We have had rats in our compost bin, so we had them rebuilt out of concrete blocks with stainless mesh for the floor. I then found they were going over the top, so have built a bifold lid with a wooden frame and some polycarbonate sheeting in the middle. The wood frame is just some decking boards chopped down with a rebate routed in and joined with some dominoes in the corners. I then gave the wood a couple of coats of Cuprinol garden colours which has worked pretty well on other things. However from the photos hopefully you can see that most of the corners are starting to split after a couple of weeks. So the question is what did I do wrong, and more importantly how should I correct it?