Mount 6x6 Deck Rail Posts Flush and Rock-Solid

The Problem

A guardrail post takes its load sideways — someone leaning, a crowd against it — and a post lag-screwed flat to the band board works loose exactly that way. The mount needs the post locked into the deck's framing, not stuck on its face, and the waterproofing under it has to go on before the post does.

The Technique

1. Prep the rim first: Z flashing and flashing tape across the top of the band board, butyl tape down the face — the post covers this forever, so it happens now.
2. Notch the post. The rail system here calls for a 4x4; the crew upsizes to 6x6 and notches 3" out of the bottom, leaving a full 2-1/2" x 5-1/2" leg. The notch laps over the band board so the post face lands flush with the deck's outside — the look a surface-mounted 4x4 can't give.
3. Locate and tack. Find the post's centerline on the band board, set the notched post over the rim, and drive one 5" structural ledger screw at an angle — pitched so it doesn't blow out the back of the band board. Five inches roughly matches the 2-1/2" leg plus the band; the screw holds the post while you work it plumb.
4. Block around the post. Fit blocking against the post's sides inside the rim and nail it off, pulling the post to plumb as the blocking goes in. Even one screw and one block makes the post noticeably rigid.
5. Through-bolt to finish: 1/2" carriage bolts through the band board, blocking, and post — the creator's word for the result is bomb-proof.