There is another post on this same thing. The problem is the pressure treated wood you are using has shrunk as it dried out. This is a common problem. One way to overcon it is to use composite decking material. another material to use is the newer plastic wood facia material that is out. Comes in white and some other colors, as easy to work with as wood, but does not shrink, crack or check.
finding one of the composites that works with the color of your house paint color may be the best choice there, or caulk and paint the rail to cover the cracks and extend the life of the rail. you can also get the cpvc lumber, a 5/4 x 6 to replace that pressure treated 5/4 x6 and then paint it
Last edited by hand drive; 06-10-2012 at 08:25 PM.