A king-size loft bed is certainly an unconventional piece of furniture, but it saves more than 40 square feet of floor space. This project will take a few hours. These plans are for pine and plywood, the cheapest woods available, but you can substitute more expensive lumber for a fancier finished product.
Assemble the mattress frame
Set both 80-inch beams and two 73-inch beams in a rectangle, lying on the narrow ends. The short beams should be inside the longer beams.
Screw the rectangle in place using two screws at each joint. Drill the screws through the faces of the long beams and into the center of the ends of the short beams.
Set a 73-inch beam inside the rectangular frame, parallel to and 15 inches from one of the 73-inch beams in the frame. Screw it in place as you did the other beams.
Repeat step three for the other three 73-inch beams, placing each beam 15 inches from the beam installed before it.
Lay the plywood sheet on top of the frame, edges flush with the edges of the beams. Screw it in place with one screw at each corner.
Reinforce the plywood. Place two screws in each 76-inch end, placed 25 and 50 inches from the end. Place three screws in each 80-inch side, placed 20, 40 and 60 inches from the end.
Attach the support posts
Cut your 8-inch cubic blocks in half down the diagonal, making a total of 10 triangular blocks.
Move all components to the room where you will install the bed. Once you add the posts, it will be difficult to move.
Set the frame on its side with one 80 inch side on the floor. Position the frame so you can access both the plywood side and the side with the beams exposed.
Set one support post on either of the corners of your frame nearest the floor, nestled in the angle formed by the two beams.
Slide the piece of two inch scrap lumber under the far end (the end not nestled into your frame) of the post to keep it level.
Screw the post in place with four screws, one for each corner of the post. Screw through the plywood and into the end of the post.
Reinforce the post's attachment by drilling two screws through the face of the 73-inch beam and into the side of the post.
Slide one triangular block into position at the corner formed by the post and the 73-inch beam. Screw it to the beam with two screws drilled through the beam and into the block. Screw it to the post with two screws through the tapering side of the block and into the post.
Repeat step eight to attach a second triangular block to the corner formed by the post and the 80-inch beam.
Repeat steps four through nine to attach the other four posts. Flip the frame once you've installed the second post so that the 80 inch side without posts attached is now at the bottom.
Attach the fifth post in the corner formed by one 80-inch beam and the center 73-inch beam, otherwise following the same method as the other four posts.
Hoist your bed frame upright and into place, with the center post next to a wall. It should be safe anywhere in the room, but will be slightly more stable against a wall.