Installing Beams
The beams go over the posts directly opposite the ledger board, and together they  support the ends of the joists.
Beams
at the same level as the joists are known as flush beams, and they usually require
joist hangers, which are easiest to mark out and nail on before the beam goes
up.
You
want a beam to have the exact same layout as the ledger board, and the easiest
way to do that is to cut and mark the beam before the ledger board goes up.
 OPTION: A beam that is positioned to support the joists from below is known as a dropped
beam, and it goes in place without joist hangers, since the joists rest on top
of it.
Codes
will usually require that beams be double or triple the size of the joists for
proper support, and that's usually accomplished by sandwiching 2 or 3 joists together
to serve as the beam.
But
if you assemble longer beams on the ground, they're difficult to raise on a tall
deck. One way
to get around that is to raise the beam members one at a time.
Positioning
Beams on Posts
 On a 6x6 post (which is actually 5-1/2 inches square) a triple 2x10 beam will only
cover 4-1/2 inches of the top. Set the 1st beam member an inch from the inside
edge, to make sure the 2nd and 3rd members end up flush with outside edge.
Toenail
the 1st beam member in place to hold it temporarily.
TIP: The ends of a beam can be cantilevered a short distance beyond the post supporting
it, which makes the posts less noticeable. Check local codes for the maximum cantilevers
allowed.
Installing Rim Joist
Where the rim joists meet the ledger board (and if it’s a flush beam, where it meets the beam) you may want to use an inside corner bracket.
Checking for Square
If
the diagonals are not equal, pry up the toenails holding the beam in place and
push the beam down toward the corner with the shorter diagonal. Then re-measure
the diagonals to see if that did the trick.
TIP: It doesn't have to be a guessing game on how far you push the beam to get
into square. Take half the difference between the two diagonal measurements, measure
that far back on the beam from where it crosses the post toward the long corner
and then push the beam until the mark lines up over the edge of the post. That
should bring it close to square, but re-check the measurements to be sure.
Once
the beam and rims are square, toenail the beam back into the top of the post.
Then
set in the other members needed to bring the beam to full strength, screwing or nailing the
second into the first and the third into the second.
 Codes might require more than toe-nailing to connect the beam to the top of the post. Several different pieces of hardware are available, depending on the size of the lumber.
OPTION: Use 1/2-inch lag bolts or carriage bolts to bind the beam members together.
On a triple 2x10 beam, the bolts should be at least 5 inches long to get through
all 3 members
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