The bit or wheel you are using is not designed for wood cutting or this specific application. I would recommend a sawzall (reciprocating saw) with a wood bi-metal blade. This way if you hit any small nails, it won't damage the blade. Bi-metal blades cut thru small nails as the term Bi (two) implies. The blades or the wheels on the rotozip are burning up the wood and looking into another tool for your job may be your answer.