Spacer between dobie metal bracket and 4x4 PT support post
My deck design uses dobie concrete blocks embedded in concrete piers for mounting 4x4 support posts.
A contractor friend told me to put a piece of cardboard between the metal bracket and the post. He claimed that it slows rot by preventing rain water from getting wicked up into the post when the post doesn't touch the metal bracket at the bottom. I assume the cardboard just rots away and a small space is left for the water to drain away.
I guess that that makes sense, but it seems like there is less support. If the end of the post rests directly on the metal bracket doesn't that provide significantly more support? Otherwise the support depends entirely on just the nails.
The idea of putting a water absorbing biodegradable material like cardboard underneath a support post set into a support post bracket is counter intuitive to me.
Firstly, a support post should be fully resting on the supporting bracket to distribute the load down to the pier effectively. As you have described this, the support post is elevated above the bottom of the supporting bracket and held in place by horizontal fasteners - a very bad idea for so many reasons.
Secondly, if you are concerned about water absorption into the bottom of the post there are better ways to address it. You can buy a one inch support post stand off base that may be a bit smaller than the size of the post and so water would never have a chance to get under the post.
If the metal bracket consists of just two vertical extended plates and resting surface for the post is just the concrete then I would recommend a spacer of some sort that is synthetic like a block of plastic. You can buy this material at any plastic supply shop. High density polyethylene or similar material would work fine and your post would be dry forever.
Unless I am missing something your friend's suggestion is not a great idea.
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