Measure the site for the greenhouse. If you don't have room for the 8-by-8-foot greenhouse described in this article, adjust the size according to the space available. Use a calculator to scale the greenhouse. For example, if your space is only 6-by-6-feet, divide six by eight to find the percentage; the greenhouse will be 75 percent smaller. Multiply each item's length by 75 percent (0.75) to find the size of the boards. In other words, the 12-foot board becomes a 9-foot board, the 8-foot boards become 6 feet and so on.
Remove the grass with a shovel, making an 8-by-8-foot square. Using a level and one of the 8-foot boards, make sure the ground is even. Dig a 2-foot-deep posthole at each corner. Pour 4 inches of gravel into each hole. Measure the diagonal across the square, from hole to hole. Fill each hole with water and allow it to drain into the soil.
Measure and mark 20 inches up on one end of the 12-foot boards. Lay out two boards, 6-inch sides down, with the tops overlapping and the bottom--at the 20-inch mark--the exact measure of the diagonal. Mark the angle of the overlap on the top and cut all four boards at the same angle. Drill a hole through the top of each board, 8 inches from the top and centered on the board.
Lay out the first pair of 12-foot boards, with the angled cut of the tops tightly butted together. Using the 2-foot board, screw it to each of the boards, temporarily attaching them together. Repeat further down with the 6-foot board. You now have the frame of a large triangle.
Raise the frame carefully with at least one helper and another on a ladder to help steady it. Drop the bottom into the post holes, diagonally across the square. Use the level to keep the frame vertical while one helper pours dry concrete mix into each hole. Carefully add a gallon of water to the concrete.
Lift another 12-foot board into place, with the bottom in the post hole and the top butted tightly to the top of the frame. Use an L-bracket on each side, attaching the board to the frame. Fill the hole with concrete mix and add water. Repeat with the last 12-foot board. Thread cable through the holes and connect them with two wire rope clamps.
Install the 8-foot boards with 3-inch screws around the base of the pyramid frame. Add more water if necessary to the postholes, then allow the concrete to dry for at least 24 hours.
Measure the storm door. Build a door frame with scrap lumber or 2-by-4-inch boards. Measure and install a 2-by-4 board horizontally across one side of the pyramid frame as a header for the door frame. Attach the door frame to the header and one side of the pyramid, or center it on the wall according to your personal aesthetics. Install the storm door according to the package directions.
Cover the exterior of the greenhouse with flat polycarbonate panels. Install them horizontally, carefully cutting the sides of the panels to fit the angles of the pyramid. Leave a space at the top of the pyramid or install a vent so heat can escape.
Roll weed cloth across the pyramid's floor. Install wood chips, gravel, pavers or bricks over the weed cloth. Hang a large outdoor thermometer inside your new pyramid greenhouse.