A hexagonal deck takes a lot more time to build than a rectangular one but it is a lot more interesting to look at.
Step 1: Leveling and Outside Frame
Level the area where the deck is going to go. Laying out the outside frame is really important. If you get the angles exact, you will be able to mass cut all of your deck boards. If you are off even slightly in your angles, you will have to measure and then cut every angle on the deck boards. If your deck is sitting on the ground, you can use pressure treated 2 x 4's as framing.
Decide how big across your deck is going to be. Here are some distances. The diagonal is the distance from a corner to the opposite corner. The deck in the picture has a 14 foot diagonal.
A diagonal of 96 inches (8 feet) needs a side length of 48 inches
A diagonal of 108 inches (9 feet) needs a side length of 54 inches
A diagonal of 120 inches (10 feet) needs a side length of 60 inches
You'll notice that for every additional diagonal foot, the side length increases six inches.
Cut the ends of six side pieces exactly 60 degrees. Remember to put end cut paint on all newly cut ends. Screw them together with deck screws, making sure that they are exactly matched up. Put this hexagon in place on the ground and make sure that each angle is 120 degrees. Cut one end of the diagonal piece into a V with the angles being 60 degrees. Place it against one inside corner of the hexagon and then measure and cut the other end of the diagonal piece by putting it on the opposite corner and drawing a pencil line underneath. Screw this diagonal in place when it is cut and painted.
Step 2: Inside Framing
Mark the center by finding half of the diagonal. Cut and screw in rays that go from the center to each corner. Make a smaller hexagon by cutting cross pieces 16 inches long and screwing them in between the rays. Fill in the additional framing by adding joists and more angled pieces so that there is not a span of over 16 inches. This is for deck boards which are 1 1/4 inches thick. If you are using thicker stock for the deck, your span can be greater. Screw in all the pieces and check all the outside angles again to see if they have shifted.
Step 3: Deck Boards
Measure one outside piece of the frame and add 1/2 inch to it. Cut six pieces of deck board with 60 degree angles to this length. Place them along the outside of the framing and screw them on. They should extend past the edge of the framing a bit. Put a pencil mark 1/2 inch inside two adjacent rays and in the middle of the rays. Measure the distance between these two pencil marks and cut your next set of six deck boards that length. Screw them on and continue in towards the inside of the hexagon. When you are about four feet from the center, measure the distance from the last board to the center. Each deck board should be 5 1/2 inches wide with a 1/2 inch gap between them. If your measurement is slightly off a multiple of six inches, make the gap between all of the next boards slightly more or less so that you will end up with the last boards close to the center.