Choosing a decking and finish

[Mitch Kronowit]

This is for our own home. I've started constructing a low-level deck in our backyard that encircles a beautiful shade tree. The deck will be about 19" above grade at the highest, so no permit or railing is required. It's free-standing, i.e., does not connect to the house in any way. In fact, it's about 25 feet away from the nearest point. The beams are made of doubled up 2x treated lumber to make an equivalent of 4x beams. I'm planning to use treated 2x6's for the joists as well. Is treated lumber necessary for the joists or could I use regular lumber?

Lastly, I need to select a decking material. I plan on laying the deck in a 45 degree "herringbone" pattern. I am tempted to use that composite material that lasts forever with virtually no maintenance, but that stuff is often 1" thick and requires joists 12" on center to support them at a 45 degree angle (my current plan calls for the joists at 16" OC). My other idea is use 2x6 redwood (about the same price as that composite stuff), but I want to treat it with something to preserve the color of the redwood. Thoughts and experiences battling the elements?

Finally, if I do go with wood, any suggestions for a finish? I once recommended by friends use McClosky's Man-of-War Marine Varnish on their wood deck, but discovered afterwards it's NOT for decks. D'oh! I believe Thompson's Water Seal will help protect the wood, but it needs to be reapplied every year, correct? I know the tall ships I sail on use a mix of linseed oil and Penetrol for protecting wood, but I don't think they use it on the decks. Then there's stain, but won't they taint the natural color of the wood and make it look like something else?

Thanks for any and all advice.