HSS Steel Posts for Basements and Decks: Sizes and Options

HSS steel posts for houses are square, rectangular or round steel tubes, typically in the 3 to 5 in (76 to 127 mm) range, fitted with welded base and cap plates. The exact section and wall thickness depend on the load and the post height, so they come from the designer or engineer. The 2024 Ontario Building Code sets the floor: steel pipe columns at least 73 mm in outside diameter with a 4.76 mm wall, and end plates of at least 100 mm x 100 mm x 6.35 mm, with other sections allowed where their capacity is shown to be adequate.

This guide covers the HSS sizes that show up under basement beams and decks, the code rules for steel posts, and the details that matter when ordering. For the full Canadian HSS chart, see our HSS sizes guide.

Common HSS post sizes for houses

HSS (hollow structural section) is designated by outside dimensions and wall thickness in millimetres, for example HSS 89x89x6.4. Here are some sizes in the range used for residential posts. Masses are per metre, from the CISC section tables.

The round 73.0 x 4.8 section meets the OBC minimum for pipe columns. None of these is a default choice: a post's capacity drops as it gets taller, because slender columns buckle before the steel itself yields, so the same load on a taller post can need a bigger section.

Canadian HSS is commonly made to CSA G40.21 grade 350W. Some tube is made to the US standard ASTM A500, which uses different design properties; your engineer should design for the material that's actually supplied. Our HSS sizes guide explains the difference.

What the Ontario Building Code says about steel posts

Section 9.17 of the 2024 OBC (O. Reg. 163/24, Division B) covers columns in houses and small buildings. Its scope (Article 9.17.1.1) includes:

• Columns supporting beams that carry no more than 2 wood-frame floors, where the supported length of joists doesn't exceed 5 m and the live load on any floor doesn't exceed 2.4 kPa.
• Columns supporting beams or header joists that carry no more than 2 levels of wood-frame balconies, decks or other accessible exterior platforms (or 1 level plus the roof), with joist lengths up to 5 m, a combined snow and occupancy load of not more than 4.8 kPa, and a platform serving a single residential suite.
• Carport roofs.

Columns outside that scope are designed under Part 4. Within it, the rules for steel columns are:

Two points that trip people up:

• Square HSS isn't named in the code. The 73 mm and 4.76 mm minimums are for pipe columns. Square and rectangular HSS posts are used under Sentence 9.17.3.1.(2), where their capacity is shown to be adequate, which in practice means the designer has checked them.
• The top plate can be omitted under a steel beam when the column is attached to the beam another way, such as a bolted or welded connection (9.17.3.2.(2)).

Adjustable columns (teleposts) have their own rule, including a 36 kN load limit. See teleposts: can they be permanent.

Basement posts: what to plan for

In a basement, an HSS post usually replaces a telepost or supports a new beam after a wall is removed. Things to sort out before ordering:

• The footing. Every post sits on a footing sized for its load. A post on the slab with nothing under it doesn't comply, regardless of the post. The footing size is part of the design.
• The height. Fixed posts are cut to a measured length. Measure after the beam is in its final position and the structure is properly supported, from the top of the footing or slab to the underside of the beam, and allow for the plates.
• The cap plate. Under a wood beam, the cap plate must extend across the full width of the beam, and it's usually drilled for fasteners into the beam. Under a steel beam, the plate is drilled to bolt to the flange, or the post is connected another way the designer details.
• The base plate. Drilled for anchors into the footing or slab, as the design shows. Our post on embedding steel posts vs base plates compares the options.
• Finishing. If the post will be framed in, a square section is easier to box than a round one, and a fixed post removes the temptation for anyone to adjust a screw later.

Moving, adding or removing basement posts is a structural alteration that needs a designer and, in GTA municipalities, a building permit. For spacing between posts, see how far apart basement columns should be.

Deck posts: exterior details

Steel posts under decks are increasingly common, especially for tall decks, long beam spans or a slimmer look than large wood posts. The exterior setting changes a few things:

• Corrosion protection. The OBC requires at least one coat of rust-inhibitive paint on exterior steel columns susceptible to corrosion. Many owners go further with a primer and finish system or another protective coating; we can prime or galvanize your posts to suit the job.
• Keep water out of the tube. An open-topped or poorly sealed tube can collect water, which rusts the inside and can freeze. Fully welded cap and base plates keep the tube sealed.
• Keep the base dry. A post bearing on a footing or pier that stands above grade drains better than one sitting in soil or a puddle.
• Lateral support. Columns have to be laterally supported to resist racking. Short columns (not more than 600 mm from finished ground to the underside of the supported member) under a deck with no superstructure are exempt (9.17.2.2.(3)). The code's explanatory note warns that large decks on tall columns are likely to need additional lateral support even when they're connected to the house on one side.
• Beam connection. Wood deck beams sit on the cap plate and are fastened through it; the plate must cover the full beam width.
• Footings below frost. Deck footings follow the same foundation rules as the rest of Part 9. Your designer or building department will confirm the depth for your site.

Ordering HSS posts

To get posts that fit the first time, send:

1. The section, such as HSS 89x89x6.4, and the grade if the drawings specify one.
2. Lengths, measured on site, and whether they include the plates.
3. Plate sizes, thickness and holes for the base and cap, or the drawing detail that shows them.
4. The beam above: wood or steel, its width, and how it connects.
5. Interior or exterior, so the finish suits where the post goes.
6. Quantity and delivery address.

Posts are often ordered with the beam they support, which keeps the connections and plate holes consistent. The cost side is covered in lally column and steel post cost in Canada.

Steel posts made to your drawings

We fabricate HSS and steel posts for basements and framing with base and cap plates welded on, along with the beams and plates they connect to, and we supply and install basement beams and posts. Send us your drawings or measurements for a quote, or see the steel products we fabricate.