A.Frank Woeste, P.E.,
professor of wood construction and engineering at
Virginia Tech University in Blacksburg,
responds: You certainly are not wasting your
time to be cautious in building a deck. Like roofs,
decks don't experience their full design load (such
as a heavy snowfall or a large party) every day or
every year. But you must build them to handle that
extreme design load when it does occur, because a
failure can be catastrophic (see "
Deck Disasters Spotlight Faulty Connections,"
Notebook, 9/02).
I don't know of any engineering numbers on the
type of fasteners you're talking about, but they
may not be designed to resist the forces a deck can
experience. And we haven't had 40 years of
experience with the product to learn from, as we
have with traditional 8d or 10d threaded nails that
would typically be used with 5/4 decking boards.
It's safest in this case to disregard any bracing
effect of the decking and fasteners and build the
structure to stand up without them.
However, the diagonal let-in bracing you
mention, whether it's installed within the floor
frame or between the posts and the deck framing,
probably is not going to do the job. There are two
important issues to consider: twisting of the deck
joists and racking of the deck in plane. Even with
the joists secured against any rotation at each
end, they'll tend to twist within the span when a
load is placed on top if they're not restrained
somehow. Assuming no help from the decking, you'd
be wise to install PT solid blocking at 2 feet
on-center. That probably sounds like a lot to most
carpenters, but that spacing is borne out by
experience with long-span truss chords that are
held with 2x4 purlins at 2 feet on-center. If you
wanted to space the blocking farther apart, you'd
need engineering for the specific span, joist
spacing, and lumber size, grade, and species.
As for racking, one solution when earthquake
loads aren't involved is to firmly attach the deck
to the house and to its carrying posts (which
should be minimum 6x6 posts treated for a
structural application in ground contact) and to
embed the posts firmly into the earth at least 3
1/2 feet deep. Then the supports can hold the deck
in place without the need for racking resistance
from the deck boards in the plane of the deck
itself. However, diagonal bracing of the posts
might be required to "stiffen" the system,
depending on deck height, post size, and deck
size.