A flitch beam consists of two pieces of wood with a steel plate sandwiched between them. It is a structural element used in building construction when a beam stronger than an all-wood beam is required, but the strength of an all-steel beam isn't needed.
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Measure the distance you want to span or consult blueprints to determine the length of the flitch beam.
Determine how much weight the beam needs to support. Account for structural components such as joists that will rest on the beam.
Find the total load on the beam by adding the loads you determined in Step 2.
Determine how much clearance is required beneath the beam. Choose the thickest lumber possible for the beam while still maintaining clearance. You will need two pieces of the same length to sandwich the steel.
Consult an engineering table in a structural engineering textbook to determine the required thickness of the steel plate, based on the length of the beam and the load it needs to carry. You can also find a table through the National Association of Home Builders website in the link provided below.