Using metal grill grates as a guide, determine the size and placement of the brick barbecue. If you desire a barbecue with walls thicker than one course of bricks, account for that now. Mark off the area and clear away all grass and debris from the site. Excavate the area to a depth of 8”. Use tamper to level and compact the ground. Add pea gravel to a height of 3” then compact and level with tamper.
Mix fast-drying cement according to manufacturer's directions. Pour on top of gravel until the hole is filled to ground level. Smooth and level the cement pad using a trowel. Allow cement to cure for 24-48 hours.
Dry-fit the first course of bricks before setting them in mortar. Lay the bricks end to end around the perimeter of the concrete slab, leaving a half-inch gap between each brick to account for the mortar. Make sure the bricks are perfectly square at the corners. Mark the outline of the bricks to use as a guide while laying the bricks.
Lay out the bricks and hose them down with water 30 minutes before laying them. This will prevent them from soaking up excess moisture from your mortar.
Prepare premixed mortar according to manufacturer's directions. Trowel a generous layer of mortar along the marked lines where the bricks will sit. Lay the first brick by pressing it gently into the mortar. Apply a layer of mortar to one end of the next brick before butting it against the first and pressing it into the mortar. Remove excess mortar as you work. Continue working in this fashion until the first course has been laid around the entire base of the barbecue. Check to make sure the course is square, plumb and level, adjusting a brick by tapping with trowel handle.
This and the remaining brick courses will be built only on the outside and rear walls, leaving the front open for access to charcoal, ash and grill grates. Apply mortar to the top of the previous course. Lay the first brick by pressing it gently into the mortar, making sure to stagger the joints. Bricks may need to be cut with a cold chisel or wet saw for a proper fit. Continue laying bricks until this and the following course are completed.
Starting with the fourth course, and continuing with every other course, turn some of the bricks so they jut out into the barbecue. These will serve as the grill grate support ledges. Having a few at different heights allows extra cooking space as well as various heat intensities. Continue laying bricks in this fashion until desired height is reached. Make sure the top and final course is laid with solid bricks.
After the final course is set and before the mortar has dried, "strike" the joints with a jointer to create the familiar concave depression in the mortar. Once the cement is almost cured, use a stiff brush to scrape off remaining bits of mortar.