Roasting food requires dry heat, while steaming it relies on moisture. It's possible to steam food in a roaster with a little modification to your cooking method. If you have an electric roaster with a lid, using it to steam a crab lets you heat the shellfish with water vapor while keeping most of the steam confined to the roaster, rather than letting it escape into your kitchen.
Read the directions that came with your roaster to familiarize yourself with its workings, if you haven't used it before. Some roasters come with steamer racks and specific instructions. Check how to set temperatures and any warnings about removing the lid during the cooking process.
Allow your crab to get to room temperature whether you are steaming a live or dead crab. This will help it cook evenly. Crabs are very perishable, so do not leave your crab out once it reaches room temperature, which can be 30 minutes to an hour, depending on its size and how cold it was to begin with. You can cook a cold crab, if necessary, but it should cook with an added minute or two of time, also depending on how cold the crab was.
Place a rack in the bottom of the roaster to keep the crab out of the water you will pour into the roaster. If a rack doesn't come with your roaster, or you will need to use a separate stainless steel rack that fits inside your roaster, and has feet to stand above the water.
Pour liquid into the roaster until it comes near the rack, but not over it. Use a liquid consisting of half water and half apple cider vinegar or beer, or water and a seasoning mix, or water with lemon pieces. Turn the roaster on high, uncovered, and bring the water to a boil.
Prepare your crab as you wish to eat it, usually by seasoning it with a dry mix. Do not remove the crab meat of a dead crab from the shell prior to cooking, which can result in the raw crab meat tearing and becoming shredded.
Check the water in the roaster. When the water comes to a rolling boil, place the crab on the rack and cover the roaster. Reduce the heat to approximately 400 F. When you place the lid on, set it so that there is space to allow some steam to escape.
Cook the crab for approximately 10-20 minutes, depending on the size of the crab. Do not remove the lid to check the crab frequently. It should be completely bright red when you remove it from the roaster, with none of the original blue or green color on the shell.
Break off one claw of the crab and check the meat. It should be solid white and firm. If it is at all pinkish or translucent, place the crab back in the pot and continue steaming until it's done.