A flitch beam is a steel plate sandwiched between two or more pieces of lumber and bolted through, so the sandwich bends as a single member. The steel supplies most of the stiffness while the wood gives you something to nail into and keeps the plate from buckling sideways.
It is an old solution that keeps coming back, because it answers a problem no single material solves cleanly: you need far more capacity than lumber can give, but you have to fit inside the depth of the existing floor and you would rather not hang steel connections off a timber frame.
How a Flitch Beam Shares Load
Bolt the plies together tightly enough and they deflect as one. When members of the same depth deflect the same amount, each one takes load in proportion to its own EI, the product of its modulus of elasticity and its moment of inertia. Steel is roughly twenty times stiffer than softwood, so even a thin plate dominates.
Take a common arrangement: a 1/2 in steel plate 9 1/4 in deep, between two 2x10s of the same depth.
| Ply | E (psi) | I (in⁴) | EI (lb-in²) | Share |
|---|---|---|---|---|
| 1/2 in x 9-1/4 in steel plate | 29,000,000 | 33.0 | 956 million | 78% |
| Two 2×10 (1-1/2 in x 9-1/4 in each) | 1,400,000 | 197.9 | 277 million | 22% |
The wood has six times the moment of inertia and still carries under a quarter of the load. That is the whole idea of the detail. It also explains a common misunderstanding: adding a third 2x to a flitch beam barely moves the number, while going from a 1/2 in plate to a 5/8 in plate adds real capacity.
The wood modulus above is illustrative. Actual values depend on species, grade and moisture content, and come from the NDS Supplement published by the American Wood Council design standards (checked September 2026), which is the standard referenced by the building codes for the timber half of the beam.
Why Anyone Builds One
Depth is fixed. In a remodel the new beam has to fit the existing joist depth. A flitch beam gets far more capacity into 9 1/4 in than lumber alone, without dropping a header below the ceiling line.
It stays a wood member at the edges. Joist hangers, subfloor and finishes all attach to the wood faces with ordinary nails and screws. Nothing has to be welded, drilled or shot into steel on site.
It arrives in pieces. A rolled section that matches a flitch beam might weigh 400 lb in one lift. The flitch components go through a doorway and up a stair by hand and get assembled in place, which is often the deciding factor in an occupied house.
The wood braces the steel. A bare plate that slender would buckle sideways long before it reached its bending capacity. Clamped between two plies, it cannot.
The Bolts Are Not Decoration
The bolts are what make three separate pieces behave as one member. Their job is to drag load out of the wood, where it is applied, and into the plate, which is doing most of the work. Get the pattern wrong and the plate is along for the ride while the lumber quietly overloads.
Two approaches are used in practice. The empirical one is a standard pattern that has worked historically, commonly 1/2 in or 5/8 in bolts staggered at around 16 in on center, which also keeps heads and nuts clear of joists framing in at 16 in spacing. The rational one calculates the required bolt size and spacing from the actual load transfer, using the lateral design values for bolted wood connections in the NDS, with extra bolts near each end to deliver the end reaction into the plate.
Either way it is a designed connection. Bolt pattern, edge distance and end distance all belong to the engineer who sizes the beam.
Flitch Beam vs the Alternatives
| Flitch beam | LVL or glulam | Steel W shape | |
|---|---|---|---|
| Fits existing joist depth | Yes | Sometimes | Often needs more depth or a dropped header |
| Carried in by hand | Yes, in pieces | Awkward, one piece | Usually needs equipment |
| Attaches with nails and screws | Yes | Yes | No, needs a nailer or welded clips |
| Site labor | High: drilling and bolting | Low | Low once it is in |
| Shop work needed | Plate cut and drilled, ideally coated | None | Cut, drilled, connections fabricated |
| Predictable behavior | Depends on the bolting | High | High |
If the depth constraint disappears, a rolled section is usually simpler and cheaper for the same capacity. Our wide flange beam sizes chart lists every W shape with dimensions and properties, and types of steel beams covers where each family is normally used.
Where Flitch Beams Go Wrong
The plate bears at the supports. If the steel runs full depth to the bearing, it can punch into the plate or sill below while the wood carries nothing. Making the plate slightly shallower than the lumber puts the bearing where the detail intends.
The plate rusts. A bare plate buried between two pieces of lumber is invisible for the life of the building. In a crawlspace, a garage or anywhere the wood may take on moisture, the plate should be galvanized or coated. The American Galvanizers Association notes that hot-dip galvanizing gives barrier and cathodic protection and typically two to five times longer maintenance intervals than paint alone (checked September 2026), which matters when the member can never be inspected.
The wood shrinks and the steel does not. Lumber installed wet loses depth as it dries; the plate does not. Bolts that were snug can end up loose, and the plies stop acting together. Specifying dry lumber and re-tightening is normal practice.
Drilling on site goes badly. Holes in the plate should be shop-drilled to a template. Trying to match holes between steel and lumber with a hand drill in a ceiling is how bolt patterns end up wrong.
Frequently Asked Questions
What is the difference between a flitch beam and a flitch plate?
The flitch plate is the steel; the flitch beam is the finished assembly of plate plus lumber plus bolts. The terms get used interchangeably, and “flitch plate beam” means the same thing as flitch beam.
How far can a flitch beam span?
There is no span table that answers this, because the capacity depends on the plate thickness and depth, the lumber species and grade, the number of plies, the bolt pattern and the load. A licensed structural engineer has to size it and specify the bolting.
How much does the steel plate weigh?
Steel weighs about 3.40 lb per foot for each square inch of cross-section. A 1/2 in by 9 1/4 in plate is 4.63 in², so roughly 15.7 lb per foot, or about 252 lb for a 16 ft plate. The steel beam weight calculator works the same way for any rolled section you are comparing against. That is a two-person carry, which is much of the appeal.
What steel grade is used for a flitch plate?
ASTM A36 is the usual specification for plate. It is widely stocked, easy to drill and weld, and its properties are what most flitch beam design examples assume.
Can I add a flitch plate to an existing beam to strengthen it?
Sometimes, and it is a common retrofit, but it is not a matter of bolting a plate to the side of a sagging beam. The existing member has to be relieved of load while the work happens, the plate has to be sized for the share it will take, and the bolting has to transfer that share. It is engineering work, not a repair you scale off a table.
The Short Version
A flitch beam is what you build when the depth is fixed, the load is too much for lumber and you want the member to still behave like wood at its edges. The steel does about three quarters of the work, the bolts are what make that possible, and the plate wants to be slightly shallower than the lumber and protected against corrosion. Everything past that, including the bolt pattern, belongs to a licensed structural engineer.
Reference information only, not engineering advice. Disclaimer.