Connecting a Steel Post to a Wood Beam

A steel post connects to a wood beam through a steel plate welded to the top of the post, which the beam bears on and is fastened to, either with screws or bolts up through the plate or with side plates (a U-bracket or saddle) bolted through the beam. Under Part 9 of the 2024 Ontario Building Code, that top plate must be at least 100 mm x 100 mm x 6.35 mm and must extend across the full width of the wood beam, and the post must be securely fastened to the beam. Beyond those minimums, the plate size, the fasteners and the bracket type are design decisions for your designer or engineer.

This post covers the three common ways to make the connection, how to get the bearing right, and what to tell your fabricator. If you are working the other way round, a steel beam sitting on wood posts, see setting a steel beam on wood posts.

What the Ontario code requires at the top of a steel post

For columns in houses that fall under Section 9.17 of the 2024 OBC (beams carrying no more than two wood-frame floors, decks within set limits, and carport roofs), these rules govern the post-to-beam connection:

For reference, 6.35 mm is 1/4 in. Anything outside the Section 9.17 scope, such as a post under a beam carrying a third floor, long-span joists or concentrated loads from above, is designed under Part 4, and the engineer details the connection. Our guide to basement column rules in the Ontario Building Code covers the scope and the rest of the column requirements.

Note what the code doesn't say: it doesn't specify a fastener type, size or number. Those come from the drawings.

Three common ways to connect a steel post to a wood beam

1. Flat cap plate with screws or bolts

The simplest and most common basement detail. A square or rectangular plate is welded to the top of the post, with holes drilled or punched outside the post's footprint. The beam sits on the plate, and lag screws or structural screws go up through the holes into the beam.

• Works well for: basement posts under continuous built-up or engineered wood beams, where the floor system above holds the beam in line.
• Watch for: holes too close to the post (no room for a driver), or too close to the edge of the beam.

2. U-bracket or saddle

A cap plate with two side plates welded up each side, forming a U that the beam drops into. Bolts pass through the side plates and the beam. This is what most people mean by a "steel post top bracket" or "steel post U bracket".

• Works well for: decks and exterior posts, single-span beams, beam ends, and anywhere the beam needs to be held against rolling or sliding sideways.
• Watch for: the inside width of the U must match the actual beam width. Built-up beams are rarely exactly their nominal size, so measure the real beam before the bracket is made. Outside, the U can trap water against the wood unless it is detailed to drain.

3. Knife plate (concealed)

A single vertical plate welded to the top of the post, slotted into a cut in the bottom of a solid timber or glulam beam and bolted or pinned through. It hides the steel, which is why it shows up on exposed timber beams in open-concept homes and timber-frame work.

• Works well for: architectural timber and glulam beams.
• Watch for: this is an engineered connection. The slot, the plate thickness, and the bolt or dowel layout all affect the beam's strength.

Getting the bearing right

The plate does two jobs: it spreads the post load into the wood, and it gives the fasteners something to go through. A few points matter more than people expect.

Full width means the real width. A built-up beam made of 38 mm plies is 114 mm wide with three plies and 152 mm wide with four, so a 100 mm plate is too narrow for either. Order the plate to cover the actual measured width.

Bearing length along the beam. Wood is much weaker when crushed across the grain than along it, and a heavily loaded post can dent into a beam if the plate is short. The designer checks this and may lengthen the plate along the beam rather than make it thicker.

Splices in built-up beams. Article 9.23.8.3 requires butt joints in a built-up wood beam to land over a support, except at the quarter points of a continuous beam where the code permits them. If a joint lands over your post, the plate has to support both pieces with enough bearing for each, which is another reason it is often longer than the minimum.

Even, level contact. The beam should bear on the whole plate. Gaps filled with stacks of wood shims compress over time and let the beam settle. If the post doesn't quite reach, the fix is a post made to the right length, or a shim the designer approves, not whatever is lying around.

Centred on the footing. The connection at the top only works if the post is plumb and centred on its footing, as Article 9.17.2.1 requires. Our post on lally column footing requirements covers the bottom end.

Fasteners and holes

• Shop-made holes are better. Drilling a 6.35 mm or thicker steel plate overhead in a basement is slow and rarely neat. Give the fabricator the hole size and pattern from the drawings and the plate arrives ready.
• Lag screws or through-bolts. Lag screws into the bottom of the beam are standard for flat plates. Through-bolts with washers are standard for U-brackets and knife plates. The designer sets the diameter, length, number and spacing, including distances from the ends and edges of the wood.
• Treated lumber outside. Preservative-treated wood can be harder on steel and fasteners than untreated wood. Use fasteners and coatings that the fastener and lumber treatment manufacturers list as suitable.
• No site welding against wood. Brackets should be welded in the shop. Welding a bracket onto a post with the beam already sitting on it is a fire risk and usually produces a poor weld.

If you are deciding between plate styles, our guide to lally column cap and base plates goes through plate sizes and options at both ends of the post.

Basement posts vs deck and exterior posts

Basement posts carry gravity load from the floors above. The floor system usually holds the beam in line, so a flat cap plate fastened to the beam, plus a properly secured base, is the usual detail.

Deck and porch posts are exposed to wind, frost and uplift, and Article 9.23.6.2 requires exterior columns and posts to be anchored against uplift and lateral movement, with limited exceptions for low, small platforms. A U-bracket bolted through the beam does that job at the top. The steel needs at least one coat of rust-inhibitive paint (Article 9.17.3.3), and many owners choose a fuller coating system. Our post on HSS steel posts for basements and decks covers sizes and exterior details.

Timber and glulam feature beams often use knife plates or custom brackets, and they are engineered connections even in a house.

When the connection needs an engineer

Bring in a designer or engineer, not just a contractor, when:

• the column is outside the Section 9.17 scope, or the beam carries posts or point loads from above
• you are replacing a wood post with a steel one under an existing beam and the load or footing is uncertain
• the post supports a deck that is high, large or carries a roof
• the beam is a glulam, LVL or timber member with its own manufacturer bearing requirements
• the work is part of a permit, such as a load-bearing wall removal, where the connection will be inspected

What to send for fabricated posts

To get posts that fit the beam the first time, send:

1. Post height from the top of the footing or slab to the underside of the beam, measured at each post location
2. Beam width and type (built-up plies, LVL, glulam, timber), measured, not nominal
3. Connection type (flat plate, U-bracket, knife plate) with plate sizes and hole layout from the drawings
4. Base detail at the bottom of the post
5. Interior or exterior, and any coating requirement
6. Quantity and any engineer's drawings

We fabricate steel posts with base and cap plates welded on, including drilled plates and brackets made to your drawings, and our shop welding is CWB certified. Send us your drawings or measurements for a quote, or see our steel products for posts, plates and beams.