Hi.  I managed to find two beef roasts of about 8 pounds each that together are enough to serve company coming for Easter dinner.  My question is, how long do I need to cook them?  Do I treat them like one 16 pound roast?

--Shaun

The amount of time that it takes a roast to cook depends on two major factors:  the temperature at which the meat is cooked and the thickness of the meat.  By thickness, I mean the shortest distance from side to side through the middle of the roast.  For a standing rib roast, that would likely be the distance running across the ribs at the center.  For tenderloin, the distance would the the diameter at the thickest point, not the length.

Weight is used to estimate how long it will take enough heat to reach the center, which is why in recipes there is a frequently a wide variation in the range of cooking times, and why they may give a shorter time per pound for very large pieces to cook.

If you were to place the roasts together so that they are in contact along one side, then the cooking time would be somewhere around double the time needed to cook just one of the roasts alone.  If your oven and roasting pans are large enough to accommodate both roasts at once with plenty of space between them for air to circulate well, then the roasting time for both will likely be only slightly more than the time needed to cook just one of them.

This assumes that your oven can keep its temperature high enough.  Because the oven heat is going into cooking both of them at the same time, two roasts will cool the oven down faster than one, so the heating element may be on for a greater portion of the cooking time.

You should still use a thermometer to check the temperature.  For beef, the USDA recommends cooking temperatures of 145°F (63°C) for medium-rare, 160°F (71°C) for medium, and 170°F (77°C) for well done.  The Joy of Cooking specifies temperatures that are somewhat lower, 130-135°F (54-57°C) for medium-rare, 140-150°F (60-66°C) for medium, but about the same for well done at 170-185°F (77-85°C).  The lower cooking temperatures allow, in part, for the fact that the temperature will rise as the meat rests after cooking.

A roast is usually allowed to rest 15 to 30 minutes once it is taken out on the oven.  Since the outer part of the roast will be hotter than the center as it comes out of the oven, resting allows for more even distribution of heat throughout the roast and may make carving easier.  To rest the roast, place it on a platter and cover loosely with aluminum foil.  I also drape over that one or two layers of clean cloth kitchen towels to help keep in a bit more of the heat.