A steel post with a base plate is connected to concrete with anchors through the plate: cast-in anchor rods when the concrete is new, or post-installed wedge, screw or adhesive anchors when it already exists. What the post does decides the details: a post carrying vertical load mainly needs bearing, while a railing post, light pole or sign post levers on its base and needs a thicker plate, stronger anchors and enough concrete, especially near edges. The designer sizes the plate and anchors; this guide explains the options and the problems that come up most.
For the choice between plating a post and casting it into the concrete, see embedding steel posts in concrete vs base plates. This article assumes a base plate.
What the post does decides the connection
| Post | Main load at the base | Typical connection |
|---|---|---|
| Basement column under a beam | Vertical load | Bearing plate on a footing, slab poured around it |
| Deck or porch post | Vertical load, some uplift and lateral | Plate on a pier or footing above grade |
| Canopy or carport post | Vertical, wind uplift and sideways | Plate on anchor rods in a pier, often designed as a fixed base |
| Guard or railing post | Horizontal load at the top, which becomes a moment at the base | Plate on anchors in a slab, step or landing, or side-mounted to the slab edge |
| Light pole or sign post | Wind on the pole or sign, a large moment at the base | Round or square flange on anchor rods in a concrete base, with levelling nuts |
A post that only carries weight presses the plate down on the concrete. A post that can be pushed sideways tries to lift one edge of the plate and pull the anchors on that side out of the concrete. That second case is where small plates and light anchors fail.
New concrete or existing concrete
Cast-in anchor rods are placed before the concrete is poured, usually with a template matching the plate holes. They are the strongest and most predictable option, and they suit columns, canopy posts and light poles on new bases. Plate holes, washers and edge distances are covered in anchor bolts for base plates.
Post-installed anchors go into holes drilled in hardened concrete. The common types:
- Wedge (torque-controlled expansion) anchors. An expanding clip grips the hole as the nut is tightened. Widely used and quick, but they push outward on the concrete.
- Drop-in (displacement-controlled) anchors. An internally threaded sleeve set with a tool. Shallow and also expansive.
- Screw anchors. Hardened screws that cut their own thread into the concrete. Low expansion forces, removable.
- Adhesive anchors. Threaded rod or rebar set in epoxy or another adhesive. No expansion force, good near edges when the product allows it, but sensitive to hole cleaning and cure time.
Whatever the type, use an anchor with published design data and follow the manufacturer's instructions for hole size, cleaning, embedment and torque. Those details are part of the anchor's rated capacity, not suggestions.
Edge distance: the usual problem
Many failed post connections come down to anchors too close to an edge. Railing posts are the classic case: they sit at the edge of a slab, landing or step, exactly where the concrete is weakest.
The concrete standard sets minimums. In the 2004 edition of CSA A23.3 we checked (Clause D.9.4), post-installed anchors without product-specific test data need at least 6 times the anchor diameter to an edge for undercut anchors, 8 times for torque-controlled anchors and 10 times for displacement-controlled anchors, and never less than twice the maximum aggregate size. Products with test data may be allowed closer. A 12 mm wedge anchor without product data would need 96 mm to the edge. The 2024 OBC references the 2019 edition, so confirm current values with your designer.
When the edge distance isn't there, the usual answers are:
- Side-mount the post with a plate or brackets bolted to the face of the slab edge.
- Move the post back from the edge, if the guard geometry still works.
- Core a hole and grout the post in, turning it into an embedded post.
- Cast in an embed plate or sleeve when the concrete is new.
- Use an adhesive anchor rated for the edge distance available.
- Thicken or reinforce the edge when the concrete is being poured anyway.
Is the concrete strong enough?
A slab-on-grade, a thin topping or an old step may not be able to carry what the post puts into it, even with the right anchors:
- Vertical loads from a column or beam need a footing. In houses, Article 9.17.2.1 of the 2024 Ontario Building Code requires columns to be centrally located on a footing.
- Guard loads are set by the OBC; our article on guard and handrail rules for steel railings covers them. The post, its welds, the plate, the anchors and the concrete all have to carry them.
- Old or cracked concrete reduces anchor capacity. Anchors in deteriorated steps often fail no matter how they are installed.
If the concrete is questionable, a designer can tell you whether to anchor into it, go around it or replace it.
Sizing the plate
The plate has to fit the post, the welds and the anchors with proper edge distance in the steel, and be thick enough not to bend. For posts with moment at the base (railings, canopies, poles), plate thickness is often governed by bending between the post and the anchors on the tension side, and is noticeably thicker than for a post of the same size carrying only weight.
On uneven concrete, a plate rocking on high spots loads its anchors unevenly. Columns are usually shimmed or set on levelling nuts and then grouted with non-shrink grout. Smaller posts are often shimmed and grouted the same way.
Light poles and round flanges
Light poles and sign posts usually have a round or square flange with holes on a bolt circle. They are set on anchor rods cast into a concrete base, with levelling nuts under the flange so the pole can be plumbed, then top nuts. The pole supplier normally provides the bolt circle, rod size and a template. If you are fabricating a custom light support, get the anchor layout from the design before the base is poured.
Wood posts on steel plates
Searches for "base plate for 4x4 post" are usually about wood posts. Standard wood post bases are proprietary galvanized connectors with published load ratings, and are installed to their manufacturer's instructions. A custom steel saddle or plate for a wood post is possible, but it should be designed like any other connection, including how the wood is fastened and kept off wet concrete. For steel posts under wood beams, see connecting a steel post to a wood beam.
Keeping water out
Outdoor post bases stay sound longer when:
- The plate is welded all round, so the tube is sealed at the bottom.
- The base sits on a pier or grout pad that sheds water, not in a puddle.
- The steel is coated. The OBC requires exterior steel columns susceptible to corrosion to have at least one coat of rust-inhibitive paint (Article 9.17.3.3). We can prime or galvanize to suit your exposure.
- Anchor nuts and washers are protected or compatible with the post's finish.
For more on outdoor posts, see HSS steel posts for basements and decks.
Posts with plates, made to order
We fabricate HSS and steel posts with base and cap plates welded on, cut and drill plates to your details, and build miscellaneous steel such as railings and light supports, all in a CWB certified shop with delivery across the GTA and Ontario. Tell us what the post supports, what it sits on and the exposure, and we'll quote to your drawings.
Send your post details or drawings for a quote, or see the steel products we supply and fabricate.
Frequently asked questions
How do you attach a steel post to a concrete slab?
Usually with a base plate welded to the post and anchors into the concrete: cast-in anchor rods when the concrete is new, or post-installed mechanical or adhesive anchors in existing concrete. The anchor type, size, embedment and edge distance should come from the designer and the anchor manufacturer's data.
Can a steel post sit on a slab-on-grade?
A light post, such as a railing or small canopy post, often can, if the slab and anchors are checked for the loads. A post carrying a beam, floor or roof needs a footing designed for that load. In houses, the Ontario Building Code requires columns to be centred on a footing.
Why do railing post anchors crack the concrete?
Usually because they are too close to the edge of a slab, step or landing. Expansion anchors push outward as they tighten, and a railing post levers on its anchors when someone leans on the rail. Close to an edge, that can split off a wedge of concrete.
What is the difference between a top-mount and a side-mount railing post?
A top-mount post sits on a base plate on top of the slab or step. A side-mount (fascia-mount) post is bolted to the vertical face of the slab edge, which keeps the anchors back from the top edge and leaves the full walking surface clear.
