam or wall in the basement. When you open the main floor, that basement beam keeps doing its job, but the new beam's ends deliver concentrated loads that have to reach a footing. More on that below.
Main floor beam carrying only the second floor
When a main floor wall carries the second floor joists and nothing else, the new beam supports one storey in Table 9.23.4.3. That table covers steel beams supporting floors in dwelling units, assuming uniform load through joists, a laterally supported top flange, Grade 350W steel and a deflection limit of span/360.
The supported joist length is half the sum of the joist spans on both sides. Second floor joists of 3.4 m and 3.8 m give 3.6 m.
Basement beam carrying two floors
In a two-storey house, the main basement beam typically carries the main floor joists and a bearing wall above that carries the second floor. That is the two storeys supported half of Table 9.23.4.3.
Here's how the two halves compare at a 4.2 m supported joist length. Each cell is the shallowest section in the table that reaches the span.
| Span | One storey supported | Two storeys supported |
|---|---|---|
| 4.0 m | W150x22 | W200x21 |
| 5.0 m | W200x21 | W200x27 |
| 6.0 m | W200x27 | W250x33 |
Derived from Table 9.23.4.3, 2024 OBC, Division B.
One caution: the appendix note behind Table 9.23.4.3 lists floor loads only (1.9 kPa live, 1.5 kPa dead). If the roof also bears down through the same wall line, ask your designer whether the table still applies or whether the beam needs a Part 4 design. For full lookups by span, see our metric guide for 3 m to 6 m spans and our 20 to 24 foot guide.
When the roof comes down on the beam too
The 2024 OBC has a second set of steel tables, Tables 9.23.4.3.-A to -J, for beams supporting a roof and one floor in dwelling units, where the beam also carries a stud wall. They're arranged in pairs by specified roof snow load (1.0 kPa to 3.0 kPa):
- A, C, E, G and I: beams supporting exterior stud walls with brick veneer.
- B, D, F, H and J: beams supporting interior stud walls, or exterior stud walls with siding.
Each table has two inputs: supported roof length and supported floor length, each half the sum of the framing spans on both sides of the beam. Your designer picks the table from the snow load for your location, which comes from MMAH Supplementary Standard SB-1, "Climatic and Seismic Data."
To see how much the roof adds, take an interior wall on the main floor carrying the second floor and the roof, with 3.6 m of supported floor and 3.6 m of supported roof, at a 1.5 kPa snow load (Table 9.23.4.3.-D). Compare it with the same beam carrying the floor only (Table 9.23.4.3, one storey, 3.6 m supported joist length):
| Span | Floor only | Floor and roof (Table -D) |
|---|---|---|
| 4.0 m | W150x22 | W150x37 |
| 5.0 m | W200x21 | W200x42 |
| 6.0 m | W200x27 | W250x49 |
Derived from the 2024 OBC, Division B. Shallowest section in each table that reaches the span.
Brick veneer is heavier again. Its tables use a deflection limit of span/600 for brick and live load, because brick cracks if its support sags. In Table 9.23.4.3.-C (brick, 1.5 kPa snow), the same 3.6 m roof and floor lengths need a W250x33 to reach 4.0 m. Openings in brick exterior walls are covered in our post on steel beams for additions and exterior wall openings.
Point loads, flush beams and other reasons to engineer it
All of the code tables assume load arrives evenly along the beam. Real second floors often don't cooperate:
- A post or girder truss above. A second floor beam or roof girder that bears on the wall you're removing puts a concentrated load on the new beam.
- A wall running across the beam rather than along it, or a stair opening framed off it.
- Heavy floors such as a tiled bathroom on a concrete topping. The appendix note to Article 9.23.4.4 says 51 mm of concrete topping can add 0.8 to 1.2 kPa of dead load.
- A flush beam set up into the ceiling with joists hung off its web. That changes the top-flange lateral support the tables rely on (Sentence 9.23.4.3.(3) describes joists bearing on the top flange at 600 mm or less with 19 mm x 38 mm strips on both sides).
- Three or more storeys, or a beam continuous over a post.
Any of these sends the beam to a Part 4 design. Our post on whether you need a structural engineer to remove a wall covers who can do that work.
Don't forget the load path down
A beam carrying a second floor gathers load that used to be spread along a wall and delivers it to two points, its ends. Those points need:
- Bearing that is even and level, at least 89 mm long at end supports (Sentence 9.23.8.1.(1)).
- Posts sized for the load. The Part 9 column rules in Section 9.17 apply to columns supporting beams that carry no more than two wood-frame floors, with a supported joist length up to 5 m. Our post on sizing a steel column for a house explains the rest.
- Continuous support to a footing. If a new post lands on the main floor, its load has to travel through blocking or a post in the basement and onto a footing sized for it. Skipping this step is one of the most common reasons a new beam sags.
Getting the beam and posts made
Once the engineer or designer has sized the beam, we fabricate it to length with the cap plates, bolt holes and connections on the drawing, along with the posts and base plates, and deliver to site. We also supply and install basement beams and posts. Send us your drawings or beam schedule for a quote, or see the steel beams and posts we fabricate.
Frequently asked questions
What size steel beam do I need to support a second floor?
It depends on the span, the length of the joists bearing on the beam, and whether a roof also bears on it. Under the 2024 Ontario Building Code table, a 5 m main floor beam carrying only second floor joists with a 3.6 m supported joist length can be a W200x21; if the same wall line also carries the roof, the roof-and-floor tables call for a much heavier section.
How do I know how many storeys a beam supports?
Count the floors whose joists bear on the beam or on walls standing directly on it. A basement beam under a bearing wall that carries the second floor supports two storeys; a main floor beam carrying the second floor joists supports one.
Does a beam supporting a second floor need an engineer?
Usually, yes. The Part 9 tables only cover beams with uniform load through joists and specific conditions. Posts landing on the beam, flush framing, heavy floors or roof loads outside the tables mean the beam is designed under Part 4, typically by a structural engineer.
What size steel beam for a 2 storey house basement?
For a basement beam carrying the main and second floors with a 4.2 m supported joist length, Table 9.23.4.3 gives a W200x27 at 5.0 m and a W250x33 at 6.0 m. Your designer confirms the size against the actual loads.
