Laying out and constructing a curved railing requires a basic understanding of geometry and the ability to make detailed drawings using a compass, protractor, and a special measuring tool called a scale ruler.

The method for constructing a curved cap rail works only for symmetrical, circular curves—quarter circles, half circles, or full circles. If your deck has irregular or elliptical curves, creating a cap rail is very difficult. For these curves, it is best to limit the railing design to include only balusters and a laminated top rail.

Curved cap rail is created from mitered segments of 2 × 12 lumber. After positioning the 2 × 12 segments end to end, the shape of the 6"-wide cap rail is outlined on the pieces. For a semicircle with a radius of up to 7 ft., four 2 × 12 segments will be needed, with ends mitered at 22 1/2°. For a semicircle with a larger radius, you will need eight segments, with ends mitered at 11 1/4°.

1. To create a curved top rail, use exterior glue to laminate four 1 1/2"-wide strips of 3/8"-thick cedar plywood together, using the curved rim joist of the deck as a bending form. First, cover the rim joist with kraft paper for protection. Then, begin wrapping strips of plywood around the rim joist. Clamp each strip in position, starting at one end of the curve. The strips should differ in length to ensure that butt joints will be staggered from layer to layer.

2. Continue working your way around the rim joist, toward the other end. Make sure to apply clamps on both sides of the butt joints where plywood strips meet. Cut the last strips slightly overlong, then trim the laminated rail to the correct length after the glue has set. For extra strength, drive 1" deck screws through the rail at 12" intervals after all strips are glued together. Unclamp the rail, and sand the top and bottom edges smooth.

3. Install prenotched 4 × 4 posts at the square corners of the deck. Then, cut 2 × 2 balusters to length, beveling the bottom ends at 45°. Attach the balusters to the rim joist with 2 1/2" deck screws, using a spacer to maintain even intervals. Clamp the curved top rail to the tops of the balusters and posts, then attach it with deck screws.

4. After the top rail and balusters are in place, attach 2 × 2 top rails to the balusters in the straight sections of the deck. The ends of the straight top rails should be flush against the ends of the curved top rail. Now, measure the distance between the inside faces of the balusters at each end of the curve. Divide this distance in half to find the required radius for the curved cap rail.

5. Using a scale of 1" equals 1 ft., make a diagram of the deck. (A scale ruler makes this job easier.) First, draw the arc of the deck with a compass, using the radius measurement found in step 4. Divide the curved portion of the deck into an even number of equal sections by using a protractor to draw radius lines from the center of the curve. For a semicircular curve, it is usually sufficient to draw eight radius lines, angled at 22 1/2° to one another. (For a deck with a radius of more than 7 ft., you may need to divide the semicircle into 16 portions, with radius lines angled at 11 1/4°.)

6. From the point where one of the radius lines intersects the curved outline of the deck, use the scale ruler to mark points 5 1/2" above and 5 1/2" below the intersection. From these points, use a protractor to draw perpendicular lines to the adjoining radius lines. The polygon outlined by the perpendicular lines and the adjoining radius lines represents the shape and size for all of the 2 × 12 segments that will be used to construct the cap rail.

7. Draw a pair of parallel arcs 5 1/2" apart, representing the curved cap railing, inside the outline for the 2 × 12 segments. Shade the portion of the drawing that lies between the straight parallel lines and the two adjacent radius lines. This area represents the shape and size for each of the angled 2 × 12 segments. Measure the angle of the miter at the ends of the board; in this example, the segments are mitered at 22 1/2°.

8. Measure the length of the long edge; this number is the overall length for each of the 2 × 12 segments you will be cutting. Using this highlighted area, determine how many segments you will need to complete the curve. For a semicircular curve with radius up to 7 ft., four segments are required, with ends mitered at 22 1/2°. For curves with a larger radius, you will need eight segments, with ends mitered at 11 1/4°.

9. Measure and mark 2 × 12 lumber for the cap rail segments, with ends angled inward at 22 1/2° from perpendicular. Set the blade on your circular saw or tablesaw to a 15° bevel, then make compound miter cuts along the marked lines. When cut to compound miters, the segments will form overlapping scarf joints that are less likely to reveal gaps between the boards.

10. Arrange the cap rail segments over the curved deck railing, and adjust the pieces, if necessary, so they are centered over the top rail. When you are satisfied with the layout, temporarily attach the segments in place by driving 2" deck screws up through the curved top rail. Measure and install the 2 × 6 cap railing for the straight portion of the railing.

11. Temporarily nail or clamp a long sturdy board between the railings at the start of the curve. Build a long compass, called a trammel, by nailing one end of a long 1 × 2 to a 1 ft.-long piece of 1 × 4. Measure from the nail out along the arm of the trammel, and drill holes at the desired radius measurements; for our application, there will be two holes, 5 1/2" apart, representing the width of the finished cap rail. Attach the 1 × 4 base of the trammel to the temporary board so the nail point is at the centerpoint of the deck rail curve, then insert a pencil through one of the holes in the trammel arm. Pivot the arm of the trammel around the cap rail, scribing a cutting line. Move the pencil to the other hole, and scribe a second line.

12. Remove the trammel, and unscrew the cap rail segments. Use a jig saw to cut along the scribed lines, then reposition the curved cap rail pieces over the top rail. Secure the cap rail by applying exterior adhesive to the joints and driving 2 1/2" deck screws up through the top rail. Remove saw marks by belt sanding.