Found a total rot disaster under my siding, can I sister these studs?

You're looking at a classic case of the "builder grade" special from the early 2000s where crews thought fiber cement was a standalone barrier and skipped the house wrap. If the OSB is mush, the moisture has been trapped against those studs for a long time. You definitely need to pull the drywall on the inside of the garage. There’s no way to judge the structural integrity untill you see the back side of the wall cavity and the sill plate.

If the rot is localized to the outer face, you can often get away with **sistering** new studs to the old ones, provided there’s enugh meat left to bite into. But if the rot goes more than halfway thru or the wood is soft all the way to the interior, you’re looking at a full replacement. Since this is near a garage door, the jack and king studs are likely carrying a heavy load from the header. If those are soft, you’ll need to shore up the header with a temporary 4x4 post before you start ripping things out. Use pressure-treated lumber for any new bottom plates that sit on the slab. Once the framing is solid, get some real **WRB** up there—don't just patch the siding back. A lot of these failures happen because of a missing kick-out flashing where the gutter meets the wall. Check that transition carefully or you'll be doing this again in five years. It feels overwhelming because you’re seeing it all at once, but it’s just sticks and sheets. Since you do kitchens, you already have the precision needed; you just have to shift your focus to managing water flow instead of cabinet reveals. It's a messy job, but it's definitely doable if you take it one section at a time and ensure your drainage plane is continuous.

I'm pretty new to this, and the thougt of 'mushy' wood behind the walls is terrifying. When you say to take it one section at a time, how do you decide where to stop cutting? If the OSB feels dry but looks stained, shold that go too, or is it okay once it's not soft anymore?

Also, I'm curious about the drainage plane. Is that just the wrap, or do I need a gap between the paper and the siding so it can breathe? I'd hate to fix the studs just to have the same rot happen again because I didn't understand how the water is supposed to escape.

Grab a flathead screwdriver and poke those stained areas. If it sinks in even a little, rip it out. OSB is cheap insurance compared to sealing in rot. For the drainage gap, look into a rainscreen—basicly furring strips over the wrap. It creates an air space so the back of the Hardie can actually dry out. Without that gap, moisture just sits there. Also, check your sill plate. If the studs are bad, the plate on the concrete is usally a sponge. Hit it with a hammer to see if it thuds or crumbles. Since you do kitchens, you know how much a tiny sag matters; if the wall dropped, that garage door will start binding soon.