A.Frank Woeste, P.E., professor emeritus
in the Department of Biological Systems Engineering at Virginia
Tech University, Blacksburg, responds: While most I-joist
companies publish fastener schedules and details for attaching
deck ledgers to code-approved engineered rim boards, these
details assume that the I-joists and rim boards are
continuously supported and bear directly on a sill plate. But
to my knowledge, no I-joist manufacturer has a detail for
connecting a deck to an I-joist cantilever, for two main
reasons. First, design loads from both the entire structure
and the deck that loads the cantilever have to be
accounted for when specifying I-joists and calculating their
load-bearing capacity at the foundation wall. But since
it’s impossible to publish design tables and other design
rules without knowing actual deck loads, I-joist industry
literature is based on the assumption that decks (and other
structures) are not connected to cantilevers. Second,
nail and screw connections into the end grain of I-joist
flanges don’t provide adequate strength in this critical
application.
The 2-by blocking used to reinforce this cantilevered I-joist
has been split by the lag screw connecting a PT deck ledger to
an engineered rim joist. Under substantial load, the split
blocking will tear off the bottom flange of the I-joist and
cause the remaining I-joist section to fail.
Whenever floor framing projects beyond the sill, deck loads
that would otherwise be carried by the supported rim joist and
sill plate are carried instead by the fasteners that connect
the rim joist to the ends of the cantilevered I-joists. Hence
the loads are strictly limited by those fasteners’
capacity. As the photo at left shows, even when blocking has
been used to beef up that connection, end-grain fastening is a
woefully inadequate method for transferring deck design
loads.
Of course, the problem occurs with dimensional lumber as well,
which is why the Prescriptive Residential Wood Deck
Construction Guide (awc.org)
prohibits attachment of decks to house overhangs and other
dimensional lumber cantilevers. As with code-approved rim board
and I-joists, an engineered solution would be required for a
safe deck connection.
Another option in both cases would be to design the deck so
that it is freestanding.