Step 3: Secure 2"x6" board between the posts

Start by measuring the distance between the 4"x4" posts. For greatest accuracy, be sure to measure from the bottom of the 4"x4" posts. (as 4x4's might be slightly off plumb or warped). Then cut the 2"x6" board to length.

Mark a line 34 1/2" above the deck surface on the 4"x4" post. Position the top edge of the 2"x6" flush with this line, then position the 2"x6" face flush to the inside edge of the 4"x4" post.

Lastly, secure the 2"x6" board to the 4"x4" post by placing one deck screw at an angle from the bottom of the 2"x6" board into the 4"x4" post, and one deck screw at an angle from the top of the 2"x6" board into the 4"x4" post.