The deck on my house faces the southwest. No morning sun. Treated wood construction. The only wood that I have had to replace has been stair treads and the top railing. Both the stair treads and the top railing is 1"X8". Original construction was probably 1994, we have owned this since 2004.

The rails curve up at the ends and on the sides, almost forming a trough. Kind of irritating, the top rails were replaced about five years ago.

So any suggestions here? The rest of the deck is in good shape for it's age, some cracks in the 4"X4"'s, rails are secure, deck surface is good with no rotting or warped boards. If it was in bad shape I would replace it. I read somewhere to run a groove down the center with a router, they inferred that this would keep water from pooling on low spots but it would seem to keep it in the center and still warp over time.

Replace it with non-treated wood, maybe? That treated stuff always turns to spaghetti when it gets wet.

Get pressure treated wood to replace it but don't replace it right away, first put the wood in a protected place to completely dry out (stack it with air spaces) then before you put it in place apply something like Thompson,s water seal. May not stop the curling but it should slow it down.

Or, where it's just the railings, replace them with a composite material like Trex or Timber Tech. Available in molded-in colors, should be able to find something that look nice.

If you use pressure treated wood it will most likely warp again. Cedar may last a little longer but can still warp. I would suggest either a composite material like Trex, use vinyl, or metal. Obviously if you change this one section with anything other than what you currently have you will want to change the other railings to match.

I always thought you had to place it with the bark side out or the center of grain down so the ends would not curl up.

All pressure treated wood is green. Meaning its not dried or aged prior to the treatment. Its probably the cause of the OPs problems. I have the same issue with my top rails.

I was tempted to suggest he just turn the rails over and nail it back down. Or screw it down.

I have the same problem on my slats or 1x1s. They curve worse than an old cowboys legs.