Q. How much strength does plywood and construction adhesive add to the strength of a built-up header?
A.Robert Randall responds: Not much. Plywood used in a built-up header adds little to the strength of the beam (see "Sizing Built-Up Wood Headers" 4/92). At best, if the plywood extends the length of the header in one piece, it only adds about 8% to the beams load-bearing capacity.

Construction adhesive, such as PL, adds nothing to the strength of a beam when the loads are vertical. In some cases, when significant lateral, or sideways, loads are anticipated, adhesives might be part of an engineer’s design. However, since conditions on site make it difficult to control dirt, moisture, temperature, and other factors that affect an adhesive bond, I rarely depend on adhesives for any load-bearing applications.

A properly installed header should be well supported at each end by jack studs (with long spans, double jack studs may be required) and the plies should be well nailed. I recommend spiking headers together with a row of three 16d nails every 12 inches.

Robert Randall, P.E., is a structural engineer in Mohegan Lake, N.Y.