Curved decks are interesting and unique but somewhat difficult to build because deck framing materials are straight and rigid. 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 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 lay out and mark the radius across the ends of your joists. You can use a compass stick. Use your plan to locate the pivot position and fix your compass to the center of the deck. Arc the compass across the front ends of the joists to mark the radius cut with a construction pencil. 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.
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" thinkness.
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.
To finish a curved deck you will need to install blocking along the curved ends of the deck to support the decking. 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.
Building a curved deck frame
Curved decks can be unique and beautiful outdoor living spaces. However building a curved deck can be quite challenging because lumber is straight and difficult to bend. We will show you some tips for building a curved or round deck. It is important to start with a good deck plan. You can layout your footing positions and determine the radiuses for your curved sections. Notice the (3) 8' radius sections and the 42" radius going around the tree to the right. In this presentation we will be explaining how to build the 42" radius section. Curved decks usually require cantilevering your joists over a series of beams. You can then cut the joists back to follow the radius of the deck. Notice the angled beam that was designed to support the arc at the corner of the deck. We built an 8' x 8' gluing table to stage the radius laminated rim joist. Set a nail at one end of the compass. Your entire radius should fit onto the plywood. Measure from the nail to the inside radius of the joists. For our 42" radius curve we will draw our radius line at 40.5". Cut your compass stick to the correct length and swing the compass across the gluing table with a pencil to draw you arc. Build clamp brackets from 3/4" plywood to attach along the arc to bend the radius rim joist. Use a table saw to rip down 3/8" thick strips from 2x material. Run the strips through a planer to create 1/4" lamination strips. For our 42" radius we used (6) 1/4" strips. For the 8' radius we used (3) 1/2" pressure treated plywood strips. Lightly dampen both sides the strips to activate the glue. Use polyurethane or gorilla glue on one side of each lamination strip. Draw a center line on each strip and align at 90 degrees to your pivot point. Start at the center and clamp the glued strips together attaching to the clamp brackets working from the center to the end of the rim joist. You only have about 10 minutes before the glue starts hardening. Try to keep the lamination strips level with each other. You may need to double clamp the ends of the radius rim joist. Allow the lamination to harden for 24 hours. You may need to run the laminated rim joist through a table saw to cut to the desired width. Attach the laminated rim joist to the deck frame with structural screws. Notice the 8' plywood lamination radius rim joists across the front of the deck. You can use the same lamination process to create custom radius railings or benches to follow your curves. This bench was made using a series of 1/4" x 2-1/2" laminated cedar strips. We used temporary removable 1/2" plastic strips between cedar strips to create the gaps. Afterwards we installed 1/2" cedar spacers to add strength every 16" along the radius.
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