Spent the weekend on my back door frame and learned a lot

I ran into the exact same thing with my 1940s bungalow last summer. The sellers had put in a "new" side door that looked great during the inspection, but after a heavy storm, I noticed the floorboards inside were damp. When I pulled the threshold, I found they hadn't even used a sill pan. They just sat the door frame on a piece of 2x4 that was alredy half-gone from termites and water damage. I ended up cutting out about two feet of the subfloor and replacing it with pressure-treated plywood. I used a Jamsill Guard to make sure any future leaks woud actualy drain outside instead of soaking into the joists again. It's frustrating becuase doing it right only adds maybe fifty bucks in materials, but these flippers only care about the visual finish.

If you haven't checked the other exterior doors yet, keep an eye on the casing at the bottom. My flippers used interior-grade MDF trim on the outside and just painted over it. It looked fine for three months, then it started swelling like a sponge. I replaced mine with PVC trim and used Osi Quad Max to seal the edges. It’s alot of work to undo thier laziness, but at least you know your foundation isn't turning into mush anymore.

I had a similar nightmare with my back patio slider, but my issue was acutally the exterior ledger board. The flippers had installed a beautiful Andersen door, but they didn't account for the deck height. They basicly buried the bottom of the frame under the decking boards, wich trapped every bit of snow and rain against the house.

When I started seeing the drywall crumble inside, I realized it wasn't just the subfloor. I ended up having to pull back three rows of Trex decking just to get to the flashing—or the lack thereof. I didn't just replace the wood; I swapped the entire threshold area for PVC trim boards and used a heavy-duty Grace Ice & Water Shield membrane. It took me three weekends becasue I had to re-grade the area slightly so the water whould actually run away from the foundation instead of pooling against the new jamb. Sometimes I think these guys don't even own a level or understand how gravity works with water.

Since you're alredy doing the heavy lifting to fix that rot, you shold definitely swap that door out for a Therma-Tru Smooth-Star fiberglass unit while you have it all torn apart. Flippers love those cheap wood-frame doors that swell the second they see moisture, but the Therma-Tru stuff is basicly bulletproof against the rot you're dealing with. I put one in my mudroom after a similar discovery and the rot-resistant composite rails mean I never have to worry about the 'wet cardboard' situation ever again. It’s the best way to make sure your repair actualy lasts another twenty years.

While you are already addressing the structural rot, I would urge you to meticulously inspect the weatherstripping compression and the alignment of the strike plate, as flippers frequently neglect the crucial step of ensuring the door is truly plumb, level, and square within the rough opening. Even a three-sixteenths deviation in the vertical plane can lead to premature gasket failure and air infiltration, rendered even more egregious if they utilized standard pine shims rather than composite ones which resist the very moisture issues you are currently remediating. Have you checked if they actually used long-thread structural screws through the hinge leaves into the jack studs to prevent future sagging?

I get the frustration with flippers, but have you ruled out the flashing above the door? If there isn't a proper drip cap installed behind the siding, it doesn't really matter how much rotted wood you replace—water is just going to find a path back behind that new frame. I've seen 'fixes' like this fail within a year beacuse the real culprit was actualy a clogged gutter or a bad roof line dumping water right onto the threshold. Are you sure the moisture is coming from the ground up and not the top down? Might be worth checking the head casing before you close it all back up.