Lay two planks of 14-inch 2-by-6 lumber flat on a work surface with a half inch between them. These are two legs planks. Place a 12 1/2-inch 1-by-2 lumber plank across both legs at one end for the top brace of the legs and center a 15-inch plank of 1-by-2 lumber on the other end for the base brace of the legs.
Drill two countersunk pilot holes through both brace pieces into each leg plank and secure the braces to the legs with 1 5/8-inch screws. Turn the leg unit over and attach a second top and base brace piece as you did the first. Repeat with four more legs, top and base brace planks so you have three leg units.
Stand the legs upright on their base braces, in a line with 20 inches between each leg. Place two planks of 60-inch 1-by-8 lumber across all three leg units with a 1/2-inch gap in the middle. There should be approximately 10 inches over on each of the outer leg units.
Drill two countersunk pilot holes through the two top planks into each of the top braces on the leg units and screw together with 1 5/8-inch screws.
Sand and finish the bench with the desired wood finish and let it dry before positioning where you want to install it on the deck. Drill two countersunk pilot holes through each of the base braces into the deck and screw down with 3-inch screws.