A steel column for a house is sized from the load it carries, its unbraced height and the steel section chosen. The load comes from the floor area the column supports (its tributary area) times the floor loads, plus anything bearing on it from above. The height controls buckling, which is usually what limits a slender column. In Ontario, columns within Part 9 of the Building Code have a minimum steel pipe size, and anything beyond that scope is designed by an engineer under Part 4. Treat this guide as an explanation of the process, not a substitute for a designer or engineer.
Step 1: trace the load path
Before any numbers, work out what the column actually holds up. In a typical house, the load path runs like this:
- Roof and snow onto rafters or trusses, which bear on exterior walls (and sometimes interior bearing walls).
- Upper floors onto joists, which bear on walls and beams.
- Beams onto columns.
- Columns onto footings, and footings onto soil.
A basement column under the main beam usually carries floor load from one or two floors. It can also pick up much more: a bearing wall from upstairs landing on the beam above it, a point load from a roof beam, or the end of a beam that replaced a removed wall. Those concentrated loads are the most common reason a column needs engineering.
Step 2: work out the tributary area
The Ontario Building Code's explanatory notes describe the Part 9 approach in plain terms: the load on a column is the supported area times the live load per unit area. The supported area is:
- Along the joists: half the joist span on each side of the beam.
- Along the beam: half the beam span on each side of the column.
Example. A basement column sits under the main beam. Joists span 4.2 m on each side of the beam, and the columns are 3.0 m apart.
- Supported joist length: half of 4.2 m plus half of 4.2 m = 4.2 m
- Supported beam length: half of 3.0 m plus half of 3.0 m = 3.0 m
- Tributary area: 4.2 x 3.0 = 12.6 m² per floor
Table 4.1.5.3. of the 2024 OBC lists a specified live load of 1.9 kPa (kN/m²) for residential areas. So:
- One floor: 12.6 x 1.9 = about 24 kN
- Two floors: 2 x 24 = about 48 kN
That second number is worth noticing. Adjustable steel columns (teleposts) built to CAN/CGSB-7.2 are limited by Article 9.17.3.4. to imposed loads of 36 kN or less, and the code's own note warns that Part 9 spans and loads can push a column past that. A column under two floors at this spacing is already over the limit. Our article on whether teleposts can be permanent covers what to do about it.
Spacing matters just as much as storeys. Move the columns from 3.0 m apart to 4.5 m and each one carries 50% more. Our guide on how far apart basement columns should be works through that trade-off.
Step 3: add dead load and factors (engineered design)
The Part 9 approach above uses live load only, on the stated assumption that the full live load won't act over the whole floor at once. A column designed under Part 4 has to consider all the loads:
- Dead load: the weight of the floors, walls, finishes and the structure itself.
- Live load: occupancy, from Table 4.1.5.3.
- Snow load: where roof load reaches the column, from the climatic data for the location.
Part 4 then applies load factors. For the main gravity case, Table 4.1.3.2.-A of the 2024 OBC combines 1.25 x dead load + 1.5 x live load, with snow as a companion load. The column's factored resistance has to equal or exceed the factored load. This is the engineer's job, and it is where point loads, eccentric connections and unusual layouts get handled properly.
Step 4: check height and slenderness
A steel column rarely fails by crushing. It fails by buckling: bowing sideways under load. Two things control that:
- Unbraced length. The height between points where the column is held sideways. In a basement this is usually the full height from footing to beam, unless something braces it partway.
- Radius of gyration. A property of the cross-section that measures how well the steel is spread away from the centre. Wider sections with the steel at the outside (square HSS, round tube) do well; small solid or narrow sections do badly.
The ratio of the two, called slenderness, drives the capacity. The same post carries noticeably less as it gets taller, which is why a column that works in a 2.1 m crawlspace may not work in a 3.0 m basement. CSA S16 sets the compressive resistance of steel columns as a function of area, yield strength and slenderness, and it treats Class C and Class H HSS differently because of their residual stresses.
The end connections matter too. A column that is properly fastened at the top to the beam and at the bottom to the footing behaves as designed. One wedged in with shims is a different, weaker column.
