How to Build a Pentagon Deck

Henri Bauholz

Making a five-sided wooden deck is an interesting mathematical challenge and an adventure in carpentry skills. However, with some thoughtful planning, good general knowledge of wood and good wood-cutting skills, you can add a five-sided wooden deck to your backyard. Whether the pentagon platform is a small backyard patio or a larger outdoor creation, the principles are the same. Here, you can find out just how it is done.

How to Build a Petagon Deck

  1. Cut the five stringers first. If one considers a pentagon shape to be similar to a wheel, then it will be easier to understand than the first step will be cutting the "spokes of the wheel". These spokes are nothing more than five straight pieces that run out from the center to form the base of the pentagon. Once you have chosen the dimension of your framing joists (i.e. 2-by-6, 2-by-8 etc.) then go ahead and cut five pieces to that length. Please note the diameter of the deck will be approximately twice the size of the length of each spoke. For our deck we will cut the five "spokes" at a length of six feet. Go ahead and cut the five pieces and make sure each end is square.

  2. Cut five pieces for the inner circle. The magic number here is 54 degrees. Each piece will be one foot long and it should have a 54-degree angle cut on each end. Be sure you use a circular saw that can be set to this angle. (Calculating the magic number is not all that difficult, if you know that a pentagon has a total of 540 degrees. That means each angle is equal to 108 degrees and each cut will be 54 degrees.)

  3. Put the inner circle together. Do this by making a mark on one edge of the 2-by-6 four inches from the end of the board. Do this with a carpenter's square. This will be your point of attachment. Now go ahead and nail the angled pieces to the five long "spokes" at the pencil marks. You can use #12 or #16 galvanized framing nails. When all five crosspieces are nailed to the longer spokes you should have a completed circle that is just over two feet across with five long protruding pieces. This is the beginning of your frame.

  4. Cut the five pieces for the outer circle. They should all be the same length and have the same 54 degree angle cut on each end. For simplicity, we will cut five pieces at the length of eight feet, but you can take a measurement after step 3 is complete for a more accurate length.

  5. Go ahead and nail the outer row of 2-by-6s to the five spokes.

  6. Put the concrete piers in. Ordinarily this is the first thing that is done, but if you are building a pentagon frame for the first time, it might be best to wait until the frame is built. Put in a total of ten piers at all the junctures that you have created so far. ( a pier is nothing but a two foot hole in the ground filled with a concrete mixture) All piers should be level and dry before you continue. It may take a few days to make the piers and let them dry. (You can buy ready-made piers and place them in the ground. Try to have the top of each pier level with the other piers or close to it. Actually in the great outdoors, "close-to-level" is often better because it provides a natural drainage.

  7. Complete the frame. Run 2-by-6 joists at two foot intervals starting along the outside ring and run them towards the center of the five-sided wheel. You will have a frame that can support the deck. Remember, it is imperative that each joist is attached to the outside ring at a 90 degree angle at two foot intervals Keep this pattern and your frame will be functional. This will involve a lot of angled cutting, since most joists will splice into the spokes of the wheel.

  8. Lay down the planking. Use special deck boards that are just over an inch thick and don't forget that this deck is like a giant pie that is sliced into five pieces. You can nail down the deck boards doing one "slice of pie" at a time. Each board will need an angled cut. Have the blade circular saw set at 90 degrees, but instead angle each cut across the surface of the board, so it splits the center of the five "spokes" This across-the-surface angle will also be 54 degrees. Fill each section of the pentagon with the planking and don't forget to keep all boards parallel with the perimeter. Also remember you must leave small gaps between each deck board. (1/2 inch or thereabouts is good)