I currently have a deck with 2x6 cedar floor. I am doing away with this and going with a composite but its only 5/4x6 so that leaves about a 1/2" difference. My issue is that my siding and my post will have a 1/2" gap where it has been cut. My thought and some others recommended was to rip 1/2 strips of 2x10 pressure treated boards and glue/nail those down and then put the new boards on top of that? Does anyone see any issues with this or recommend something better. the flooring i am doing is most likely fiberon horizon.

This is one of the projects that starts with the phrase "how hard can it be?". This seams really simple. First take off the old deck boards then replace with new deck boards. You have found one of the problems in replacing deck boards. There are several other reasons why this is more difficult than it sounds.

I think you might have a bigger problem on your hands than you think. In most cases when a deck used 2x6 decking the support boards (or joists) are spaced at around 24 inches apart. Most composite lumber requires that the joists be 16inches on center. So if your joists are 24 inches apart then you cannot use composite decking.

Another issue is what to do with the old joists. Over time they will rot. There is a chance that you could be putting composite decking on top of bad lumber.

I don't have much experience with that 1/2" gap created by the difference in deck boards and this topic is a little out of the scope of our work. I think the least expensive way to build would be to find trim that matches your siding.

Deckman

Thanks Deckman, the joist are on 16" centers. Most of the local deck shops i talked to saw no issues with the 1/2" filler. They said it would be more hastle to trim around everything in their opinion. One installer came out and said that he would do it that way as well. so hopefull this works out. thanks for your comment. i will let you know how it turns out in a few weeks.