A steel post retaining wall uses vertical steel posts, usually wide-flange I-beams, set in concrete-filled holes in the ground, with concrete or treated timber panels (sleepers) slotted between the flanges to hold back the soil. Each post acts as a cantilever, so its size, spacing and embedment depth depend on the wall height, soil, water and any load behind the wall, and are set by an engineer. In Ontario, a retaining wall over 1,000 mm of exposed height next to public property, a building access or other places the public goes is a designated structure that must be designed under Part 4 of the Building Code.
This guide explains how these walls work, how they compare with concrete walls and what goes into the steel.
How steel post retaining walls work
There are two common types, and the same idea under several names:
Post and sleeper walls. I-beam posts are set at regular spacing in drilled holes that are filled with concrete. Precast concrete or treated timber sleepers drop into the channels formed by the flanges of neighbouring posts, stacking up to the wall height. The soil pushes on the sleepers, the sleepers span sideways to the posts, and the posts carry the load down into the ground. Gravel and drainage go behind the sleepers.
Soldier pile and lagging. The same principle, used more often for excavation shoring and larger walls. Steel H-piles or W-shapes are drilled or driven, and timber or concrete lagging is placed between them as the excavation goes down. Permanent versions are faced with concrete or precast panels.
You may also hear "king post wall", a term used in some places for the same arrangement of steel posts with infill panels.
Because the posts are cantilevers fixed in the ground, the critical point is just below the surface, where the bending moment is greatest. That is why embedment depth and the concrete around the post matter as much as the steel section.
Steel posts vs a concrete cantilever wall
| Steel post and sleeper | Cast-in-place concrete cantilever wall | |
|---|---|---|
| Foundation | Drilled holes with concrete around each post | Continuous footing with heel and toe |
| Excavation | Small holes, little disturbance behind the wall | Full excavation for the footing |
| Construction speed | Fast once holes are drilled | Forming, rebar, pouring and curing |
| Tight sites | Good near property lines and existing structures | Footing heel needs room behind the wall |
| Appearance | Exposed or clad steel, concrete or timber sleepers | Formed or finished concrete |
| Durability concerns | Steel corrosion at and below grade, timber decay | Rebar cover and drainage |
For the concrete alternative, see retaining wall rebar for footing and stem.
What sizes the posts
An engineer sizes the posts from the forces the soil puts on them. The main inputs:
- Retained height. Soil pressure grows with depth, and the bending in the post grows faster than the height. A wall twice as tall needs much more than twice the post.
- Soil type. Clay, sand and fill push differently, and the soil in front of and below the wall decides how well the post is held.
- Water. Water pressure behind a wall can exceed the soil pressure. Drainage (clear stone, filter fabric and a drain at the base) is part of the structural design, not an extra.
- Surcharge. A driveway, parked cars, a building footing or a slope above the wall adds load. Walls next to driveways and houses are rarely "just landscaping".
- Post spacing. Wider spacing means fewer posts but each carries more soil, and the sleepers must span further. Sleeper strength often limits the spacing.
- Frost. Ontario's freeze-thaw cycles move saturated soil. Drainage and embedment below frost penetration help keep the wall from jacking.
- Deflection. A post strong enough not to break may still lean too much. Stiffness can govern on taller walls.
The section is usually a W-shape, chosen so the flange width suits the sleeper thickness and the depth gives the bending strength and stiffness needed. Our W-shape size chart lists flange widths and depths. We don't publish "rule of thumb" post sizes because the same section can be fine on one site and badly undersized on the next.
Ontario rules
Designated structures. Under Article 1.3.3.5 of Division A of the 2024 Ontario Building Code, a retaining wall exceeding 1,000 mm in exposed height adjacent to public property, access to a building, or private property to which the public is admitted is a designated structure, which makes it a building under the Building Code Act. Part 4 (engineered structural design) applies to it (Article 1.3.3.2).
Guards. Retaining walls that are designated structures must have guards on open sides where the public has access to the top (Article 4.4.4.1). Our article on guard and railing rules covers guard heights and loads.
Wood sleepers. Where the OBC's Part 9 rules apply, structural wood in retaining walls must be pressure-treated to resist decay if the wall supports ground critical to the stability of building foundations or is more than 1.2 m high (Sentence 9.3.2.9.(4)).
Local rules. Municipalities, conservation authorities and fence or grading bylaws may add requirements, especially near slopes, watercourses and property lines. Before drilling, contact Ontario One Call for utility locates.
Smaller walls not caught by these rules still need to be built properly. A failed wall next to a driveway or neighbour's yard is expensive however tall it is.
Corrosion and durability
The weak point of a steel post wall is the steel at and just below grade, where soil stays damp and oxygen is available. Things that help:
- A protective coating suited to buried and exposed steel. We can prime or galvanize your posts to suit the job.
- Concrete encasement of the embedded length, with the top of the concrete brought up to or above grade and shaped to shed water.
- Good drainage so the backfill around the posts isn't saturated.
- Extra steel thickness where the designer allows for some corrosion over the wall's life.
- Inspection. Check the posts at grade every few years, especially on the soil side if it can be exposed.
Our article on embedding steel posts in concrete explains why the concrete line is where steel posts rust first.
Fabrication and ordering
Retaining wall posts are simple pieces, but a few details save time on site:
- Lengths include the embedded depth plus the exposed height, cut to the design. Posts on a sloping wall may each be a different length.
- Cap plates on top keep water out of the web area and finish the post.
- Holes or clips for tie-backs, guard posts or fence brackets, if the design has them, are much easier to add in the shop.
- Marking each post with its location keeps a stepped or sloping wall in order.
- Material certification. New steel with mill test certificates gives the designer a known grade. Used beams and rail of unknown origin are a gamble.
Retaining wall posts made to your design
Once your engineer has set the post size, length and spacing, we cut and fabricate the posts with cap plates and any holes or attachments on the drawings, weld in a CWB certified shop and deliver across the GTA and Ontario. Engineering services are available for projects that need them; tell us the wall height, length and what sits behind it when you ask.
Send your wall drawings or details for a quote, or see the steel products we supply and fabricate.
Frequently asked questions
What size I-beam do I need for a retaining wall?
It depends on the retained height, the soil, water, any surcharge such as a driveway or building, the post spacing and the embedment depth. Each post works as a cantilever holding back a strip of soil, and the size, spacing and depth in the ground should come from an engineer.
Does a retaining wall need a permit in Ontario?
Under the 2024 Ontario Building Code, a retaining wall exceeding 1,000 mm in exposed height next to public property, access to a building, or private property to which the public is admitted is a designated structure designed under Part 4. Municipalities may have their own rules too, so check with yours before building any wall.
How deep do steel retaining wall posts go?
There is no fixed ratio. The embedment depth is designed to resist the overturning from the retained soil, and it depends on the soil, the wall height and the post spacing. Posts are normally set in drilled holes filled with concrete.
Can I use old steel beams for a retaining wall?
Only if a designer accepts them. Used beams have unknown grade and history, may be corroded or damaged, and their size has to match the design. New steel with mill certificates removes the guesswork.
