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The deck has a plywood subfloor covered by vinyl sheeting, which has a square cut-out at the corner perfectly fitting the deck post through it. As such, the 6"x6" post sits directly on the plywood. The caulk between the post and the vinyl slowly seeped water which has caused at least 80% of the post to rot at the base, as has the plywood.

Covered deck

Rotten post base with rotten plywood directly under it

Water damaged plywood from underneath

I'm planning to replace the post with a pressure treated lumber, carefully cut the rotten plywood from below, and replace with a new clean sheet.

My question, is if the vinyl decking material supposed to completely cover the plywood where the post sits, or should I recreate what was originally here? Should I be laminating more vinyl material over top the plywood and underneath the post? Thanks in advance.

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Vinyl decking must be lapped at the post.

Apply the floor portion and cut at the post leaving an extra 1 or 2 inches so that it forms a collar (upward) around the post. This will leave the post's corners exposed of course.

Then apply additional decking as a wrap around the post and let it skirt (downward) an extra 2 inches to lap over the vinyl floor. Seal the lapping as any other seam. Pay extra attention to sealing at the corners.

How high the wrap around the post is depends on your rain and snow conditions and whether you'll have post facia lapping over it.

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The referenced video does not show the upward collar. Always go with the manufacturer's recommended method if unsure.

Ref: https://www.youtube.com/watch?v=yU1yesKiJB0

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