A steel plate joist repair means bolting a steel plate to the side of a damaged or overloaded wood joist or beam, or sandwiching a plate between two plies of lumber to make a flitch beam, so the steel and wood carry the load together. It's a common way to fix joists that were cut too deeply for plumbing or ductwork, cracked, or are sagging, without making the member deeper. Because the strength comes from the plate size and the bolt pattern, every flitch or repair plate should be sized by an engineer for the actual damage and loads.
This post explains what these plates are, where they're used, what the Ontario Building Code says about cut and drilled framing, and what goes into fabricating them.
Flitch plates, repair plates and connectors: what's what
People search for "steel plate joist" and "steel plate joint connector" when they mean quite different things. The three families are:
- Flitch beam. A steel plate sandwiched between two or more pieces of lumber, through-bolted at intervals along its length. The wood braces the thin plate against buckling sideways, and the plate adds a lot of strength and stiffness without adding depth. Flitch beams show up as headers, short beams in renovations and as replacements for a single overloaded joist.
- Side repair plate. A plate bolted to one or both faces of an existing joist or beam to bridge a crack, a notch or a hole, or to stiffen a member that sags. On a damaged joist, the plate is usually long enough to reach well past the damage on both sides so the load can transfer through enough bolts.
- Connectors. Joist hangers, mending plates, nail plates and light-gauge straps are catalogue hardware. They join members together and are selected from the manufacturer's load ratings. They are not a substitute for a structural repair plate, and nailing a thin mending plate over a cracked joist doesn't restore its bending strength.
The rest of this post is about the first two: structural plate, cut and drilled to a design.
When joists and beams need a steel plate repair
Plated repairs are usually the answer to one of these situations:
- Joists cut beyond the code limits. Plumbers, electricians and HVAC installers sometimes notch or bore joists deeper or in worse places than the code allows. A plate can restore the lost section where replacing the joist would mean tearing out a ceiling or floor.
- Cracks and splits. A joist that cracked over a heavy load, or split along a large knot, can often be plated rather than replaced.
- Sagging beams. A built-up wood beam that has deflected over the years can be stiffened with a flitch plate, sometimes as an alternative to adding a post in the middle of a room.
- New loads. A stone countertop island, a cast iron tub, a partition wall moved onto a joist line or a new opening above can push existing framing past what it was built for.
- Limited depth. Where a deeper beam would hit a door head or eat into basement headroom, a flitch beam can carry more load at the same depth as the existing joists.
Plates don't fix everything. Rotted or insect-damaged wood has lost strength and gives the bolts nothing to grip, so decayed sections are cut out and replaced, and the cause of the moisture is fixed first. Fire-damaged framing is assessed by an engineer before any repair is chosen.
What the Ontario Building Code says about notched and drilled framing
The 2024 Ontario Building Code (O. Reg. 163/24) sets the basic limits for cutting framing members in Subsection 9.23.5 of Division B:
- Holes (9.23.5.1). Holes drilled in floor, roof or ceiling framing must be no larger than one quarter of the member's depth and at least 50 mm from its edges, unless the member's depth is increased by the size of the hole.
- Notches (9.23.5.2). Floor, roof and ceiling framing can be notched only on the top of the member, within half the joist depth from the edge of bearing, and no deeper than one third of the joist depth, unless the depth is increased by the size of the notch.
- Wall studs (9.23.5.3). Loadbearing studs must keep at least two thirds of their depth undamaged unless the weakened studs are suitably reinforced.
- Roof trusses (9.23.5.5). Truss members can't be notched, drilled or otherwise weakened unless the truss design allows for it.
So the code's own fix for an oversized hole or notch is extra depth. Anything else, including a steel plate, is a repair that needs to be designed. Trusses are a special case: a damaged or cut truss is normally reviewed by the truss designer or an engineer, who issues a specific repair detail.
The Part 9 span tables cover sawn lumber joists and beams, built-up wood beams, glued-laminated timber and steel W-beams. There are no tables for flitch beams or plated joists, so those are designed under Part 4 by an engineer. Structural repairs can also need a building permit, so check with your building department before work starts. Our guide to permits for load-bearing wall removal in the GTA explains how the permit rules work for structural changes in houses.
How a steel plate shares load with the wood
Steel is many times stiffer than lumber. When a plate and a piece of wood are bolted together and bend as one, the steel attracts load in proportion to its stiffness. That is why a fairly thin plate makes a big difference, and also why the details matter:
- Bolts do the work. Load passes between the wood and the plate only through the bolts. Their diameter, number, spacing and distance from the ends and edges of the wood are what make the two parts act together. A plate with too few bolts mostly goes along for the ride.
