Pressure-treated pine is often one of the more economical choices for decking materials. Even on pricer decks, deck builders use the material to build the support systems for decking. The main thing you need to bear in mind is that you cannot use it indoors — it is strictly an exterior product; you cannot burn it owing to the chemicals, and you’ll need special nails and screws. If possible, look for higher-grade pine that is pressure treated at the mill with water-repellents and sometimes pre-stained. Since many of the higher grade options are kiln dried both before and after pressure-treatment, they’ve less tendency to warp, twist, or shrink.