So what is a collar tie (the technical term is actually collar beam) and why is it so important?
First, look at the diagram to the left here. See that short, horizontal framing member at the top? That is a collar tie. It looks pretty unassuming at first. In fact, it looks relatively unimportant. But that little framing member carries a lot of stress and movement on its shoulders. When collar ties are removed, a gradual sag and unevenness in the roof occurs. At first it is not too bad, but eventually can become a serious issue (albeit many, many years down the road typically). To be safe, make sure you have collar ties if you have a rafter roof system.