Re: Replacing exterior stair treads
fifteen years ago I went to the trouble of milling pressure treated 2x12's into 1" thick stair treads, with a nice nosed (rounded) edge just like interior treads, only in PT. The material never actually rotted, but the 1-piece design cupped (even though I cut kerfs (grooves) in the underside to reduce cupping), puddled water and weathered to a very splintery and unsightly condition. I changed over to two-piece treads this year on my steps. There isnt any way for water to puddle because it can drain through the space between.
It is ordinary 5/4" decking that I recommend to you for treads. 2-piece them and you won't be sorry. If you need less width, you can rip one or both down to achieve the width you are after. Leave a 3/4" overhang beyond the riser.
Remove not the ancient landmark, which your fathers have set.