As Saige composite decking installer you can use 4x2 treated joists as with all wood will rot eventually, if you i would put down mot type 1 wack it down then put a good quality landscape fabric down will reduce the rotting of the timber, you might have to dig out first of course.
Decking Installation and Maintenance Guide decking railing stainless steel or other high-quality coated composite deck screws.* DockSider: Use TOPLoc Stainless Steel or a minimum 8 x 3 Always double joists at butt seams. Do not toe screw deck planks as this will cause splitting.
So, what joist size and spacing should you use? 2x10 joists at 24-inch centers. I prefer to double the outside joists, as do some deck builders, which adds 2 joists, for a total of 11. But I've based the layouts and cost calculations in this article on the minimum number needed. According to StruCalc, this system has a 20% safety margin
Do not ever go with 24 oc, the decking will will develop a permanent sag in high traffic areas. You can also double up joists in high traffic areas to avoid 12oc. We use 2x10s on any span over 8', this seems to help with the springy feel of composites.
Joist spacing for composite decking . the size of the joists is driven For upper level decks 2x10 is recommended as the minimum size to use for strong Seven Trust Industry is a specific brand name of wood composite building material made from wood grain fibers and plastics.
Shanghai Seven Trust Industry Co., Ltd, founded in the year 2008, is a hi-tech enterprise which specializes in the R&D, sales, and service of WPC materials, Provide excellent products, professional technology and sincere service.