A flitch beam (or flitched beam) is a compound beam used in the construction of houses, decks, and other primarily wood-frame structures. Typically, the flitch beam is made up of a steel plate sandwiched between two wood beams, the three layers being held together with bolts. In that common form it is sometimes referenced as a steel flitch beam. Further alternating layers of wood and steel can be used to produce an even stronger beam. The metal plate(s) within the beam are known as flitch plates.
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Advantages
Because flitch beams are significantly stronger than wood alone they require less depth than a wood-only beam of the same strength, are much lighter than a steel beam of the same size, and can still be nailed to the rest of a wooden structure. Flitch beams can also be created from existing in-situ joists or beams permitting easier renovations.
Disadvantages
Due to the high cost of labor, use of this type of beam has greatly declined. The advent of high-strength engineered lumber which uses modern adhesives and lower cost wood fibers has rendered this system largely obsolete. For example, compare the capacity of 2 beams spanning 18 feet: the allowable strength of a 3½ʺ × 11¼ʺ laminated veneer lumber (LVL) member has a tabulated allowable applied load of 398 plf (pounds per linear foot); compare this to a 2 × 12 flitch beam constructed of ½ʺ steel plate with two laminations of #2 SPF with an allowable applied load of 386 plf.
Engineered lumber is cut to length and installed similar to sawn lumber; the flitch requires shop fabrication and/or field bolting. This coupled with a much increased self weight of the beam (11.4 pounds (5.2 kg) for engineered wood vs. 25.2 pounds (11.4 kg) for a flitch beam) decreases the viability of the system.