Lay out your deck. Decide whether the 20- or 14-foot side will run parallel to the house. Mark the corners by driving in a wood stake at each corner location. Tie mason's line between the stakes at the height of your deck top, minus 1¼ inches for the decking, to mark the outline. Use a string level to level the mason's line.
Measure along one 20-foot side and wrap a piece of colored tape to the mason's line every 5 feet. Repeat the process on the other 20 foot side. Mark each 14-foot side with tape in its center. Tie mason's line between each tape mark on the first 20-foot side to the corresponding mark on the opposite 20-foot side, and one piece between the center marks of the 14-foot sides to form a grid. Dig a post hole directly beneath each tape mark, under every intersection on the grid and at each corner. Make your holes 10 to 12 inches across and 24 inches deep.
Measure from the bottom of each hole to the mason's line. Cut a 4-by-4 post to that length and set one post and a half bag of concrete in each hole. Bolt 14-foot 2-by-8s to each post in the three 14-foot rows of posts formed. The two outside 2-by-8s should be attached on the outside of the posts, the center 2-by-8 may run on either side. Drill two 1/4-inch by 2-inch pilot holes through the 2-by-8 for each post and attach with 1/4-by-3 inch lag bolts.
Attach two 10-foot 2-by-8s to each each 20-foot side in the same manner, with the middle joint in the center of the center post. Attach 2-by-6 joist hanger brackets along the insides of the outside 14-foot 2-by-8s and both sides of the center 2-by-8 every 16 inches. Make sure the hangers on the center 2-by-8 are directly across from the outside hangers. Cut 2-by-6s to fit between the 2-by-8s. Install one for every pair of hangers using 1-5/8 treated deck screws.
Add water to the holes and mix the concrete, fill the remainder of the holes with dirt and pack it tight.
Start along one 14-foot side to attach the 5/4 treated decking. Use two 1-5/8 inch treated deck screws in each 2-by-6 joist. Attach the second board full length and let it hang off. Start the second row from the opposite end so that the end joints stagger. Space between rows 1/4-inch for swelling. Continue laying until the deck is covered. Use a chalk line to mark a straight line across the overhanging boards at the deck ends. Cut them off along the line with a circular saw.