[KS]

Wayner:  any oil-based stain should work well on drycedar.

Bigfish:  Composite or wood 5/4 decking need joists on 16" centers, or 12" centers for diagonal patterns.  2-by material (wood or composite) can be on 24" centers.  If 5/4 decking is used on existing 24" centers, lighter joists can be used between the existing joists...if it now has 2x12, 2x8s between them would work and save some money.  (5/4 (five-quarters) wood is about a full inch thick.  1-by (4/4)is about 3/4" thick; 2-by is about 1-1/2" thick.)

I'm just finishing a Home Depot "Veranda" brand composite deck.  It rated well in "Consumer Reports," was lower cost than most other choices, and looks really good with the Phantom hidden fasteners.  The fasteners are pricey and time consuming to use, but look good.

[byteme]

If you install this yourself, get those colored screws with the funny looking heads. Don't use the normal looking colored  deck screws. The "special" one's will not cause the composite material to squish out creating unsitely appearance.
Sorry I don't know the name of the screw type or brand, but the folks that sell you the boards should guide you to the proper screw type. The heads look like inverted cups.

Bob

EDIT: I found a Google image.
"Regular" deck screw will look like the left side after they're driven in, the "good one's" will be the right side.

I used these - http://www.fastenmaster.com/product.aspx?catID=13&prodID=15 - and had no issue w/ mushrooming.  Looks great and every box comes with two drill bits to fit.  For our 20x24 deck, two flower boxes and the benches I think I went through 8 boxes with about 1/2 left.  16" on center joists.

I can tell you from working with it, Azek is great and if you can get it for $43 for 20' deck board and can get enough of it to finish the job I'd jump all over that.

[awe-d-o-file]

I live in a very shaded forest. Moss,mushrooms and mold are constant problems here. To help a deck dry out after rain don't space the boards so close together. In 2003 I used a very good pressure washer and got the PT wood back to its natural state. I then used a good Olympic stain, two coats, not that Thompsons crap. Over 5 years later and I have no mold or slippery spots. Some of the vertical slats on the railing showed green mold this year but a solution of water and Clorox removed these easily. 

                                                           ET

[jermmd]

Well, my deck is just about done. I went with the grey Azek decking and it's really looking good. I only have the stairs to go and that should be done by tomorrow. I have an extra six 20 ft boards left over and I can't return them. I'm thinking of using them for a boat rack for my canoe and kayaks.