If it is the length of your car, and you have tie down straps you should be able to do this perfectly safely. Ideally, get yourself canoe brackets to attach to your roof bars - these are curved to match your canoe and allow you to put more tension in the straps without damaging the canoe.
It is recommended (especially when you have one longer than your vehicle) that you also attach a strap at bow and stern if possible, as well as a warning flag or banner at each end. These straps will avoid excessive movement up and down, and the flag will help make others aware of the potential risk at head height (eg cyclists)
Some countries mandate flags at specific length of cargo overhang, others don't, so you should check your local regulations.