Base (plinth) for rotted post?

I have to fix up six posts holding up a roof overhang, the bottom few inches are rotted. They look like an I-Beam with a 4x6 core and 2x6 sides and are rotted at the bottom. The top goes into a beam pocket which I prefer not to mess with. The posts sit on an elevated wood deck that I am re-building--so assume the new substrate is 2 layers of ply, which I will then add cement board, water-proof membrane, and tile to.
My idea was to saw off the bottom 4 inches and build a little square plinth underneath. But what to use?
One idea is to simply cast in place with concrete, then get the waterproof membrane a couple of inches or so up the concrete sides before tiling. Anyone used this method before? Any problems to anticipate?
Another idea is to put an undersize block of wood under the post, then seal the sides and smooth to match with Bondo, WoodEpox or some such. I have never tried this before either?
I would most appreciate your thoughts and experience on this.
Cheers, Shawn
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How old was it?
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on the main beam.
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That's a career in a wet place. You can shorten the post with a block at the bottom if it's all fastened together. A piece with the grain running horizontally will not suck up as much water at the beam top, but the shrinkage across the grain can be appreciable. If the whole thing is clad, do yourself a favor and use something to stop the water from wicking up the post. (gasket/treatment)
--


MichaelB
www.michaelbulatovich.ca
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of the post, but build a melamine form around the area (only about 4in high) and fill it the entire footprint with fiberglass resin. That should keep the endgrain from soaking up any more water.... unless there is an easier way for this?
The whole deck will get two layers of ply, cement board, waterproof membrane, and then tiles. But the underside of the ply (Sturdifloor T&G) will be open to the elements. I won't be able to paint before installation, but should I paint the underside before installation to guard against insects and moisture absorption?
Thanks, Shawn
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In a previous post Rima Neas wrote...

Use a piece of HDPE cut to the same size as the post. It will never rot and you can use wood working tools to get it to the right size.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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