Steel Beams and Posts for Decks: Spans and Sizing

A steel beam for a deck is sized by a designer or engineer from the deck's span, the length of joists it carries and the deck loads in the Ontario Building Code; unlike house floors, there's no steel deck beam span chart in the code. Steel makes sense on decks that need long clear spans, fewer posts, a slim beam line or tall posts. The code sets the loads, the deflection limit and the minimum corrosion protection, and the rest comes down to good exterior detailing.

This post explains how steel deck beams and posts are sized, which code rules apply, and the details that keep steel lasting outdoors. For steel post sizes specifically, see our guide to HSS steel posts for basements and decks.

Why use steel under a deck

Wood is the default deck structure in Ontario, and for most low decks it's the right one. Steel earns its place when:

• The span is long. A steel beam can clear a wide patio or walkout below with fewer posts.
• The deck is high. Tall decks off a second storey or a walkout basement need stiff posts and solid lateral support.
• Space underneath matters. Fewer, slimmer posts leave room for a patio, parking or a hot tub area below.
• The look matters. A slim steel beam line reads cleaner than a deep built-up wood beam.

Steel beams typically carry wood joists, so the deck surface and framing above stay conventional.

Why there's no steel deck span chart

Searches for a "steel deck beam span chart" come up empty in the Ontario code for a reason. Article 9.23.4.3 of the 2024 OBC gives steel beam spans for floors (Table 9.23.4.3) and for beams supporting a roof and one floor (Tables 9.23.4.3.-A to -J), all in dwelling units. Decks aren't in those tables, and the floor table's design basis (1.9 kPa live load, 1.5 kPa dead load, span/360 deflection) doesn't match deck conditions.

What the code does give is the loads and limits a designer works to:

The snow load depends on location and comes from MMAH Supplementary Standard SB-1, "Climatic and Seismic Data." Snow piles up on decks below roofs and against walls, so the design load can be well above the 1.9 kPa floor value.

How a steel deck beam gets sized

The designer works from the same inputs as any beam:

1. Span between posts, or between a post and the house.
2. Supported joist length, half the sum of the joist spans on both sides of the beam. A beam at the outer edge of a deck carries only half the joist span on one side.
3. Loads: the deck design load above, the dead load of the framing and decking, and anything special such as a hot tub, planters, a privacy wall or a roof over part of the deck. Concentrated loads like a hot tub are designed for specifically.
4. Deflection at 1/240 of the span for a deck serving one dwelling unit.

From there the beam is selected and checked for bending, shear, deflection and lateral stability. Because a deck beam is usually exposed, the designer may also prefer a section with a wider flange for joist bearing or a particular depth for appearance. Our W-shape size chart lists dimensions for common sections.

Section 9.17's column rules apply to posts under beams that carry no more than two levels of wood-frame decks, or one level plus the roof, where supported joist lengths don't exceed 5 m, combined snow and occupancy loads don't exceed 4.8 kPa, and the deck serves a single residential suite. Outside those limits, the posts are designed under Part 4.

Posts: HSS, lally column or something else?

Most steel deck posts are square HSS with a welded base plate and cap plate. A few points specific to decks:

• Lally columns (concrete-filled steel pipe) are mainly a basement product. Outdoors, the concern is water getting into the pipe or around the plates. A sealed HSS post is the more common exterior choice. Ask your designer before using any interior column product outside.
• Seal the tube. Fully welded cap and base plates keep water out. An open tube collects water, rusts from the inside and can split when the water freezes.
• Lateral support. Columns must be laterally supported to resist racking (Sentence 9.17.2.2.(2)). Posts not more than 600 mm from finished ground to the underside of the supported member, under a deck with no superstructure, are exempt (Sentence 9.17.2.2.(3)). Taller decks need bracing or a designed connection to the house.
• Anchorage. Exterior columns and posts must be anchored to resist uplift and lateral movement (Article 9.23.6.2), with limited exceptions for small, low, freestanding decks.

Whether the post sits on a base plate or is cast into the pier is covered in embedding steel posts in concrete vs base plates.

Footings under steel deck posts

Steel posts concentrate load onto fewer footings, so each footing works harder. Deck footings follow the Part 9 foundation rules, including the minimum foundation depths in Table 9.12.2.2, which depend on soil type and drainage and often mean going below the depth of frost penetration. Sentence 9.12.2.2.(7) exempts decks that are not more than one storey, not more than 55 m² in area, not more than 600 mm from finished ground to the underside of the joists, not supporting a roof and not attached to another structure (unless differential movement is shown not to harm it).

Many decks with steel beams are attached to the house or stand higher than that, so the exemption doesn't apply. Your designer or building department confirms the depth for your site. If the design calls for reinforced piers, our post on rebar in sonotubes and concrete piers covers how they're reinforced.

Keeping steel durable outdoors

The code minimum is shop primer on exterior beams and one coat of rust-inhibitive paint on exterior columns. Decks sit in rain, snow and de-icing splash for decades, so most designers go further. Options include a full primer and finish paint system or hot-dip galvanizing; we cover the trade-offs in does structural steel need paint or galvanizing. We can prime or galvanize your beams and posts to suit the job.

Detailing matters as much as coating:

• Avoid water traps. Pockets where a beam meets a post, or a ledger meets the house, should drain.
• Protect the top flange. A wood nailer bolted to the top flange traps moisture against the steel. A membrane or tape between them, and pressure-treated lumber for the nailer, help.
• Keep posts off wet ground. Bear posts on piers that stand above finished grade.
• Touch up after install. Bolt holes, field cuts and scratches from handling are where rust starts.

Steel deck beams and posts made to order

We fabricate steel beams and HSS posts with welded cap and base plates, drilled for your deck framing and anchors, to your designer's drawings, and deliver across the GTA and Ontario. Send us your drawings or sketch for a quote, or see the beams, posts and plates we fabricate.