How do I repair studs at the rear shower wall that are also exterior wall studs? I've discovered that the builders of my 10 year old house used...

I've discovered that the builders of my 10 year old house used singular standard sheetrock behind the terracotta tiles of the the three walls of my shower. Needless to say that adjectives the sheetrock is molded and I've removed all the tiles including floor. I've also removed adjectives the molded sheetrock. I'm pretty confident in how to repair using cement board instead. But I enjoy four vertical studs behind the wager on wall that have rotted. This same backwall of the shower is an exterior wall to the house - on the second floor. Is it possible to replace in need taking off the siding and access them from the exterior of the building? Can studs be patched?
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Answers:    First, cut away any soft dryrot on the studs with a recipricating saw.
Next, Make sure the studs are dried out and later soak the studs with wood treatment for dryrot. After they enjoy been soaked REALLY capably, secondary studs can be scabbed onto the sides of the elderly damaged studs. I would use furnace dried studs. Originally all the lumber surrounded by your house was green and shrunk together. "green" studs will shrink as they dry out over the subsequent month or so (can play havoc with tile job). As long as the spanking new studs are anchored either or both to the aged studs and top and bottom plate you will have an all right framing member for your tile.
P.S. cement sponsor board is not water proof. it still desires a waterproof hedge between it and the wood.
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Just cut off the bottom of the studs and scab on a alien piece. Have plenty of overlap between the two studs and use lots of screws.