cedar deck rot repair question?

OK all you deck people. Several years ago I had my rather large deck completely replaced due to rot. The new deck used white cedar from the great northwest. I was told that it is extremely resistent to the normal deteriorating factors. So be it.
The carpenter poured new piers but kept them close to the ground. Then he mounted the end pieces (what ever they're called) on edge across the piers. On one side of the deck near a flower garden the mulch was placed up to the deck touching the side. Over the past 5-7 years the mulch decayed when had the effect of this end piece and all the bannisters attached to it to be below the surface of the mulch. As this was on the garden side no one really paid any attention.
This year, however, someone sat on the railing and most of the ss screws attaching the bannisters to the end piece detached exposing the fact that the end piece of the deck is quite rotted through as are the lower 6-10" of all the bannisters.
I purchased 2 x 10 x 18' of pressure treated lumber to attached to the old piece as I am not familiar with how to remove it if it happens to be a supporting end. I'm hoping that I can "sister" the treated lumber to the old one with ss screws and rebuild the railing using all new bannisters. Oh, and I've already shoveled out all the mulch and dirt to a good 3" below the wood now.
So, my question: is this approach feasible? Is there anything that I could/should do to the old piece prior to attaching the new wood?
Any tips and advice or even actual instructions, pointers to youtube, etc will be greatly appreciated. Unfortunately as a retired person I can no longer afford to call in the "Big Guns".
Thanks
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Decks are pretty simple. Go out and attack with pry bar for nails, or screw gun. Anything you can take off, you can cut a piece the same size and put back.
--
Dan Espen

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On Friday, August 30, 2013 9:55:13 PM UTC-4, net cop wrote:

+1
Unless something special is going on, which without seeing it we don't know, I don't see the point to sistering rotted wood. I would just replace it. I'm also not to keen on the concept of sistering to a piece of rotting wood. Sistering usually means the whole piece of whatever is there is left and then you secure another similar piece to it, like when a joist cracks or a plumber has foolishly cut big holes in it. If you put rotting wood in contact with new wood, you now have wet decaying wood, which is similar to what? mulch. So, it would seem to some extent you're starting the process over again.
Sounds like something was wrong from the design phase on this, either with height, soil grading, water runoff, etc. A few inches of mulch shouldn't wind up being in contact with the deck, certainly not to the extent that it can structurally decay a new deck in just a few years.
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