How to Fasten a Deck Railing Top Cap

The top cap of the deck is a broad rail that extends the entire length of the upper edge of the framing rail.

The First Section of the Top Cap

Check local building codes for required height of a deck railing.Check local building codes for required height of a deck railing.
It is an optional ornamental feature that can serve as a shelf to rest potted plants, lamps and beverages. Deck railing is typically built from 2-by-4-inch weather and rot-resistant milled planks. The top cap is constructed of 2-by-6-inch planks. Like trim molding used inside the home, a professional-looking finish is achieved by using mitered joints in the corners.

Step 1

Measure and note the length of the first section of railing where you plan to install the top cap. Start on a side adjacent to an entrance to the deck. The measurement is taken from the edge of the first post to the center of a diagonal line across the adjacent corner of the framing rail. One end of the top cap will be cut at 90 degrees. The other end will be cut at 45 degrees to form a mitered joint in the first corner.

Step 2

Lay the 2-by-6 plank on a flat surface. Look at the end of the plank. Turn the plank, if necessary, so the concave sides of the growth rings face downward. You will cut and install all of the planks in this orientation.

Step 3

Add 1 inch to the length of the first framing rail to calculate the length of the top cap. Measure and mark the 2-by-6-inch plank to this length. Place a carpenter's square on the mark, then draw a line across the 6-inch surface of the plank. Measure and mark the center of the line. The mitered cut must pass through the mark in the center of the line.

Step 4

Rotate the blade on the miter saw to make 45-degree cuts. Lay the plank on the saw table with its 6-inch surfaces horizontal and a 2-inch surface against the guide fence. Align the plank so the blade will pass through the mark. Support the other end of the plank with scrap lumber so the plank is horizontal. Holding the plank firmly against the guide fence, trim the plank.

Step 5

Lay the plank on the deck rail. The center of the mitered cut is centered to the corner of the deck rail. The opposite end of the top cap extends 1 inch past the opposite end of the rail. Use a drill and driving bit to screw the top cap to the framing rail using 2 1/2-inch deck screws. The screws should be evenly spaced, about 8 inches apart.

Trimming Planks with Two Miter-Cut Ends

Step 1

Measure and note the distance between two adjacent corners of the railing. The measurements are taken from the centers of diagonal lines across the corners or posts.

Trim one end of the plank with a 45-degree mitered cut. Measure and mark the center of the cut edge. Measure from the center of the trimmed edge to mark the length of the top cap on the plank.

Step 2

Draw a perpendicular line, using a carpenter's square, across the plank at the mark. Measure and mark the center of the line. The mitered cut must pass through the mark in the center of the line.

Step 3

Rotate the saw blade to make a 45-degree cut. The cut ends of the plank are divergent, not parallel. Lay the plank on the saw table with its 6-inch surfaces horizontal and a 2-inch surface against the guide fence. Align the plank so the blade will pass through the mark. Support the other end of the plank with scrap lumber so the plank is horizontal. Holding the plank firmly against the guide fence, trim the plank.

Things You Will Need

  • Measuring tape
  • Pencil
  • 2-by-6-inch weather and rot-resistant lumber
  • Carpenter's square
  • Scrap lumber
  • Miter saw
  • Drill with driving bits
  • Deck screws, 2 1/2 inches long

Tip

  • Draw a diagonal line across the board in the direction of the cut before trimming to avoid confusion regarding the direction of the mitered cut.

About the Author

Denise Nyland "Denisen" is a long term resident of Panama City, Fla. She studied radiologic sciences and education and has published articles in multiple professional journals and contributed to various educational texts.