Should I use pressure treated lumber on my deck?

Pressure treated lumber has a tremendous amount of chemicals in it. There are certain scenarios where treated lumber can’t be avoided. Local building codes may state that if it comes within 6″ of grade it has to be treated. We try and avoid treated lumber whenever possible. Regular Hem Fir is fine here in Colorado as long as you protect it with a penetrating stain, and flash the tops of the joist.