Step 5: choose the section
With the factored load and the unbraced length known, the designer picks a section that works:
| Option | Typical use in houses | Notes |
|---|---|---|
| Steel pipe column | Part 9 basement columns | Minimum 73 mm outside diameter and 4.76 mm wall under Article 9.17.3.1. |
| Square HSS | Engineered posts, replacements for teleposts | Equal strength both ways, flat faces for plates and fastening |
| Rectangular HSS | Posts inside walls | Fits a wall cavity; weaker about its narrow axis |
| W-section column | Heavier loads, larger openings | Easy to connect to steel beams; needs bracing about the weak axis |
The code allows sizes other than the minimum pipe where the capacity is shown to be adequate (Sentence 9.17.3.1.(2)), which is how engineered HSS posts end up on house drawings. Engineers use load tables such as those in the CISC Handbook of Steel Construction to check a section quickly against a factored load and length; our explainer on HSS columns and load tables shows how those tables work.
Wall thickness follows the same logic. A thicker wall adds area and stiffness, but it also adds weight. The designer picks the lightest section that carries the load at the height required, then checks it is practical to connect.
Step 6: detail the ends and the footing
A correctly sized column can still fail at its ends. The Part 9 rules cover the basics:
- End plates: at least 100 mm x 100 mm x 6.35 mm at each end (Article 9.17.3.2.), with the top plate across the full width of a wood beam. Over a steel beam, the top plate can be omitted where the column is attached to the beam another way.
- Fastening: the column must be fastened to the member it supports and laterally supported.
- Footing: the column must sit centred on a footing. Table 9.15.3.4. gives minimum column footing areas of 0.40 m² supporting one floor and 0.75 m² supporting two floors, for columns spaced 3 m on centre, adjusted for other spacings.
The footing is often the weak link in older houses. A bigger load on the same small pad is a common cause of settlement. Our full summary of the basement column rules and our column footing guide cover these in more detail.
When you need an engineer
You can understand the process from this guide, but the column size on your permit drawings has to come from a qualified designer or engineer. Expect engineering when:
- The column is outside the Part 9 scope in Article 9.17.1.1. (more than two wood-frame floors, supported joist lengths over 5 m, or live loads above 2.4 kPa).
- A wall, post or beam is being removed, so loads are being moved.
- A concentrated load lands on the beam above the column.
- The column is taller than usual, or not braced at the top.
- You want to replace a telepost carrying a load over its rated limit.
In Ontario municipalities, changing a structural column is normally a building permit matter.
Getting steel columns made
Once the size is set, we fabricate steel and HSS posts with base and cap plates welded on, to your drawings, with CWB certified shop welding. We also supply and install basement beams and posts, and engineering services are available for projects that need them.
Send your drawings or measurements for a quote, or contact us through the contact page with questions about your project.
Frequently asked questions
How much weight can a steel post hold?
There is no single figure. A steel post's capacity depends on its cross-section, wall thickness, steel grade and unbraced height, and on its end connections and footing. A short, thick-walled post can carry far more than a tall, thin one of the same outside size. Capacity comes from CSA S16 design or published load tables, checked by a designer or engineer.
What size steel column do I need for a 2 storey house?
It depends on how much floor area the column supports, not just the number of storeys. Two floors over the same tributary area roughly double the load, which can push a column past what an adjustable steel column is rated for. A designer or engineer sizes it from the actual layout.
What is the minimum steel column size in the Ontario Building Code?
For columns within the scope of Part 9, Article 9.17.3.1. requires steel pipe columns to have an outside diameter of at least 73 mm and a wall thickness of at least 4.76 mm, unless another size is shown to have adequate capacity. Each end needs a steel plate at least 100 mm x 100 mm x 6.35 mm.
How thick should a steel column be?
Wall thickness is chosen with the outside size to give the column enough area and stiffness for its load and height. Under Part 9 the minimum pipe wall is 4.76 mm, but engineered columns often use HSS with walls of 6.4 mm or more. The design, not a rule of thumb, sets it.
