Do Steel Beams Sag? Deflection, Bending and Level

A properly sized steel beam doesn't sag over time the way a wood beam can. It deflects a small, calculated amount when load is applied, springs back when the load comes off, and at normal building temperatures it doesn't keep creeping downward year after year. When a steel beam in a house looks like it's sagging, is out of level or isn't straight, the cause is usually something around the beam: a settling post or footing, an overloaded or undersized beam, a missing support, or how it was installed. Those are worth tracking down, because the fix depends on which one it is.

This post explains the difference between normal deflection and a real problem, the common reasons a steel beam ends up out of level or out of straight, and when to bring in an engineer.

Deflection, sag and bending: three different things

People use these words loosely, but they mean different things for a steel beam:

• Deflection is the elastic bending every beam does under load. Steel behaves like a very stiff spring: put load on, it bends a little; take it off, it returns. Designers limit deflection so floors don't feel bouncy and finishes don't crack.
• Sag usually describes a permanent, visible drop in the middle of a beam. In wood, part of that can come from creep, where wood keeps deforming under sustained load. Structural steel at room temperature doesn't creep in any meaningful way, so a "sag" in a steel beam is almost always movement of the supports, a beam that was too small or overloaded, or damage.
• Bending in the permanent sense means the steel has yielded: it's been stressed past its yield strength and won't spring back. That takes overload, impact, buckling or fire, not ordinary household use.

How much deflection is normal

The 2024 Ontario Building Code sets maximum deflections for Part 9 houses in Table 9.4.3.1. For floor beams, floor joists and floor decking, the limit is 1/360 of the clear span in all cases. Sentence 9.4.3.1.(2) says dead loads need not be considered in that check, so it's the deflection under live load (people, furniture) that's limited.

Some worked numbers at span/360:

Two things to keep in mind. First, those are maximums under full design live load, which a house rarely sees, so actual day-to-day deflection is usually smaller. Second, beams sized from the OBC steel beam tables in Article 9.23.4.3 already have this limit built in. Our post on steel beam deflection limits and floor bounce goes further into how stiffness and bounce are designed.

A deflection you can see from across the room, especially one that grows, is more than normal elastic movement.

Why a steel beam is not level

When a beam is lower at one end or in the middle, look at the supports first. Steel beams are stiff; supports are where most of the movement happens.

• A post or footing has settled. A column on an undersized footing, or on a basement slab with no footing under it, can push down over time. One low post makes the beam low at that point.
• A telepost was adjusted. Adjustable columns get turned, sometimes to "fix" a squeak or a door, and the beam follows. Our post on replacing lally columns covers when posts should be swapped out.
• Bearing shims crushed or shifted. Wood shims or loose packing under a beam end in a foundation pocket can compress, rot or slide out. Steel shims and a proper bearing detail avoid that. See steel beam bearing length on concrete and masonry for what a good bearing looks like.
• The masonry pocket deteriorated. Old block or brick under a beam end can crack or crumble, especially with water around it.
• A support was removed. A post taken out during a renovation, without a designed replacement, leaves the beam spanning further than it was sized for.
• It went in that way. A beam can simply be installed out of level, or installed with its natural mill camber pointing down. Common practice is to place any natural camber upward so the beam doesn't start life with a dip.

There's also a pattern that looks like a beam problem but isn't. Wood joists and sill plates shrink as they dry, while the steel beam doesn't. Where the wood around a steel beam shrinks more than the framing over the beam, the floor can show a slight hump along the beam line. The OBC recognizes this: Sentence 9.23.9.2.(5) requires a gap of at least 12 mm between the joist splice and a flush steel beam to allow for joist shrinkage.

Why a steel beam is not straight

"Not straight" usually means one of two things:

• Camber, a curve up or down in the plane of the web.
• Sweep, a curve sideways in the plane of the flanges.

Every rolled beam leaves the mill with a small amount of each, held within tolerances set by the material standards. A beam can also be deliberately cambered so it settles flat under load. Our post on steel beam camber tolerances and natural mill camber explains where the tolerances come from and how camber is measured.

A beam that's visibly bowed sideways, twisted, or kinked at one point is a different story. That suggests damage in transport or handling, a beam that buckled sideways because it lacked lateral bracing, or overload. Those need an engineer.

Can steel beams bend? On purpose, yes

Steel beams can be bent, but in a controlled way in a fabrication shop, not by the loads in a house. Fabricators camber beams by heating or mechanical means and roll sections into curves for arches and curved frames. Our post on heat cambering vs cold cambering covers how it's done. Field bending or heating a beam to "straighten" it, without an engineer's procedure, can change the steel's properties and is a bad idea.

How to check a beam and when to call an engineer

You can get a good first look without any special tools:

1. Run a string line or a laser along the underside of the bottom flange from end to end, and measure the gap at the middle and at each post.
2. Check the posts. Are they plumb? Is the floor cracked or sunk around their bases? Are teleposts screwed down to the bottom of their range?
3. Look at the bearings. Are the ends sitting fully on steel or solid masonry? Are there crushed or rotted shims, or rust at the bearing?
4. Note the symptoms above. Doors that stick, new drywall cracks, or a floor that slopes toward the beam line.

Call a structural engineer if you find any of the following:

• A drop that's getting worse over time.
• A kink, twist or sideways bow in the beam.
• Cracked or sinking footings or slab around posts.
• Heavy rust with flaking or lost thickness at the ends or flanges.
• A post that was removed, or new loads added above (a kitchen island on a new upper floor, a removed wall).
• Fire damage of any kind.

The usual fixes, once the cause is known, include a new post and footing, re-supporting a bearing with proper steel shims and grout, reinforcing the beam, or replacing it. Each one is a designed repair, usually with a permit. If you're wondering whether the beam was simply too small, our post on how much weight a steel beam can hold explains what goes into a capacity check.

Beams, posts and cambered steel to your drawings

We fabricate steel beams cut, drilled and fitted to your engineer's drawings, including heat-cambered beams where camber is specified, and HSS posts with welded base and cap plates for new supports. We supply and install basement beams and posts and deliver across the GTA and Ontario. Send us your drawings for a quote, or see the beams and posts we fabricate.