There are several
things to consider, and some things you need to investigate.
Let’s start at the bottom. Your concrete footings need to do several things.
The bottoms need to be below the frost line (as defined by your local building
code). They need to distribute the weight that they are supporting, so they
need broad bases (also defined in local codes or specified by an engineer).
They need to be solid, and generally you would put a framework of rebar in the
bottom. They need to extend above ground level so that any wood framing that is
supported does not come in contact with the ground. If pressure treated lumber
is used (highly recommended), this may not be necessary. The posts need to sit
in saddles or brackets designed to anchor them to the concrete piers.
If you are going to set the hot tub on the deck, you will want to make sure
that the ledger on the house is securely attached with bolts or lag screws that
are tight into the house framing. A 2 x 6 is a bit light for a ledger, but may
be ok. I usually support a hot tub on a mini deck that is then surrounded by
the walking deck. This way, you can recess the hot tub into the deck to allow
for easier entry and exit. The benefit of this will depend on the design of
your hot tub model. If you do set it down into the deck, be sure to allow for
equipment access panels.
The decking itself needs to be of a material that will not deteriorate. The
decking under the hot tub will likely be wet almost all of the time, depending
on the climate where you live. You don’t mention the spacing on your joists,
but be sure that it is sufficient to support the load.
Check the specifications on the underside of the hot tub. Some models
concentrate the load and some distribute it. Your tub support should take this
into consideration.
Make sure you have cross bracing, bolted ledger, and joist hangers on the
ledger. When pumps and blowers are running, there will be vibration and over
time things can come loose if they are not properly connected.
You should be able to find a reference book or info on the web that will give
specific engineering data so that you can be absolutely certain that you have
designed strong enough.