How do you make sure the roast chicken and the sides are done at the same time?
Simply put the sides in before or after the meat for an appropriate length of time such that everything is ready at once. 

For example, say my roast chicken takes 1.5 hours to cook including resting. Roast potatoes take 1 hour. Therefore I put my roast potatoes in the oven 0.5 hours after I put the chicken in, and they are both done at the same time.