A steel beam can sit on wood posts, as long as each post is sized for the load, is at least as wide as the beam it carries, stands on a proper footing, and is tied to the beam with a designed connection. The usual detail is a steel cap or saddle on the post, fastened into the wood and bolted to the beam's bottom flange. Whether wood is the right choice is another question: under a steel beam, many designers prefer steel posts because they're slimmer for the same load and connect more simply.
This post covers what the 2024 Ontario Building Code says about wood columns, how steel to wood connections are detailed, and when to choose steel posts instead. Any beam and post layout in a house needs to come from a qualified designer or engineer, and changing one usually needs a building permit.
When wood posts end up under steel beams
The combination comes up more than you might expect:
- Exposed timber posts in a renovated main floor or open basement, chosen for looks, carrying a steel beam that replaced a wall.
- Decks and porches, where a steel beam spans further than wood but the owner wants wood posts to match the rest of the structure.
- Existing wood posts that a renovator hopes to reuse under a new steel beam.
- Garages and outbuildings framed in wood with a steel beam over a wide door or open bay.
In each case the question isn't whether wood can carry a steel beam (it can) but whether the specific post and connection have been checked for the load.
What the Ontario Building Code says about wood columns
Section 9.17 of the 2024 OBC covers columns in houses and small buildings, within its scope: columns under beams carrying no more than two wood-frame floors with supported joist lengths up to 5 m, deck beams within set limits, and carport roofs. Columns outside that scope are designed under Part 4. For wood columns, the key rules are:
| Rule | Requirement | Article |
|---|---|---|
| Width | Not less than the width of the supported member | 9.17.4.1.(1) |
| Minimum size | 140 mm x 140 mm rectangular or 184 mm round, unless calculations show smaller sizes are adequate | 9.17.4.1.(2) |
| Material | Solid, glued-laminated or built-up; built-up columns have specified bolting or nailing | 9.17.4.2 |
| Contact with concrete | Separated from concrete in contact with the ground by 0.05 mm polyethylene or Type S roll roofing | 9.17.4.3 |
| Footing | Centrally located on a footing conforming to Section 9.15 | 9.17.2.1 |
| Fastening | Securely fastened to the supported member | 9.17.2.2.(1) |
| Lateral support | Laterally supported to resist racking, directly or through the supported members | 9.17.2.2.(2) |
The width rule matters with steel
The supported member is the beam, and for a steel beam the width that bears on the post is the flange. Flange widths vary a lot between sections of similar depth:
| Section | Flange width |
|---|---|
| W200x27 | 133 mm |
| W250x33 | 146 mm |
| W150x22 | 152 mm |
| W310x39 | 165 mm |
Flange widths from the CISC section tables. See our W-shape size chart for other sections.
A dressed 6x6 post is 140 mm square. That meets the minimum column size, but it's narrower than the flange of a W250x33 or a W150x22, so under those beams a 6x6 doesn't satisfy the width rule on its own. Options include a larger post, a built-up post, or a different section; the designer resolves it.
How a steel beam to wood post connection is detailed
The connection has three jobs: spread the beam's load onto the end grain of the post, hold the beam in place, and keep the beam from rolling over at its support. A typical detail includes:
- A cap plate or saddle. A steel plate at least as wide as the flange sits on the post. On a saddle, side plates run down the faces of the post, forming a U that is lagged or through-bolted into the wood.
- A beam connection. The cap is bolted to the beam's bottom flange through drilled holes, or welded to it in the shop.
- Restraint at the support. A steel beam needs to be held against twisting where it sits on a support. Side plates on the saddle, web stiffeners, or blocking between the beam and adjacent framing do this, as the engineer details.
- Base detail. At the bottom, the post sits on a footing or pier with a post base that keeps the end grain off wet concrete and anchors the post against movement. Indoors, Article 9.17.4.3 requires a moisture barrier where wood sits on concrete in contact with the ground.
For the reverse situation, a wood beam carried on steel posts, see connecting a steel post to a wood beam.
Bearing on the wood
Wood is strong along the grain, which is how a post carries load. The cap plate's job is to spread the beam's reaction across the full post section. A narrow flange bearing directly on wood concentrates load on a small area and can crush the fibres or split the post top. That's another reason the plate, not the flange, should bear on the wood.
Steel post to steel beam: the simpler connection
Under a steel beam, a steel post is often simpler. Section 9.17.3 requires steel columns to have end plates of at least 100 mm x 100 mm x 6.35 mm, but Sentence 9.17.3.2.(2) lets the top plate be omitted where the column supports a steel beam and is attached to it another way. In practice, the post is fabricated with a cap plate drilled to match holes in the beam flange and the two are bolted on site.
Compared with wood, a steel post:
- Is smaller for the same load, which matters in a finished basement.
- Doesn't shrink, rot or split at the top.
- Connects steel to steel without lag screws into end grain.
Our comparison of lally columns and wood posts covers the trade-offs in more detail, and basement column rules in the Ontario Building Code covers the steel column requirements.
When wood posts are the right call
Wood posts under steel beams make sense when:
- The posts are exposed and the look matters, such as timber posts in a great room.
- The rest of the structure is wood, such as a deck, porch or garage, and matching materials keeps detailing consistent.
- An existing, sound, properly founded wood post can be reused after the designer checks it for the new load and width.
They make less sense in tight basements, where a 140 mm or larger post takes floor space a steel post wouldn't, or where moisture at the base is a problem.
Steel beams with the right plates
We fabricate steel beams with drilled flanges, welded or loose cap plates and saddles to your engineer's details, along with steel posts and base plates if you go that way, and deliver across the GTA and Ontario. Send us your drawings or connection details for a quote, or see the beams, posts and plates we fabricate.
Frequently asked questions
Can a steel beam sit on wood posts?
Yes, when the posts are sized for the load and the connection is designed. Under the 2024 Ontario Building Code, a wood column must be at least as wide as the member it supports, sit on a proper footing, and be securely fastened to the beam. Many designers still choose steel posts under steel beams.
How do you connect a steel beam to a wood post?
Usually with a steel cap or saddle on top of the post: a plate wide enough to carry the beam flange, fastened down into the post and bolted or welded to the beam. Side plates or a U-shaped bracket stop the beam from rolling and the post top from splitting. The engineer details it.
What size wood post for a steel beam?
The 2024 OBC sets minimums of 140 mm x 140 mm for rectangular wood columns and 184 mm for round ones unless calculations show smaller sizes work, and the column must be at least as wide as the supported member. The actual size depends on the load and height.
How do you connect a steel post to a steel beam?
A steel post is normally fitted with a cap plate that is bolted or welded to the beam's bottom flange. The 2024 OBC allows the top plate to be omitted under a steel beam where the column is attached to the beam another way.
