Mount Deck Rail Posts

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.

The Problem

A rail post has to end up plumb in two directions while carriage bolts — which want their holes drilled with the post already in position — lock it down. Drill both holes with the post leaning and the bolts faithfully preserve the lean.

The Technique

1. Tack the post with one structural screw, placed where a bolt hole will later land — the bolt erases the screw's hole, so the finished post shows no stray "face hole."
2. Drill and set the bottom carriage bolt first, snug but not final. Starting at the bottom makes the plumbing easier: the post pivots from a fixed base instead of swinging freely. Drive the bolt through with the sledge, run the nut down with a wrench.
3. Check plumb left-and-right — this is the two-person part: one below on the bolt, one above sighting the level — and hold it while drilling the second hole.
4. Set the second bolt. The post is now locked plumb side-to-side.
5. Shim before final tightening. Toward-the-house / away-from-the- house plumb gets corrected with shims behind the post, then the nuts get their final tightening against the shimmed position.

Working alone, the creator's suggestion is a site-built jig to hold the post where the second person would — the sequence stays the same.