DIYbanter - View Single Post

In article .com,
says...
yeah, that is a trick you can use on interior wood, but on a deck, it
would take too much of a beating and wouldn't last. and IMHO the putty
looks worse than the gap in this case. why don't you try doing nothing
for a few months...if it still bothers you after that amount of time,
then you need to fix it.

The best fix I've come up with is to wait until all shrinkage stops.
Then back up at least two joists from the separation, cut off one of the
boards with a circular saw and replace the cut off part. Be careful
with the cuts as they're kinda difficult to keep square. Also, it's
unlikely you'll be able to make the full width of the cut without
nicking/cutting the adjacent boards. You'll probably have to do the
finish cut at the edges with a sharp chisel.

It's a pain and you're gonna lose some wood but you can get reasonably
tight fits. With 3 joists (minimum) to nail/screw into you can get
joints that will last. Note, though, that you'll need to let the
replacement boards age/dry just like the board(s) you're replacing or
you'll have the same problem after you put them in.

Good luck
Tex