- Stiffness has to match. The engineer checks deflection as well as strength, because a floor that is strong enough can still feel bouncy or crack a ceiling.
- Balance. A plate on only one face of a joist loads it off-centre, which can make it twist. Repairs often use plates on both faces, or a plate in the middle of a flitch beam, to keep the load centred.
- Bearing at the ends. If the repaired member carries more load than before, the posts, walls or hangers it sits on, and the footings below them, may need checking too.
- Depth of the plate. Plates are often detailed a little shallower than the lumber so the steel doesn't stand proud of the wood and push up on the subfloor or down on the ceiling as the lumber dries.
None of this can be read from a chart. The plate thickness and bolt pattern for a 2x10 floor joist cut for a drain are different from those for a beam carrying a bearing wall.
Flitch plate vs other ways to fix a joist or beam
| Fix | Where it fits | Trade-offs |
|---|---|---|
| Sister with new lumber or LVL | Cracked or over-notched joists with room to fasten a full-length member alongside | Simple and familiar; needs access along the joist and may still fall short for heavy loads |
| Steel side plates | Damage near bearings or services that can't move; members that must stay the same depth | Engineered bolt pattern; plates are heavy to handle in tight spaces |
| Flitch beam | New or replacement beams and headers where depth is limited | More capacity at the same depth than wood alone; designed and fabricated to order |
| Steel W-beam | Main beams, long spans, removed bearing walls | Highest capacity for the depth; needs posts, bearing and often footings |
| Add a post | Sagging beams where a column in the room is acceptable | Shortens the span cheaply but needs a footing and costs floor space |
For main basement beams, replacing an undersized wood beam with steel is often cleaner than plating it. See replacing a wood basement beam with steel and our comparison of steel vs wood beam sizes for the same span.
What goes into fabricating flitch and repair plates
Once the engineer has issued a detail, making the plates is straightforward shop work:
- Plate grade and thickness come from the design. Our post on steel plate grades in Canada explains the common grades.
- Cutting to size. Plates are cut to the length and depth on the drawing. See steel plate cut to size for how cut plate is ordered.
- Holes. Bolt holes are drilled in the shop to the pattern on the drawing, so the plate becomes the template for drilling the wood on site.
- Finish. Plates going into a dry interior floor are usually supplied primed or as specified. For a damp crawl space or exterior deck, we can prime or galvanize the plates to suit the job.
- Handling weight. Steel weighs about 7,850 kg per cubic metre, so a 10 mm plate, 235 mm deep and 4 m long, weighs roughly 74 kg. Long plates may be split into pieces with a spliced detail, if the engineer allows it, so they can be carried down a basement stair.
On site, the floor is usually shored and relieved of load before the repair, so the steel picks up load as intended. That sequence, and the bolt tightening, are part of the engineer's instructions, not something to improvise.
Getting flitch and repair plates made
To quote plates accurately we need the engineer's detail or sketch showing plate size and thickness, the hole size and pattern, the quantity and any finish requirement. If you don't have a design yet, tell us about the problem when you ask for a quote: engineering services are available for projects that need them.
Send us your drawings or repair details and we'll cut and drill the plate to suit. For the plates, beams and posts we supply, see our steel products page.
Frequently asked questions
Can you repair a cracked or over-notched joist with a steel plate?
Often, yes. A steel plate bolted to the side of the joist, or sandwiched between two plies, can restore or add strength without making the member deeper. The plate size and bolt pattern have to be designed by an engineer for the actual damage and loads.
What is a flitch beam?
A flitch beam is a steel plate sandwiched between two or more pieces of lumber and through-bolted so the steel and wood act together. It carries more load than the lumber alone at the same depth, which is why it is used where headroom is tight.
Does the Ontario Building Code have span tables for flitch beams?
No. The Part 9 span tables cover sawn lumber, built-up wood, glued-laminated timber and steel W-beams, not flitch beams or plated joists. Those are designed by an engineer, and the drawings go to the building department with the permit where one is needed.
Are joist hangers and mending plates the same as a steel repair plate?
No. Joist hangers, nail plates and light-gauge straps are catalogue connectors with load ratings set by their manufacturer. A structural repair plate is usually thicker hot-rolled plate cut and drilled to an engineer's detail and fastened with bolts.
