How to Build a Square Concrete Pit for a Hot Tub

Mark the perimeter of the planned pit with stakes and string. Make the perimeter of the pit 4 feet wider than the hot tub you will install, to compensate for the thickness of the concrete and to leave room for equipment. For this example, the hot tub is 8 feet by 8 feet by 4 feet, so the pit will be dug 12 feet by 12 feet by 4 feet. Use the carpenter's square to ensure that the corners are perfectly 90 degrees. Mark under the string with the spray paint as well, to aid in getting accurate pit dimensions.

Dig out the pit using a shovel and wheelbarrow. Start from one corner, digging down 4 feet as straight as possible. Dig your way diagonally across the pit, so you have a basic ramp to use to cart out dirt. Keep the sides as straight as possible, although they don't have to be perfect, because the poured concrete will compensate.

Cut 2-by-4-inch wood to the following dimensions: 12 pieces 11 feet, 11 inches long, and four pieces 4 feet long. While standing in the pit, nail these together to create a four-sided box frame around you. The four sides will have an 11-foot, 11-inch piece at the top, middle and bottom, with a 4-foot piece at each side. Hammer nails through the 4-foot pieces, into the 11-foot, 11-inch pieces. Cut plywood sheets to 11 feet, 11 inches by 4 feet. Nail them into the outer sides of the frame by hammering nails through the plywood, into the 2-by-4-inch frame. Add a nail every 3 feet along where the plywood meets the frame.

Attach the four sides by putting nails through the side of the 4-foot vertical piece, and into the ends of another 4-foot vertical piece. Make sure the frame is centered within the pit, with a 1-inch gap between the frame and the pit. Make sure the plywood fits tightly together at the seam between the wood and the floor of the pit. Push the frame down so it sinks slightly into the ground to create a tighter seal.

Pour concrete in the wheelbarrow and add water in the proportion listed on the bag. Stir with the shovel in a figure-eight pattern. Do this one bag at a time because it will become heavy. Pour concrete into the gap between the frame and the pit. Work quickly, as the concrete will start to harden. Allow the concrete to dry as instructed on the packaging, at least overnight.

Remove the frame from the pit after the concrete has dried. Pour a 2-inch slab of concrete on the bottom of the pit, using the same method you used for the walls. Use a concrete float on a pole to level the pad. Allow it to dry overnight.