Scribing decking so that it fits tightly around guardrail posts is a fussy job that requires repeated measuring and marking of cut lines. While marking the sides of the posts using a square is pretty straightforward, it’s trickier marking the depth of the cut parallel with the decking, especially if a post is slightly out of alignment with the other posts. To simplify the process, I use a post scribe card—a simple jig that I make with a piece of aluminum flashing to help me to mark the parallel line precisely every time.
After lining up the border deck board against the run of guard posts, I measure the distance from the edge of the deck board to the desired overhang line. For example, if the distance from the inside edge of the posts to the edge of the deck fascia is 1 1⁄4 inches and I want the decking to overhang the fascia by an inch, the total would be 2 1⁄4 inches.
Then I take an aluminum flashing card and bend it at a 90-degree angle so that one leg measures 2 1⁄4 inches. I press the inside edge of the bend against each deck post and scribe the cut line along the 2 1⁄4-inch leg of the card. If the post is twisted or slightly out of line with the other posts, the jig will automatically account for the variation. So when I cut out the notches for the border, each post will have a perfect fit.
This technique also works with posts that are mounted inside the fascia. Often, with the internal posts, the joint between two boards falls somewhere in the middle of the posts, so you have to scribe both pieces of decking. With the post scribe card, the process is a lot faster and easier.