I don’t have a deck, but I saw a deck sofa made from 2x4’s on Instructables and liked the idea of the build. I thought I would give it a try. Though it is going to weigh at least 80 pounds (each 2” x 4” x 8’ weighs ~ 10 pounds, up to 17 for treated - this uses “8” of them). I modified and built a “deck throne”. Basically it is a chair and a half. The sides will be 33” tall and 27” deep and the seat will be 35” wide. I used almost a pound of 2½” deck screws.
I had 6 salvaged 2” x 6” x 8’ish from someone's deck. They were doubled up for support, so I had to cut them apart with a sawzall, then remove the half nails from each side (old fashioned twisted deck nails). I used 3 of these for the sides of the chair. I cut to length and ripped the edges off to neaten them up and get them down to 4.5”. I didn’t feel like cutting them down to 3.5” and have a bunch of short 1” x 2” strips left. There are 4 - 30” (verticals) and 4 - 27” (front to back) and 2 - 24” cross support that will also hold the seat. I sanded a little to clean them up. I contoured the inside of the top/arm rest...just because. I also added a 2nd, 24” front to back piece from a piece of 2” x 3”ish that I had. The top of the main cross support was placed 18” up, the other cross piece was placed 5” above that.
I bought 2 treated 2” x 4” x 8’ for the seat frame. I cut 2 - 35” for the width and 3 - 23” for the depth. I used a ½” rounded edge in the router for the outward facing edges of the front & back boards (35” ones). I had some 1” x 3” x 8’ for the top. These were cut just shy of 35” and it took 8 pieces with a ¾” gap between each. They were put on with a brad nailer. I set the front one back just a touch for comfort and the last one was just a bit shy of the back edge. The seat was placed flush to the top of the cross support on the sides, as well as flush to the front of the sides and it will be about an inch short of the back.
The final piece was something for the back. I had a leftover piece of 5/4” decking that was almost the exact length. It also almost fit perfectly between the top cross piece and the arm rest. I had to sand about a ¼” off so it would fit in (but just the part that is under the arm rest, not the whole thing)
I can have my feet up or I can sit sideways. Or just regular with some space on the sides. Two little kids could easily fit.
Now I should think about cushions...make them? Or just throw a blanket on it when using?