What Is a Flitch Beam?
A flitch beam (flitch plate beam) is a composite beam sandwiching a steel plate between wood members, bolted together so the three act as one — the steel carries most of the load while the wood provides nailing surfaces and fire protection. Flitch beams were the standard solution for long residential spans before LVLs and PSLs took over. They still appear in renovations where matching existing wood framing depths matters.
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How they work
The steel plate (typically 1/4 to 1/2 inch) is sandwiched between two wood members (2x10s, 2x12s) with through-bolts at intervals; the bolts force composite action. The transformed-section analysis assigns load by stiffness — steel, being ~20x stiffer than wood, carries the lion's share. The wood sides let carpenters nail joists and hangers with standard framing hardware, which is the detail advantage over bare steel.
Where they are still used
Renovations matching existing dimensional-lumber depths (a flitch beam fits where an LVL of equal capacity would be deeper), historic structures where steel must be concealed, and field situations where a lumber yard and a steel supplier are both handy but an LVL is a week out. New construction has largely moved to LVL/PSL — cheaper, lighter, and no steel fabrication.
Design and field notes
The engineer sizes the plate and wood per the NDS/AISC composite provisions, specifies the bolt pattern (diameter, spacing, edge distances — the bolts are the composite mechanism), and details bearing and connections. Field fabrication needs care: the plate must be straight, the wood dry (shrinking wood loosens the bolts), and the bolts torqued to spec. Inspect the bolt pattern — widely spaced or missing bolts mean the beam is not composite.
The bottom line
Flitch beams sandwich steel plate between wood for composite action — useful in renovations matching existing depths. Size per composite provisions, detail the bolt pattern carefully, and use dry lumber.
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