As a guide, use a rasp (shown above) or power sander to shape the three top members so they’re aligned and flat.
Determine the Length and Angle Cuts
For the handrail. Screw four temporary blocks to the top rail assembly, then clamp the handrail to them. Mark the angle on both ends, then cut to length. Use knot-free straight-grained wood for easier bending.
The Final Steps
Check for high spots in the sub-cap assembly with a square and plane or sand down any uneven spots, since the handrail needs to lie flat (Photo 9).
Screw four 7-inch pieces of 1×3 scrap to the sub-cap, then clamp the handrail down and scribe the length and angle onto the handrail (P). Cut the handrail to length at the angles as marked.
Squeeze a bead of adhesive down on the sub-cap and handrail supports. Clamp down the outside end of the handrail first, then slowly push down the other end (at the center post), using clamps to help muscle the wood down the last few inches; the fit will be very tight. When you get to the last 1/2 inch, tap the end of the handrail down snug with a hammer and block of wood, then hold it in place with clamps at the ends until you’re finished securing it in place.
Using a countersink bit with an extension, drill two holes from below in each space between balusters, angling the bit toward the outside of the handrail, so the 1-5/8- inch screws will have more wood to bite into. Drive in the screws.
Use 4d casing nails to secure the trim pieces (J) to cover the gap between the ends of the deck boards and outer support arch. Apply caulk along these deck trim pieces, as well as at the ends and edges of the handrails. Pour pea gravel around the footings to provide a smooth gradual transition from the ground to the bridge walkway. Cut and glue the top caps (available at most home centers) onto the tops of the posts. Apply an exterior finish to the wood, let it dry, then take a stroll across your new bridge.
Install the Handrail