A.Christopher DeBlois, a structural engineer with Palmer
Engineering in Tucker, Ga., responds: You're right. Lag
bolts don't get the same purchase in a 1 1/4-inch-thick
engineered LSL (laminated strand lumber) rim joist as they do
in 1 1/2-inch-thick framing lumber. If a lag bolt is properly
installed, with its tip well through the rim joist and only the
threads engaged in the band, its capacity relative to pull-out
forces depends on the thickness of the band and the density of
the wood. Because LSL rim joists are typically built up of the
same wood species used for framing material (and thus have the
same approximate density), the big variable is the thickness. A
1 1/4-inch-thick LSL is five-sixths as thick as 1
1/2-inch-thick 2-by stock, and thus has 16 percent less holding
capacity. To provide the same total pull-out strength, you'd
need to provide six-fifths the number of bolts, an increase of
20 percent.
Although there may be other variables, the end result for shear
and pull-out strength to carry the weight of a deck or porch
will be similar — 20 percent more bolts in a 16 percent
thinner band will provide about the same capacity. (See
"Load-Tested Deck Ledger Connections" [3/04] for bolting
schedules for 2-by ledgers.)
Instead of using more lag bolts, an alternate approach (if you
can plan ahead) would be to use 1 3/4-inch LVL material for
bands or rim joists wherever you will be bolting a deck or
porch to the house. That's the method I used for my own house,
and the small increase in materials cost was well worth it when
I added the deck a few years later.
To me, a bigger concern than pull-out strength is the strength
of the connection between the band and the joists and subfloor
when an LSL rim joist is used with wood I-joist floor framing.
Because I-joist webs are so thin, the connection between each
joist to the LSL band is weak; therefore, you must rely on the
connection of the subfloor to the top of the band to keep from
pulling the rim joist off the house. But since this band is
thinner than 2-by material, it's a little bit easier for screws
or nails from the subfloor to miss the band entirely, or to
catch very little solid material. I've seen a deck literally
collapse away from a house, taking the LSL rim joist with it.
And if the rim joist is at the end of a cantilever or overhang
with no wall below, the potential for this type of failure
increases dramatically.
To ensure that the rim joist itself is well secured to the
floor framing (at least where joists bear on the outside wall),
I recommend reinforcing the connection from the joists to the
band. In new construction, this can be done by adding plywood
web stiffeners on both sides of each joist at their outer end
so that they finish flush with the I-joist chords, and by
nailing from one side through the OSB web into the stiffener on
the opposite face (clinching over any nails that poke all the
way through). Then install joist hangers upside-down over the
top of the end of each joist before you set the subfloor,
nailing the hanger off to both the I-joist and the rim joist to
complete the connection (see illustration).
For existing framing, install light-gauge framing angles (such
as Simpson's L70s) to connect the web of each I-joist to the
rim joist.