The reason why is that when timbers check they dry faster on the outside than they do on the inside and they open up and crack. That is the natural process of timber. As it dries the rest of the way through the center the gap can actually close back up a little bit. So, you want to wait until it is done moving and is completely dried out before you try this process because anything that you fill these cracks with could tend to try to push it back out or hold it before coming back together as it fires the rest of the way through center the timbers.
As I have mentioned the larger the timbers are, checking can be greater, especially in bigger timbers and also in cedar timbers.
So, what we have in front of us here today is a piece of wood that’s been in our yard for over a year. This is a piece of cedar and as you can see it has a huge crack in it. This still, if I were to show you the ends, this goes in about an inch, inch and a half at the most. It is not a structural issue. It is just cosmetic. So, here is how we would fix something like this.