Curved decks are interesting and unique but somewhat difficult to build because deck framing materials are straight and rigid. It is always best to start with a good plan. Curved decks are easier to draw on paper than to build in the real world. You will need to use an AutoCAD program or graph paper to deterimine the center of the circle and the lenght of the radius (the distance from the center of the cirlce to any point along the edge of the circle).
In order to build a curved deck you use a series of angled dropped beams, posts, and footings to support cantilevered joists that will extend across the curved or round section of arc. Joists are usually allowed to cantilever a beam by 2′ without hiring an engineer. You can run the ends of the joists wild past the beam because you will be cutting them back all at once.
Next you will lay out and mark the radius across the ends of your joists. You can use a deck board as a compass. Use your plan to locate the pivot position and fix your compass to the center point with a screw. Leave the screw a bit loose so the compass board can turn. If the center point lands in between joists you will need to install cross blocking to attach the compass pivot point. Arc the compass across the front ends of the joists to mark the radius on each side of the joist with a construction pencil. Then you will cut the joists to the proper lengths and prepare your front rim board to cover the ends of the joists against the curved section of deck.
If the pivot point is located in the house you will need to layout the circle in the yard. Lay pieces of plywood along the circumference of the circle. You can trace and cut these plywood pieces into templates that you can layout on top of the deck to trace your radius.
It is always a good idea to install blocking between joists along the curved ends of the deck. This will strengthen the frame and create a good clamping surface to bend your fascia board across the curved radius.
The rim board can be bent using kerfed cuts. This involves scoring the 2x material with a circular saw to allow it to bend across the radius. Or you can use 1/2″ plywood in layers built up to 1.5″ thickness.
Another method for installing a curved rim or header board is to soak strips of pressure treated plywood in hot water for about an hour to increase its flexibility. You can then bend the pressure treated plywood strips across the front of the joists and screw it into place.
Composite decking is flexible enough to be bent across curved sections of deck. If the decking is hot it will be even easier to bend into the desired shapes. Fascia board is flexible and can be installed over the rim board to clean up the edges. Some composite and metal railing companies offer curved railing that can be custom ordered to match your decks radius.
Source: https://www.decks.com/how-to/509/building-a-round-or-curved-deck
