Bird,
On the columns, I can see that layers (2X maybe) have been laminated together and then maybe turned. I can't see the end, so I don't know if there is a space in the middle or not but they are not thin walled.
Rob and Ray,
The porch is about 22 years old and built by the house builder. He used untreated 1X4 pine tongue and groove, didn't seal anything, and then painted the exposed upper surface only. The moisture is absorbed from the bottom and the paint blisters as expected. I tried to find treated 1X4 tongue and groove but could not find any that was straight. According to the lumber yard, none of it is straight and they aren't buying any more of it. I did some research and one acceptable method would be to replank with untreated 1X4 tongue and groove but seal all six sides before assembly and then paint the tongues and grooves and assemble them wet. I choose not to do that.
I am replacing it with 5/4 1x6 treated pine decking. I will decrease the height of the column blocks by 1/4" to compensate for the increased deck thickness. I attempted to find decking that had been kiln dried after treatment (KDAT) but it only is available here in the 2X6 dimension. So, I bought the treated decking, not kiln dried after treatment, and have stacked it with spacers on the porch to dry. I want it to shrink before I install it so there is not a significant gap between the boards as you might have in a deck. When I bought it, it was very wet and heavy. I've dried it for about 6 weeks now and it is significantly lighter and the width is now about 5 1/5 as opposed to 5 5/8 to 5 3/4 when it was wet