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.