Because of the weight of the chimney, you will need to support the entire barbecue on a thick, reinforced slab of concrete. Check with your local building codes for the proper thickness. If the local code calls for less than 6 inches, make the base at least 6 inches. The steel reinforced drain pipe can be purchased at a sewer supply company or salvage yard. If you can, have someone cut it for you. If the place you purchase it won't or cannot cut it, you will need a diamond-bladed masonry saw to cut it.
Dig the hole for the cement footing. Make a round hole for the chimney. Connect a second hole to the round hole for the barbecue pit. The hole for the chimney footing should be 6 inches bigger than the drain pipe. The hole for the barbecue pit should butt up against the hole for the chimney so that it looks like one large hole. The barbecue pit hole can be square or round, and can be whatever size you wish. Keep in mind that the heat of the fire is determined by how close to the chimney you start the fire.
Pour the cement for the footing. If you have a solid-type soil, you may not need to use forms—just make sure the sides are packed tight and are even. Allow the footing to cure for at least seven days.
Stand the drain pipe up on the slab and center it. Face the square opening you cut in the drain pipe to the barbecue slab. If you want to make a smoker in the chimney, drill ½-inch holes about 6 inches down from the top of the drain pipe and insert two pieces of ½-inch rebar. Meat will be hung from the rebar.
Use the masonry saw to cut bevels in the bricks. Lay the bricks around the chimney to be sure the angle of the bevel is correct and the bricks line up properly against the curve of the pipe. When you find the correct angle, cut enough bricks to brick the outside of the chimney. Layer the bricks flat-side down around the chimney, and mortar them into place. Be sure to stagger each row of bricks. Do not cover the 12-inch opening.
Lay the bricks for the fire pit. If you chose to make a round fire pit, cut the bricks with the masonry saw to fit the angle of the curve of the fire pit. Stagger two rows on top of the first row of brick.
Lay fire brick upright against the inside of the brick in the fire pit. If you have a round fire pit, you will need to bevel cut the fire brick to the angle of the circle. Make a layer of cap bricks for the fire pit using the same angled cut as the bricks below, and lay them on top. Clean the excess mortar from the bricks.
Measure across the top of the chimney. Use this measurement to cut a piece of metal for the cover. Weld a handle onto the cover. The cover can be moved to let all of the smoke out or can be set so that only some of the smoke is released if you will be smoking meat inside the chimney.