Replacing a wood basement beam with steel means supporting the floor on temporary shoring, removing the old built-up beam and its posts, and setting a steel W-beam on posts and footings sized for the new layout. It's a common fix for a sagging or damaged wood beam, and the main way to get rid of basement posts, because steel spans further for its depth. It's structural work that needs a design from a qualified designer or engineer and a building permit.
This post covers why owners make the swap, the details that are specific to replacing an existing beam (as opposed to installing one where there was none), and how the job is sequenced.
Why owners replace a wood beam
Most wood basement beams in older Ontario houses are built-up beams: several pieces of dimension lumber nailed together on edge. They work well when sized and supported properly. The reasons to replace one usually fall into a few groups:
- Sag. A beam that dips between posts, often with cracked drywall or sticking doors upstairs. It may be undersized, overloaded by later renovations, or have a post that has settled.
- Damage. Splits, crushing at the posts, rot at the foundation wall pocket where the end sits against damp concrete, or insect damage.
- Earlier cuts. Beams notched or drilled for ducts or plumbing by a past owner.
- Joints in the wrong place. In a built-up beam, the 2024 OBC requires butt joints in the individual pieces to fall over a support, except for continuous beams where they may be placed near the end quarter points of the clear spans (Article 9.23.8.3). Joints mid-span without that detail are a weak point.
- Too many posts. The beam is fine, but the owner wants an open basement for finishing, and a longer span needs a stronger beam.
Not every problem needs a new beam. A sagging beam can sometimes be fixed by adding or correcting a post and footing, and damaged plies can sometimes be reinforced. The designer decides between repair and replacement after looking at the whole load path.
What changes when wood becomes steel
Span. For the same load and depth, steel reaches further. Our post comparing steel and wood beam sizes for the same span puts the 2024 OBC tables for each side by side. The sizing logic for a main basement beam is covered in basement steel beam size: choosing the main girder.
Depth and height. A built-up beam of 2x10s is about 235 mm deep; a W200 steel beam is about 200 mm. If the joists sat on top of the old beam, they need to land at the same level on the new one, otherwise the floor above ends up with a dip or a hump. The usual answers are:
- A wood nailer bolted to the top flange, which also gives the joists something to be nailed to. See attaching a wood top plate or nailer to a steel beam.
- Shims between the beam and joists at each bearing.
- Raising the bearing at the beam ends and posts so the top of the nailer meets the joists.
Lateral support. The OBC steel beam span tables assume a laterally supported top flange. Sentence 9.23.4.3.(3) describes one way: wood joists bearing on the top flange at 600 mm or less, the load delivered through those joists, and 19 mm x 38 mm wood strips in contact with the top flange nailed to the joists on both sides. If your joists sit on a nailer instead, the designer shows how the flange is restrained.
The ends: beam pockets and bearing
The old wood beam usually sits in pockets in the foundation walls at each end. The steel beam often uses the same pockets, but they need attention:
- Bearing length. Beams need even, level bearing at least 89 mm long at end supports (Sentence 9.23.8.1.(1)). Our post on steel beam bearing length on concrete and masonry covers pockets, bearing plates and when the wall itself needs checking.
- Level and solid. Old pockets are often uneven or packed with wood shims. Steel goes on a steel bearing plate, solid shims or grout as the drawings show, not on loose wood.
- Moisture. Rot at the ends of wood beams is common because the pocket is damp. Steel doesn't rot, but it still corrodes if it sits in water, so the end gets primed and the pocket kept dry.
Posts and footings
Replacing the beam is the right time to look at what's under it.
- Teleposts. Many basements still run on adjustable steel columns. Under Article 9.17.3.4, adjustable columns conforming to CAN/CGSB-7.2 are limited to an imposed load of 36 kN; above that they must be designed under Part 4. Most owners swap to fixed steel posts at this point.
- Fewer posts, bigger loads. Removing a post spreads its load to the posts that remain, which then need bigger footings. Section 9.17 requires columns to sit centrally on a footing conforming to Section 9.15.
- Fastening. Columns must be securely fastened to the member they support (Sentence 9.17.2.2.(1)). Steel posts under a steel beam are usually bolted through a cap plate to the bottom flange.
How a beam replacement is sequenced
Every job is different, and the engineer's drawings and the contractor's plan govern. A typical sequence:
- Design and permit. The beam, posts, footings, bearing details and temporary shoring are designed and the drawings go in for a permit.
- Fabrication. The steel beam is cut to the measured length with cap and bearing plates and holes for the nailer. If it can't fit into the basement in one piece, it may be fabricated in sections with a designed splice.
- Footings first, where possible. New or enlarged footings are poured and cured where the posts will land.
- Shoring. Temporary supports go up on both sides of the old beam, close enough to carry the joists, bearing on something that can take the load. The basement slab alone often can't.
- Remove the old beam. The posts come out, and the old beam is cut into manageable pieces and removed.
- Set the steel. The beam is brought in, lifted onto its bearings and posts, levelled, and fastened. The nailer or shims bring the joists back to bearing.
- Release and inspect. Shoring is removed gradually so the floor settles onto the new beam, then the work is inspected before any finishing.
Steps 4 to 7 can often happen in a short window once the materials are on site, which is why the steel arriving complete, with every hole and plate in place, matters. For what goes into the price of this kind of job, see the cost to install a steel beam in a basement.
Is it worth it?
Replacing a sound wood beam just to change materials rarely makes sense. Replacing one that is failing, or one that blocks a basement plan, often does. The questions to bring to your designer:
- What's actually wrong with the existing beam, and can it be repaired?
- How many posts can the new layout lose, and what does that do to the beam size and the footings?
- How much headroom will the new beam and any nailer take?
- Can the beam come in one piece, or will it need a splice?
Get the replacement beam and posts made
We fabricate steel beams and posts to your engineer's drawings, with cap plates, bearing plates and nailer holes ready to install, and deliver across the GTA and Ontario. We also supply and install basement beams and posts, and engineering services are available for projects that need them. Send us your drawings or measurements for a quote, or see our services.
Frequently asked questions
Can you replace a wood beam in a basement with a steel beam?
Yes. The floor is supported on temporary shoring, the old beam and posts come out, and a steel beam is set on new or upgraded posts with footings sized for the new layout. It needs a design from a qualified designer or engineer and a building permit.
Why replace a wood basement beam with steel?
The usual reasons are a sagging, split or rotted beam, a beam that was cut or notched during earlier work, or wanting fewer posts. Steel spans further than a built-up wood beam of similar depth, so the same basement can often lose one or more posts.
Will a steel beam be the same height as my wood beam?
Usually not. A steel beam is often shallower than the built-up wood beam it replaces, so a wood nailer on the top flange, shims, or adjusted bearing heights bring the joists back to the right level. The designer details this.
Do I need new footings when replacing a basement beam?
Often. If the new beam spans further between posts, each post carries more load and its footing has to be checked and usually enlarged. The 2024 Ontario Building Code requires columns to sit centrally on footings that conform to Section 9.15.
