Lally columns are most often 3 in, 3-1/2 in or 4 in (about 76 to 114 mm) in outside diameter, with walls ranging from thin 11-gauge tube (about 3 mm) to Schedule 40 pipe (5 to 6 mm), cut or adjusted to the basement height. In Ontario, the 2024 Building Code sets the floor for steel pipe columns at 73 mm outside diameter and 4.76 mm wall thickness, unless a different size is shown to have enough capacity. The right size for your basement comes from the load and the height, not from the diameter alone.
Here is how the sizes work, what the numbers mean and how to read what's already in your basement.
Common lally column diameters
Steel basement columns sold under the lally name in the US usually come in three diameters. One major US manufacturer, for example, lists fixed and adjustable columns in 3 in, 3-1/2 in and 4 in outside diameter, with a standard 11-gauge wall and heavier Schedule 40 options, in lengths up to 12 ft.
| Nominal size | Outside diameter | Typical use |
|---|---|---|
| 3 in | 76 mm | Light loads, shorter basements |
| 3-1/2 in | 89 mm | The common middle size |
| 4 in | 102 mm | Heavier loads or taller columns |
Larger round sections exist, and engineered posts can be any size the designer needs. In Canada, fixed posts are often round or square HSS (hollow structural section), designated in millimetres by outside size and wall thickness. Our HSS sizes chart lists common metric sections, and HSS steel posts for basements and decks covers the sections usually used in houses.
Wall thickness: gauge vs Schedule 40
Two columns with the same diameter can have very different strength because of wall thickness. You'll see two ways of describing it.
Gauge. Light tube is often described by gauge. The 11-gauge wall used on many US columns is 0.120 in (about 3.0 mm) nominal. Higher gauge numbers mean thinner steel.
Pipe schedule. Heavier columns are often made from standard steel pipe, which is sized by nominal pipe size (NPS) and schedule. The nominal size is not the outside diameter, which trips people up:
| Nominal pipe size | Outside diameter | Schedule 40 wall |
|---|---|---|
| 2-1/2 in | 73.0 mm (2.875 in) | 5.16 mm (0.203 in) |
| 3 in | 88.9 mm (3.500 in) | 5.49 mm (0.216 in) |
| 3-1/2 in | 101.6 mm (4.000 in) | 5.74 mm (0.226 in) |
| 4 in | 114.3 mm (4.500 in) | 6.02 mm (0.237 in) |
So a "3-1/2 in Schedule 40 column" in a lally column catalogue is really 3 in nominal pipe with a 3-1/2 in outside diameter. When you compare columns, compare the actual outside diameter and wall thickness.
Notice where the Ontario minimum sits. The 73 mm diameter in Article 9.17.3.1. matches 2-1/2 in nominal pipe, and the 4.76 mm (3/16 in) wall is less than Schedule 40 for that size. A light 11-gauge tube, at about 3 mm, is well under the 4.76 mm wall minimum. That doesn't automatically rule it out: Sentence 9.17.3.1.(2) allows other sizes where the loadbearing capacity is shown to be adequate. But it does mean a thin-walled column needs a recognized rating or a designer's check behind it, rather than leaning on the code minimum.
Lally column heights
Basement columns aren't sold by height the way lumber is. They're either adjustable within a range or cut to a measured length.
- Adjustable columns (teleposts) are made in a series of length ranges, each with a limited amount of adjustment from a screw or a telescoping tube and pin. One US manufacturer's evaluation report, for example, covers adjustable columns from 6 ft to 12 ft long with a maximum of 4 in of screw adjustment. Canadian teleposts list their maximum extension on the label, and a column extended past it is no longer the column that was rated.
- Fixed columns are cut to length. Some come with one plate welded on and the other end left open for cutting on site; others are fabricated to a measured length with both plates welded in the shop.
How the height is measured
For a fixed post, the length is the distance from the top of the footing (or the bearing surface the base plate sits on) to the underside of the beam, minus the thickness of both plates. The measurement is taken after the beam is properly supported, at the column location, because floors and beams are rarely perfectly level. If the post will be embedded in the slab, the measurement is still to the footing, not to the slab surface.
Height affects capacity. A taller column is more slender and buckles at a lower load than a short one of the same section. In one manufacturer's published tables, the allowable load on the same column drops by a third or more between the shortest and the tallest length listed. That's why you can't size a column from diameter alone, and why a tall basement or a garage may need a heavier section than a standard basement. Our guide to how much weight a lally column can support goes into the numbers.
What size lally column do you need?
It depends on two things: the load on the column and its unsupported height. The load comes from the area of floor (and sometimes roof) the column holds up, which is set by the joist spans on each side of the beam and the spacing between columns.
How the size gets chosen in practice:
- Like-for-like replacement. If an existing column has a label or rating and nothing above has changed, the replacement is usually matched to it or upgraded. Check the label on an adjustable column for the CAN/CGSB-7.2 reference, the maximum load and the maximum extension.
- New or moved column. When a beam is added, a wall is removed or columns are relocated, the designer or engineer works out the load and specifies the post section, plates and footing on the drawings. That's a structural alteration and normally needs a building permit.
- Loads above 36 kN. Under Article 9.17.3.4., adjustable columns conforming to CAN/CGSB-7.2 are only covered up to 36 kN. Above that, the column has to be designed under Part 4 of the code, which usually means an engineered fixed post.
The full method for sizing a post is covered in how to size a steel column for a house.
Round lally column or square HSS post?
On Ontario renovation drawings you'll often see square HSS instead of a round column. Both work; the choice usually comes down to practical points.
| Round column or pipe | Square HSS post | |
|---|---|---|
| Code path | Pipe column minimums in 9.17.3.1., or shown adequate | Shown adequate under 9.17.3.1.(2), usually by a designer |
| Connections | Plates welded to a round end | Flat faces make plate and bracket welds straightforward |
| Finishing | Harder to box in neatly | Easy to frame around and drywall |
| Availability | Adjustable types sold off the shelf | Cut and fabricated to order |
Either way, the post needs its end plates, a proper footing and a positive connection to the beam. For the plates, see lally column cap and base plates explained.
Getting the right size made
We fabricate round and square steel posts cut to your measured length with base and cap plates welded on, and we supply and install basement beams and posts. Send us your drawings, or the section, length and plate details your designer has specified, and we'll quote it. See our steel products for the full range.
Frequently asked questions
What sizes do lally columns come in?
US lally and adjustable steel columns are commonly sold in 3 in, 3-1/2 in and 4 in outside diameters, in standard and heavier wall thicknesses, and in lengths up to about 12 ft. In Ontario, fixed posts are usually steel pipe or HSS cut to the exact height needed.
What is the minimum lally column size in Ontario?
Under Article 9.17.3.1. of the 2024 Ontario Building Code, a steel pipe column needs an outside diameter of at least 73 mm and a wall at least 4.76 mm thick. A different size is allowed only where its loadbearing capacity is shown to be adequate.
What size lally column do I need?
It depends on the load the column carries and how tall it is, since taller columns buckle at lower loads. For a replacement, the existing column's rating is a starting point; for a new or moved column, the designer or engineer specifies the size on the permit drawings.
Is a 3 inch lally column strong enough?
It can be, for the right load and height. But a thin-walled 3 in column has a wall well under the Ontario 4.76 mm minimum for pipe columns, so its capacity has to be shown to be adequate, either by a recognized rating or by a designer.
