- Q.When I need a 6x6 deck
post, I usually assemble a built-up column from three
2x6s. Is this kind of post equal in strength to a
6x6?
A.Frank Woeste, P.E.,
professor of wood construction and engineering at
Virginia Tech, responds: The built-up post you
describe may be as strong as the 6x6, depending
upon the grade and species of the lumber, and how
the 2x6s are nailed together. Assuming the post and
2x6s are No. 2 Southern Pine, the nailing used to
laminate the three 2x6s is the main issue in
determining if three pieces of 2x6 will replace the
6x6 post.
When the deck column is loaded, the nails or
screws slip or give slightly, allowing the column
to start bowing. If the nailing is inadequate, the
bowing will continue and the capacity of the deck
column is reduced. Even when such a column is
assembled in accordance with rigorous National
Design Specification for Wood Construction (NDS)
provisions (using 30d common nails 8 inches on
center and staggered 2 1/2 inches), its bearing
capacity is reduced 40% from a solid-sawn column of
the same size (4 1/2 x 5 1/2 inches) having
identical lumber properties.
In summary, substituting a post made of
laminated 2-by material for a solid-sawn post in a
free-standing application (such as a deck post) is
not recommended without an in-depth analysis based
on the NDS column provisions.