Roasted chicken bones can be used to make stock, but the result differs from stock made with a raw carcass. The bones have already been heated, and some of the gelatin has dissolved in the oven, so the stock will not gel as much as stock made from a raw carcass. On the other hand, roasted bones give it a darker color and deeper flavor, and release their flavor more quickly. The browned bits stuck to the pan go into the stock too.
For stock made with a raw chicken carcass, start the bones in cold water and simmer gently for a long time so the collagen in the connective tissue slowly turns into gelatin; a good stock gels on its own when chilled. Some of this conversion has already happened in the oven when the chicken is roasted, and the gelatin has moved into the juices that ran into the pan. That is why stock made from cooked bones gels less. In return, the surfaces browned in the oven give the stock a darker color and a roasted flavor; the stock will have less body but a deeper color.
The bones used for stock should be the carcass left on the pan after serving or set aside before being divided among plates; do not use bones collected from plates or already eaten. After the chicken is served, do not leave the carcass out for more than 2 hours (1 hour above 32°C / 90°F); cool it and refrigerate it. How long it can be held is a storage question. Break the carcass into pieces by hand or with the back of a knife, because the exposed inner surface of a broken bone releases flavor into the water more quickly. The skin and meat left on the bones go into the water too and add flavor. Put small pieces such as wing tips and the neck in the pot as well.
How the chicken was cooked also affects the stock. Bones from chicken cooked with plain salt, black pepper, and thyme work for any stock. If the chicken was cooked with a tomato-paste, honey, or heavily spiced sauce, the sauce has stuck to the bones and will transfer to the stock; scrape off thick layers of sauce on the bones, or the stock will turn cloudy and sweet. The bones are already salted, so do not add salt to the stock; adjust the salt by tasting the dish in which you use the stock. For the same reason, use spices sparingly.
Put the bones in a pot and add enough cold water to cover them, then add onion, carrot, and parsley stems. As the water approaches a boil, lower the heat and keep it, as with stock made from raw bones, at just an occasional bubble on the surface; boiling water makes the stock cloudy. Do not discard the sticky brown bits left in the roasting pan: pour a little hot water into the pan, scrape and dissolve the bits with a spatula, and add them to the pot. These bits give the stock its deepest flavor and determine its color.
Cooked bones release their flavor faster than a raw carcass, so do not apply the long cooking times given for raw carcasses to these bones. Judge when the stock is ready by the condition of the bones and liquid, not by the time: the liquid has a distinct color and chicken aroma, and the bones begin to fall apart at the joints when pressed by hand. Waiting longer after this point will not add flavor; it will only reduce the liquid. Strain the stock through a fine-mesh strainer; for a clearer result, line it with cheesecloth. Remove the fat from the surface, and if you will not use the stock right away, cool it quickly by dividing it into shallow containers and refrigerate it within 2 hours.