Frozen roast beef should be cooked 50 percent longer than thawed cuts, according to Canada Beef. For example, a 2-pound roast that would normally take approximately two and a half hours to cook instead takes three hours and 45 minutes to cook to a well-done state.
Cooking times vary according to the weight of the roast, with larger cuts of beef taking longer to cook. A 9- to 11-pound frozen roast takes about 7.5 hours to cook to well-done. Cooking times also change according to doneness as well. The same size cut of beef takes about six hours and 45 minutes to cook to a medium-rare state.